Environmental and endocrine correlates of tactic switching bynonterritorial male tree lizards (Urosaurus ornatus)

Rosemary Knapp,1,* Diana K. Hews,2 Christopher W. Thompson,3 Lauren E. Ray,and Michael C. Moore

Department of Biology, Arizona State University, Tempe, AZ 85287-1501, USA

Received 30 July 2001; revised 1 March 2002; accepted 1 August 2002

Abstract

Animals often exhibit individual variation in their behavioral responses to the same stimuli in the biotic or abiotic environment. Toelucidate the endocrine mechanisms mediating such behavioral variation, we have been studying a species of lizard with two distinct malephenotypes. Here we document behavioral variation across years in one of the two male phenotypes of the tree lizard,Urosaurus ornatus,and present hormone data that support an endocrine mechanism underlying this behavioral variation. Nonterritorial male tree lizards appearto be nomadic rovers in some years and sedentary satellites in others, whereas territorial males are always territorial. This behavioralvariation by nonterritorial males was correlated with environmental conditions. In environmentally harsher years (as assessed by rainfall),nonterritorial males appear to behave as nomads, whereas in more benign years they are more site-faithful. A between-year comparison oflevels of corticosterone and testosterone for the two male phenotypes supports a model for how hormones underlie the males’ reproductivetactics, particularly the nonterritorial males’ behavioral plasticity. In an environmentally harsher (drier) year, both types of males had highercorticosterone levels than in a milder (wetter) year, but only nonterritorial males had lower testosterone in the relatively harsher year. Wepropose that disruptive selection for individual variation in hormonal responses to environmental cues may be a common mechanismunderlying the evolution of alternative male reproductive tactics in this and other species.© 2003 Elsevier Science (USA). All rights reserved.

Keywords: Reproductive tactics; Male phenotypes; Testosterone; Corticosterone; Environment; Lizard;Urosaurus; Selection

Environmental conditions, both biotic and abiotic, can haveprofound influences on an individual’s behavior. For example,the distribution and defensibility of resources often influencewhether males are territorial (Brown, 1964) or are monoga-mous or polygamous (Emlen and Oring, 1977). Precipitationcorrelates strongly with plant productivity and in turn withterritoriality of male pronghorn antelope,Antilocapra ameri-cana (Maher, 2000), and home range size of female marsupials(Fisher and Owens, 2000). On a more short-term time scale,

storm conditions can cause individual birds to abandon nestingattempts (Wingfield et al., 1998). Such changing environmen-tal stimuli do not elicit uniform behavioral responses from allindividuals in a population (Reynolds et al., 1993; Schwabl,1995; Sku´lason and Smith, 1995; Robinson and Schluter,2000). Understanding the physiological bases of such behav-ioral variation in response to environmental stimuli is important ifwe are to understand the evolution of life history characterssuch as reproductive behavior (e.g., Sinervo and Svensson, 1998).

Species in which males exist as one of two or more discretephenotypes offer useful systems for examining the physiolog-ical, and especially endocrine, bases of individual variation inresponse to environmental stimuli. In such species, males ofthe different morphological phenotypes use alternative tacticsfor achieving reproductive success (reviewed by Gross, 1996).In some populations of tree lizards,Urosaurus ornatus, malephenotypic dimorphism in dewlap (throat fan) color correlates

* Corresponding author. Fax:�1-405-325-6202.E-mail address: rknapp@ou.edu (R. Knapp).1 Present address: Department of Zoology, University of Oklahoma,

Norman, OK 73019, USA.2 Present address: Department of Life Sciences, Indiana State Univer-

sity, Terre Haute, IN 47809, USA.3 Present address: Department of Fish and Wildlife, 16018 Mill Creek

Road, Mill Creek, WA 98012-1296, USA.

R

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with reproductively important behavior (Hover, 1985; Thomp-son and Moore, 1991b, 1992). In the central Arizona popula-tions that we study, males with orange dewlaps containing acentral blue spot (“orange-blue” males) defend territories andare very aggressive. Males lacking the central blue spot (“or-ange” males) are less likely to escalate male–male interactionsand do not defend territories (Thompson and Moore, 1991b,1992). The original description of the behavior of orange maleswas that they were essentially nomads. That is, they either haveno fixed home range or have home ranges many times largerthan the site-faithful, territorial orange-blue males.

In the course of conducting various studies, however, itbecame apparent that orange male tree lizards exhibit mark-edly different behavior in different years. In this article, wepresent data collected over several years indicating thatnonterritorial, orange males are plastic in their behavior;that is, there is plastic variation within one of the twodevelopmentally fixed male alternative phenotypes in thisspecies (sensu Moore, 1991). We then show that this be-havioral plasticity correlates with yearly variation in theabiotic environment, namely, rainfall. Viewing this yearlyvariation in behavior and environmental conditions in lightof our previous behavioral endocrinology studies (Knappand Moore, 1995, 1996, 1997), we developed a model ofhormonal mediation of the nonterritorial orange males’ be-havioral plasticity (Knapp, 1996; Moore et al., 1998).Briefly, this model proposes that orange males possess atwo-step steroid hormone stress response, whereas orange-blue males have a one-step stress response. In response tosocial or abiotic stressors, both morphs can exhibit elevatedcorticosterone levels. However, orange males are morelikely than orange-blue males to exhibit coincident de-creases in plasma testosterone levels. Such high cortico-sterone levels, low testosterone levels, or both are hypoth-esized to facilitate orange males behaving as nomadicrovers. When corticosterone levels are low and testosteronelevels high, orange males are predicted to behave as seden-tary satellites. In contrast, orange-blue males maintain highlevels of testosterone even in the face of high corticosteronelevels. High levels of testosterone may help maintain ag-gression at the level required by orange-blue males forsuccessful territorial defense (cf. Wingfield et al., 1990).Lastly in the present article, we present hormone data thattest one of the model’s predictions. Some of these resultshave been reported previously in review form (Moore et al.,1998).

Methods

Field sites and animals

Male tree lizards were observed at two localities incentral Arizona. Our main field site was located in TontoNational Forest along Arizona State Highway 87 just north-west of the Sycamore Creek–Sugarloaf Mountain Road

(111° 30� W, 33° 42� N). This site is characterized by largegranite boulders and Upper Sonoran Desert vegetation andis hereafter referred to as the “boulder site” (see Thompsonand Moore, 1991a). This site provided data for 1987 and1991–1995. Tree lizards were observed in 1990 at a secondsite located in the mesquite bosque at the Coon Bluff Rec-reation Area just west of the Bush Highway, also in TontoNational Forest. The studies reported here were approved bythe Arizona State University Animal Care and Use Com-mittee and conducted under Arizona Game and Fish andU.S. Forest Service scientific permits.

Censuses

Census data were used to compare the two male morphsfor general site fidelity across years. The census data pre-sented in this article derive from several studies (describedbelow) conducted for other reasons during seven differentfield seasons. In all years for which data are presented in thisarticle, male tree lizards were marked to allow identificationof individuals from a distance. In some years, males weremarked on the base of the tail with unique color combina-tions of paint marks. In other years, males received uniquecombinations of small colored beads under deep hypother-mia anesthesia. The beads were secured through the dorsaltail musculature via monofilament line using a less invasivevariation of the method described by Fisher and Muth(1989). This marking method had no discernable effects onlizard behavior.

Because the hypothesis that variation in abiotic environ-mental conditions may be driving yearly variation in malebehavior developed over several years as we conductedvarious studies, our analyses of behavioral data were ofnecessity retrospective. We therefore had to develop a com-parison that could be made across the different studiesdespite different specific census methods. Thus, we calcu-lated the percentage of adult male tree lizards of each morphthat were resighted at least once after marking. However,there was variation across years in whether there was asingle census or multiple censuses of marked males. In 4years, marked males were censused multiple times. In theother 3 years, there was only a single census on the dayimmediately following marking.

In 4 years, censuses of the study areas were conductedbetween 12 and 26 times. Censuses in these years (1987,1990–1992) were conducted by systematically walkingthrough all parts of the particular study area while searchingfor lizards. In 1987, 40 males received paint marks and werecensused 12–25 times between 1 May and 22 June byC.W.T. In 1990, L.E.R. and M.C.M. established a mapped100-m2 study area. Thirty-three males were marked withbeads and censused 6–26 times by L.E.R. between 14 Juneand 27 July 1990. The 1991 data derive from 29 controlmales from a study of the effects of testosterone manipula-tion on behavior (R. Knapp and M. C. Moore, unpublished);these control males received only sham manipulations. The

84 R. Knapp et al. / Hormones and Behavior 43 (2003) 83–92

study area was censused 3–5 times per week by R.K. be-tween 10 July and 9 August 1991. In 1992, males weresystematically captured, measured, and marked with beadsuntil all males seen on a 70 � 75-m study grid were marked.Any new unmarked males were captured and marked when-ever sighted during the census period, although occasionalindividuals escaped capture. The study grid was censused1–3 times per week by D.K.H. between 30 March and 26August 1992, with most censuses performed between Apriland June for the 49 marked males.

For the other 3 years, resighting data are based on asingle census for males previously marked in the same year.In 1993–1995, censuses focused on a particular subset ofmarked males; resighting data are based on results fromsearching for up to 15 min for particular males in thegeneral area where they were last seen. In 1993, 56 maleswere marked early in the breeding season with beads as partof a study of hormonal responses to male–male interactions(Knapp and Moore, 1996). Here we report new analyses onthe percentage of males of each morph that were resightedby R.K. on two sequential days between 3 May and 23 June1993. This comparison is most comparable to census meth-ods for 1994 and 1995, when male tree lizards were caughtthroughout the breeding season (23 June–15 July 1994, 18May–12 July 1995) and given unique paint marks. Resight-ing data for 1994 and 1995 are derived from censuses byR.K. 1 day after males were marked. Males in these 2 yearswere part of studies that investigated the effects of artifi-cially elevated corticosterone levels on behavior and phys-iology (Knapp, 1996; Knapp and Moore, 1997); only datafrom unmanipulated control males are included in thepresent analyses (n � 11 for 1994, n � 43 for 1995).

Despite the variation in census methods across years, webelieve that the resighting data presented here accuratelydocument behavioral variation by orange males because thecensus methodology was consistent within a given year.The effort devoted to searching for males was not quantifiedabsolutely, but was consistent for all animals within a givenyear. For example, in 1993–1995, R.K. searched for eachmale for approximately 15 min or until he was found,whichever came first; the searching was done in the generalarea in which the male was last seen. The percentage ofmales resighted would be expected to increase with increas-ing number of censuses, but the relative morph difference inresighting for any given year is not expected to change. Inaddition, we know that the morph frequencies remain con-stant over the course of the active season (Thompson andMoore, 1991a); thus differences in timing of the censusesacross years also should not affect our interpretation of theresighting data. Each census technique was used in both“wet” and “dry” years (described below under Environmen-tal Data). Variation in the number of males marked alsoshould not affect the probability of resighting males of eachmorph in a given year because the census effort was inde-pendent of morph type or total number of males marked.Finally, tree lizards are an “annual” species, with very few

adults surviving more than 1 year (Tinkle and Dunham,1983); few if any lizards contribute to data in more than asingle year. Again, although census methods differedamong years, within a year both morphs were censused in asimilar manner and thus were likely to be censused withoutbias. Further, inspection of the census data (Fig. 1) revealsthat there was no systematic association between censusmethod (single vs multiple) and type of year (wetter vsdrier).

Environmental data

To support our impression that variation in the abioticenvironment was influencing male behavior, we needed toquantify a key environmental parameter known to influencelizard populations. For several species of desert lizards,including Urosaurus, reproductive output is positively cor-related with rainfall (Parker and Pianka, 1975; Martin,1977; Ballinger, 1977). Because tree lizards are insectivores(Asplund, 1964), rainfall can influence lizard populationsvia its effects on prey activity patterns and spatial distribu-tion (e.g., Ballinger, 1977; Dunham, 1978; Vitt et al., 1978;M’Closkey et al., 1990). Thus we obtained values for totalmonthly precipitation prior to and across the tree lizard’sannual reproductive season as measured at Bartlett Dam,Arizona (located approximately 18 km west of the BoulderSite) (NOAA National Climatic Data Center Annual Clima-tological Summary, Asheville, NC). In addition, NOAAreports monthly departures from normal precipitation basedon a 30-year average (here 1951–1980); we used thesevalues to categorize years as “wetter” or “drier” based onthe values for late winter/early spring (January–March)(Fig. 2A). This time of year is important for the growth ofvegetation that supports the insects that make up the bulk of

Fig. 1. Yearly variation in percentage of males resighted at least once fornonterritorial orange (gray bars) and territorial orange-blue (black bars)male tree lizards. Orange males exhibited greater year-to-year variationthan orange-blue males. Number of males censused is shown within eachbar. Numbers above paired bars for each year are P values from Fisherexact tests of differences in the proportion of males resighted between themale morphs. Letter below each year indicates relative number of censuses:M, multiple censuses; S, single census. Modified, with permission, fromMoore et al. (1998).

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tree lizard diet (see Asplund, 1964). In years when this latewinter/early spring period is wet, the plants are green andactively growing and insects are abundant. In years whenthis period is dry, the plants are brown and dormant, andinsects are scarce. Our general notes about the appearanceof our study site in these years confirms that it is brown inyears with little late winter rain and green in years with latewinter rain.

Hormone data

Based on the results of previous behavioral experiments(Knapp and Moore, 1995, 1996, 1997) and the yearly vari-

ation in orange males’ behavior described here (see Re-sults), we developed a model for how changes in circulatingcorticosterone and testosterone levels might translate stress-ful social or abiotic stimuli into changes in male reproduc-tive behavior. Briefly, this model proposes that orange-bluemales have a “one-step” stress response as evidenced bychanges in corticosterone levels, whereas orange males havea “ two-step” stress response as evidenced by changes inboth corticosterone and testosterone levels. Following fromthis model, we hypothesized that plasma levels of thesesteroid hormones would reflect morph differences in re-sponse to the prevailing abiotic conditions, thus contribut-ing to yearly variation in the percentage of orange malesresighted. This model predicts that yearly variation inplasma levels of corticosterone and testosterone would cor-relate with yearly variation in morph differences in behav-ioral plasticity and that this morph-specific annual variationin hormone levels should be consistent with expected be-havioral responses to environmental stressors. Specifically,we tested the prediction that both morphs would have highercorticosterone levels in response to harsher conditions(higher levels in a “drier” year than in a “wetter” year), butthat only orange males would have lower testosterone levelsin a drier year. Thus, in drier years, orange males’ relativelylow testosterone levels would induce nomadic behavior,whereas in wetter years their relatively higher testosteronelevels would support sedentary satellite behavior.

Plasma levels of corticosterone and testosterone wereavailable from unmarked, control males from the studiesconducted in 1993 (a “wet” year) and 1995 (a relatively“drier” year). Hormone data were not available for malescensused in other years. Because hormone levels changeover the course of the breeding season, only hormone datafrom equivalent periods (17 May through 24 June) werecompared for 1993 and 1995. These data represent 83% ofthe unmarked control groups from 1993 reported in Knappand Moore (1996) and 82% of the 1995 control male datareported in Knapp and Moore (1997).

In both years, blood samples were collected as quickly aspossible (usually within 1 min) after capturing a lizard usinga noose made of waxed dental floss. After being caught, thelizard was gently restrained and a blood sample was col-lected from the infraorbital sinus using a heparinized cap-illary tube. Blood was transferred to a microcentrifuge vialand kept on ice until return to the laboratory. There, thesamples were centrifuged for approximately 5 min and theplasma was collected and stored at �20°C until assayed.Assays in both 1993 and 1995 were conducted by R.K.using a protocol that has been more fully described previ-ously (Knapp and Moore, 1996); each assay included allsamples from that year, but only samples from that year.Briefly, ether-extracted plasma samples were run on diato-maceous earth–glycol columns to separate testosterone andcorticosterone from each other and from neutral lipids thatcould interfere with the radioimmunoassay. Each collectedfraction was dried under nitrogen and then resuspended in

Fig. 2. (A) Departure from normal (average) precipitation for the months inwhich precipitation would most likely affect tree lizard behavior for agiven year. Variation across years during the late winter/early spring(January–March) is correlated with whether similar percentages of the twomale morphs were resighted (1991–1993) or not (other years) (see Fig. 1).Numbers below months represents the normal precipitation for 1951–1980.(B) Relationship between departure from normal precipitation for Januarythrough March and the difference in percentages of orange-blue males andorange males resighted at least once. Departure from normal precipitationwas calculated from data shown in (A). Difference in percentage malesresighted between the morphs was calculated from data presented in Fig. 1.

86 R. Knapp et al. / Hormones and Behavior 43 (2003) 83–92

phosphate buffer for use in the assay. Each sample wasassayed in duplicate and final plasma hormone values werecorrected for individual losses during extraction and run-ning on the columns. Intraassay variation for testosteronewas 23.1% for 1993 and 2.8% for 1995. Intraassay variationfor corticosterone was 27.8% for 1993 and 6.8% for 1995.Interassay variation was 18.2% for testosterone and 2.2%for corticosterone.

Corticosterone levels are known to increase and testos-terone levels tend to decrease over the course of the breed-ing season in tree lizards (Moore and Thompson, 1990).Therefore, numerical day of the year (“Season” ) was used asa covariate in ANCOVAs to test for yearly variation inhormone levels. The ANCOVA assumption of equality ofslopes of hormone level versus season was met for all datasets except orange-blue males’ testosterone levels. Testos-terone data from orange-blue males were thus analyzedusing ANOVA. Hormone data were log-transformed priorto analysis to normalize distributions and achieve homosce-dascticity.

Results

Yearly variation in percentage males resighted

The percentage of males of each morph that were re-sighted within each field season differed across years (Fig.1). In some years, there was no significant difference in thepercentage of males of each morph that were resighted;more than 80% of males of each morph were resighted in1991, 1992, and 1993 (Fisher exact tests: all P � 0.1). In theother 4 years, almost exactly half as many orange males asorange-blue males were resighted (Fisher exact tests, all P� 0.067). In each of these 4 years, more than 60% of theorange-blue males were resighted, whereas 50% or fewer ofthe orange males were resighted.

Yearly variation in rainfall

Yearly variation in whether males of the two morphswere resighted with equal frequency correlated with yearlyvariation in environmental harshness. Years in which thetwo morphs were resighted in similar proportions (1991–1993) were characterized by high rainfall during the latewinter/early spring (January–March) (Fig. 2A) and largeincrease in overall rainfall from January through Marchover normal amounts (Fig. 2B). In contrast, rainfall was lowduring this same period in those years when half as manyorange males as orange-blue males were resighted.

Yearly variation in hormone levels

Consistent with predictions from our model of endocrinemediation of behavioral variation (Fig. 3) (see Introductionand Methods), corticosterone levels reflected the environ-

mental conditions in the wet 1993 and the relatively drier1995. Both morphs had plasma corticosterone levels thatwere significantly lower during the wet year of 1993 com-pared with the drier year of 1995 (Fig. 4B) (ANCOVA:orange: F (1,31) � 6.8, P � 0.014; orange-blue: F (1,35) �9.5, P � 0.004). In contrast, only orange males showedyearly variation in testosterone levels (Fig. 4C) (ANCOVA:F (1,31) � 4.5, P � 0.04). Testosterone levels of orange-blue males did not differ significantly between years(ANOVA: F (1,36) � 0.169, P � 0.68). The data in Fig. 4are back-transformed adjusted means and standard errorsfrom the ANCOVAs or back-transformed means from theANOVA (for orange-blue testosterone).

The yearly variation in hormone levels is not likely to bethe result of interassay variation. In both years, columnchromatography and radioimmunoassays were performedby the same person (R.K.) in the same laboratory usingidentical protocols. If the yearly variation in testosteroneand corticosterone levels were the result of interassay vari-ation, the morphs’ differences in hormone levels betweenyears should be similar, rather than different. The means ofthe standards from the two hormone assays were not sig-nificantly different from each other (Mann–Whitney U tests;testosterone: U � 2.5, P � 0.14; corticosterone: U � 6.0, P� 0.55).

Discussion

Behavioral variation in nonterritorial orange males

When Thompson and Moore (1991b, 1992; Moore andThompson, 1990) first described the behavior of the twomale morphs, they documented that orange-blue males were(1) more likely to escalate agonistic male–male interactionsthan were orange males and (2) were residents on therelatively large 1-ha study area for much longer than orange

Fig. 3. Proposed model for how corticosterone and testosterone mayinfluence the expression of reproductive tactic in male tree lizards inresponse to environmental and social stressors. Orange males are proposedto have a “ two-step” stress response, whereas orange-blue males are pro-posed to have a “one-step” stress response. See text for discussion. Cort,corticosterone; Testo, testosterone. Modified, with permission, from Mooreet al. (1998).

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males. On the basis of their observations, Thompson andMoore characterized orange-blue males as territorial malesand orange males as nonterritorial nomads. We have nowdocumented yearly variation in whether or not nonterrito-rial, orange male tree lizards are resighted in the sameproportion as territorial orange-blue males (Fig. 1). Suchvariation in the resighting of nonterritorial orange malesrelative to territorial orange-blue males suggests that orangemales are plastic in their expression of two different non-territorial tactics, at least between years.

Among species with alternative male reproductive tac-tics, two nonterritorial reproductive tactics that are com-monly observed are nomads and sedentary satellites. No-

mads move among the territories of several males whereassedentary satellites are more site-faithful (e.g., Dunbar,1982; Gross, 1982, 1996; Thornhill and Alcock, 1983;Waltz and Wolf, 1984; Shelly and Greenfield, 1989; Tabor-sky, 1994). In our tree lizard populations, most orangemales may be behaving as sedentary satellites in those yearswhen the percentage of resighted orange males was similarto that of orange-blue males. In years when a much smallerfraction of orange males were resighted, most of these malesmay be behaving as nomads. Describing orange males asnomads is based in part on our observations of individualorange males moving in a straight-line distance coveringseveral orange-blue male territories and on our failure toresight most orange males, in contrast to consistent, site-specific resighting of orange-blue males (summarized inMoore et al., 1998). Direct comparison across years must bemade cautiously because of differences in censusing meth-ods. However, it is important to note that despite differentcensus methods across years, the majority of orange-bluemales were resighted each year whereas this was not true oforange males (Fig. 1). In addition, from Fig. 1 it appears thateven in years in which most orange males were not re-sighted, some orange males were site-faithful. A directconnection between the behavior of individual orange malesand the territorial behavior of surrounding orange-bluemales remains to be described in detail (D.K. Hews andM.C. Moore, in preparation). However, it seems likely thatvariation in behavior among orange males in any given yearwill be influenced by the behavior of territorial orange-bluemales, which in turn is likely to be dependent on the qualityand quantity of suitable habitat (see below). We do knowthat orange males are still significantly less aggressive to-ward another male in a benign (wetter) year (Cramer-Mel-drum, 2000).

An alternative interpretation of the morph differences inresighting (Fig. 1) is that the data do not reflect space usebut rather reflect overall activity levels, and hence visibility,of the two morphs. Such an interpretation of the yearlyvariation in morph differences is unlikely because captiverearing of hatchling tree lizards from several populationshas always resulted in morph frequencies equal to those ofmorphs sighted and captured in the parental populations(Thompson and Moore, 1991a; Thompson et al., 1993;Hews et al., 1994; Hews and Moore, 1996; Hews et al.,1997). We thus have been finding the two male morphs indirect proportion to their actual representation in the popu-lation. Regardless, interpreting the data in Fig. 1 as reflect-ing overall activity levels would still support our interpre-tation that orange males are more plastic in their behaviorthan are orange-blue males.

The difference in percentage of males resighted betweenorange and orange-blue males within a given year is un-likely to reflect morph differences in mortality. Orangemales do not become more scarce over the course of theseason, as would be predicted given the different percent-ages of males resighted in some years (e.g., Thompson and

Fig. 4. (A) Total monthly rainfall for January through March for 1993 anddrier 1995, two years for which hormone data are available for male treelizards. These months represent the late winter/early spring period thatestablishes yearly food resources for tree lizards (see text for discussion).Both orange and orange-blue males exhibited elevated plasma corticoste-rone levels in the drier 1993 (B), but only orange males exhibited associ-ated yearly variation in testosterone levels (C). The data are back-trans-formed adjusted means and standard errors from the ANCOVAs (or simplyback-transformed means from the ANOVA for orange-blue testosteronelevels). Sample sizes are shown within each bar. *P � 0.05, **P � 0.01.Modified, with permission, from Moore et al. (1998).

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Moore, 1991a). We can think of only two possible scenariosto explain our observations of orange male behavior: eitherorange males are completely nomadic or they have homeranges that are substantially larger than those of orange-bluemales. The most feasible way to answer this question wouldbe direct observational data from radiotelemetry studies.

Environmental correlates of behavioral variation

Years in which orange males were resighted in similarproportion to orange-blue males were characterized by wet-ter conditions during the late winter/early spring (January–March) (Fig. 2A). This time of year is important for thegrowth of vegetation that supports the insects that make upthe bulk of tree lizard diet (see Asplund, 1964). Thus 1991–1993 can be characterized as wet years where more foodand habitat were likely available to tree lizards (even thoughfor the whole period from January through July 1991 wasnot wetter than normal) (Fig. 2B). Changes in the amount ofavailable habitat and population density are likely to influ-ence both the frequency and intensity of social interactions.For tree lizards, we have observed that suitable habitat,based on where we regularly find lizards, is restricted to theriparian areas adjacent to dry washes during harsh years.During more benign years, suitable habitat extends furtherout from the washes (R.K., M.C.M., and D.K.H., personalobservations). This type of variability is likely to influencethe local population density and also orange-blue male ter-ritory size. Our impression over the years has been thatterritories are smaller and more actively defended in harsheryears, whereas territories are larger in more benign years.With smaller territories, individual orange-blue males arelikely to better defend their territories against intruders andthe frequency of male–male interactions is also likely to behigh. Under these conditions, orange males would be pre-dicted to behave as nomads, moving from one territory toanother as territorial orange-blue males successfully defendtheir territories. With less frequent or less intense agonisticinteractions, as might be seen when territories are larger inbenign years, orange males are predicted to be more site-faithful, behaving as sedentary satellites. Such variation inmale behavior with territory size has been documented intwo species of grasshopper (Shelly and Greenfield, 1989).Our impressions of orange male tree lizard behavior clearlyrequire additional work to be fully substantiated and toseparate, if possible, the effects of biotic and abiotic envi-ronmental stimuli on behavioral variation in male tree liz-ards.

An endocrine mechanism underlying behavioral variation

Previous studies indicate morph-specific differences inthe effects of social encounters on plasma hormone levels.Specifically, when we manipulated hormone levels or socialinteraction, we found morph differences in testosterone lev-els but only when corticosterone levels were high (Knapp

and Moore, 1995, 1996, 1997). First, the two morphs differin the sensitivity of plasma testosterone levels to elevationof corticosterone levels: orange-blue males’ testosteronelevels are more resistant to suppression by corticosteronethan are those of orange males (Knapp and Moore, 1997).Second, the two morphs differ in their hormonal responsesto agonistic interactions. Orange males exhibit elevatedcorticosterone and suppressed testosterone levels 1 day afterwinning a brief agonistic interaction (Knapp and Moore,1996). In contrast, orange-blue males do not exhibit suchhormone changes in the field (Knapp and Moore, 1996),although they do exhibit elevated corticosterone levels afterwinning an encounter under different experimental condi-tions involving more intense encounters (Knapp and Moore,1995). The observed morph differences in hormonalchanges following male–male interactions are consistentwith the predicted effects of these hormones on the expres-sion of subsequent agonistic behavior based on our knowl-edge of their actions in other vertebrates (see referencesabove).

Taken together, the results of these previous studies(Knapp and Moore, 1995, 1996, 1997) support a model ofhow steroid hormones might influence the variable expres-sion of reproductive tactics in orange males. This model(Fig. 3) (Knapp, 1996; Moore et al., 1998) proposes thatorange males possess a two-step steroid hormone stressresponse, whereas orange-blue males have a one-step stressresponse. In response to social or abiotic stressors, bothmorphs can exhibit elevated corticosterone levels. However,orange males are more likely than orange-blue males toexhibit coincident decreases in plasma testosterone levels.Such high corticosterone levels, low testosterone levels, orboth (i.e., high corticosterone levels having their effects viasuppression of testosterone levels) are hypothesized to fa-cilitate orange males behaving as nomadic rovers. Whencorticosterone levels are low and testosterone levels high,orange males are predicted to behave as sedentary satellites.In contrast, orange-blue males maintain high levels of tes-tosterone even in the face of high corticosterone levels.High levels of testosterone may help maintain aggression atthe level required by orange-blue males for successful ter-ritorial defense (cf. Wingfield et al., 1990) or relative dom-inance status (cf. Sapolsky, 1989, 1993).

Hormonal correlates of behavioral variation

This model makes several predictions. First, relevant tothe present study, it predicts a correlation between the be-havioral variation in a wet year and a drier year and levelsof two steroid hormones known to influence the expressionof agonistic and territorial behavior in vertebrates. Testos-terone facilitates the expression of aggressive and territorialbehavior (e.g., Svare, 1983; Moore, 1988; Marler andMoore, 1988; Wingfield et al., 1990). Corticosterone is astress hormone that responds to a variety of environmentaland social stressors (e.g., Leshner, 1979, 1980; Moore et al.,

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1990; Knapp and Moore, 1996; Klukowski and Nelson,1998; Wingfield et al., 1998; Romero and Wikelski, 2001)but this response can vary across individuals (e.g., seeSapolsky, 1989, 1993). Corticosterone also can reduce ag-gressive behavior and territorial defense in many verte-brates, including several lizard species (Svare, 1983; To-karz, 1995; DeNardo and Licht, 1994; DeNardo andSinervo, 1994a,b), can increase activity (Breuner and Wing-field, 2000), and can also directly inhibit testosterone secre-tion (Orr and Mann, 1992).

Consistent with corticosterone’s actions as a stress hor-mone, we found that both morphs exhibited higher plasmacorticosterone levels in 1993, a year characterized byharsher conditions, than in 1995, which was milder (Fig.4B) (cf. Romero and Wikelski, 2001). In contrast, onlyorange males exhibited significantly lower testosterone lev-els in the harsher year (1993) compared with the milder year(1995) (Fig. 4C). The lower testosterone levels and highercorticosterone levels shown by orange males in the harsheryear paralleled the lower proportion of these males re-sighted in that year. Despite orange-blue males’ relativelyhigher corticosterone levels in the harsher year, their testos-terone levels also remained relatively high, paralleling theconsistently high proportion of these males resighted eachyear. A second prediction of our model is that artificialelevation of corticosterone levels should result in orangemales switching from site-faithful satellite behavior to no-mad behavior, whereas orange-blue males’ behavior shouldbe resistant to change. Such results would be consistent withdata from other taxa in which elevated corticosterone levelsinduce increases in restlessness or locomotion (Sandi et al.,1996; Breuner et al., 1998; Cash and Holberton, 1999;Breuner and Wingfield, 2000), although the ability of cor-ticosterone to affect these behaviors varies with environ-mental conditions (e.g., Breuner and Wingfield, 2000). Ourmodel also predicts that differences in populations of glu-cocorticoid receptors and/or binding proteins should existbetween the morphs as such differences would also likelyinfluence corticosterone’s behavioral effects. These predic-tions are currently being tested. The latter prediction hasbeen partially supported by recent documentation of morphdifferences in steroid-binding globulin characteristics (Jen-nings et al., 2000).

Evolution of discrete male phenotypes

Our findings here and in previous reports suggest that theevolution of the two distinct male behavioral phenotypeswithin tree lizards could have resulted from disruptive se-lection on hormonal responses to both social and abioticstressors, with correlated or subsequent selection on mor-phology. Within-species variation in endocrine responses tostimuli is widespread. Variation in the neuroendocrine sys-tem and in endocrine responses to various stimuli has beendocumented within and among strains of domestic and lab-oratory animals (e.g., Shire, 1979; Meaney et al., 1991;

Mathias et al., 1999). Differences in hormone levels havealso been documented among distinct behavioral or mor-phological phenotypes in other vertebrates (e.g., Moore,1991; Brantley et al., 1993; Sinervo et al., 2000) and insects(e.g., Roff et al., 1997; Zera and Huang, 1999; Sullivan etal., 2000). Because behavior can be more evolutionarilylabile than morphology in some circumstances (West-Eber-hard, 1989), the evolution of distinct behavioral phenotypeslikely would have preceded subsequent selection on mor-phological correlates, eventually resulting in the discretemale morphologies associated with the reproductive tacticsextant today. Disruptive selection has been responsible forthe trophic- or resource-based polymorphisms that arose inmany species in response to niche variation (reviewed inRobinson and Schluter, 2000). Similarly, disruptive selec-tion on individual variation in hormonal responses to envi-ronmental cues and associated differences in possiblethreshold effects of one hormone on another may be acommon mechanism underlying the evolution of alternativemale reproductive tactics and phenotypes in a variety ofspecies (cf. Roff et al., 1997).

Acknowledgments

We thank E. Cramer-Meldrum, E.L. Lacy, I.T. Moore,D. Painter, A. Quain, and S.K. Woodley for assistance in thefield. J. Alcock, J.F. Harrison, E.L. Lacy, K.S. Matt, D.Painter, G.E. Walsberg, and S.K. Woodley made valuablecontributions to the development of the ideas presented inthis article. C.J. Leary, S.K. Scobell, and especially T.S.Jessop offered valuable comments on an earlier draft of themanuscript. Portions of our research were made possible bya Gaige award from the American Society of Ichthyologistsand Herpetologists to R.K., NIMH Individual National Re-search Service Awards to R.K. (MH10443) and D.K.H.(MH10074), and NIMH Grant 48564 to M.C.M.; an allot-ment of NSF REU-DIR 90-01317 provided financial sup-port for L.E.R.

References

Asplund, K.K., 1964. Seasonal variation in the diet of Urosaurus ornatusin a riparian community. Herpetologica 20, 91–94.

Ballinger, R.E., 1977. Reproductive strategies: food availability as a sourceof proximal variation in a lizard. Ecology 54, 628–635.

Brantley, R.K., Wingfield, J.C., Bass, A.H., 1993. Sex steroid levels inPorichthys notatus, a fish with alternative reproductive tactics, and areview of the hormonal bases for male dimorphism among teleost fish.Horm. Behav. 27, 332–347.

Breuner, C.W., Wingfield, J.C., 2000. Rapid behavioral response to corti-costerone varies with photoperiod and dose. Horm. Behav. 37, 23–30.

Breuner, C.W., Greenberg, A.L., Wingfield, J.C., 1998. Noninvasive cor-ticosterone treatment rapidly increases activity in Gambel’s white-crowned sparrows (Zonotrichia leucophrys gambelii). Gen. Comp. En-docrinol. 111, 386–394.

Brown, J.L., 1964. The evolution of diversity in avian territorial systems.Wilson Bull. 76, 160–169.

90 R. Knapp et al. / Hormones and Behavior 43 (2003) 83–92

Cash, W.B., Holberton, R.L., 1999. Effects of exogenous corticosterone onlocomotor activity in the red-eared slider turtle, Trachemys scriptaelegans. J. Exp. Zool. 284, 637–644.

Cramer-Meldrum, E., 2000. Environmental effects on differences in ag-gressive behavior among tree lizards. MNS thesis, Arizona State Uni-versity.

DeNardo, D.F., Licht, P., 1994. Effects of corticosterone on social behaviorin male lizards. Horm. Behav. 27, 184–199.

DeNardo, D.F., Sinervo, B., 1994a. Effects of corticosterone on activityand home-range size of free-ranging male lizards. Horm. Behav. 28,53–65.

DeNardo, D.F., Sinervo, B., 1994b. Effects of steroid hormone interactionon activity and home-range size of male lizards. Horm. Behav. 28,273–287.

Dunbar, R.I.M., 1982. Intraspecific variations in mating strategy, in: Bate-son, P.P.G., Klopfer, P.H. (Eds.), Perspectives in Ethology, Vol. 5Ontogeny. Plenum, New York, pp. 385–431.

Dunham, A.E., 1978. Food availability as a proximate factor influencingindividual growth rates in the iguanid lizard Sceloporus merriami.Ecology 59, 770–778.

Emlen, S.T., Oring, L.W., 1977. Ecology, sexual selection, and the evo-lution of mating systems. Science 197, 215–223.

Fisher, D.O., Owens, I.P.F., 2000. Female home range size and the evo-lution of social organization in macropod marsupials. J. Anim. Ecol.69, 1083–1098.

Fisher, M., Muth, A., 1989. A technique for permanently marking lizards.Herpetol. Rev. 20, 45–46.

Gross, M.R., 1982. Sneakers, satellites and parentals: polymorphic matingstrategies in North American sunfishes. Z. Tierpsychol. 60, 1–26.

Gross, M.R., 1996. Alternative reproductive strategies and tactics: diversitywithin sexes. Trends Ecol. Evol. 11, 92–98.

Hews, D.K., Moore, M.C., 1996. A critical period for the organization ofalternative male phenotypes of tree lizards by exogenous testosterone?Physiol. Behav. 60, 425–429.

Hews, D.K., Knapp, R., Moore, M.C., 1994. Early exposure to androgensaffects adult expression of alternative male types in tree lizards. Horm.Behav. 28, 96–115.

Hews, D.K., Thompson, C.W., Moore, I.T., Moore, M.C., 1997. Populationfrequencies of alternative male phenotypes in tree lizards: geographicvariation and common-garden rearing studies. Behav. Ecol. Sociobiol.41, 371–380.

Hover, E.L. 1985. Differences in aggressive behavior between throat colormorphs in a lizard, Urosaurus ornatus. Copeia 1985, 933–940.

Jennings, D.H., Moore, M.C., Knapp, R., Matthews, L., Orchinik, M.,2000. Plasma steroid-binding globulin mediation of differences instress reactivity in alternative male phenotypes in tree lizards, Urosau-rus ornatus. Gen. Comp. Endocrinol. 120, 289–299.

Klukowski, M., Nelson, C.E., 1998. The challenge hypothesis and seasonalchanges in aggression and steroids in male northern fence lizards(Sceloporus undulatus hyacinthinus). Horm. Behav. 33, 197–204.

Knapp, R., 1996. Endocrine, Environmental and Physiological Influenceson Reproductive Tactics in a Polymorphic Lizard. Ph.D. dissertation.Arizona State Univ., Tempe.

Knapp, R., Moore, M.C., 1995. Hormonal responses to aggression vary indifferent types of agonistic encounters in male tree lizards, Urosaurusornatus. Horm Behav. 29, 85–105.

Knapp, R., Moore, M.C., 1996. Male morphs in tree lizards, Urosaurusornatus, have different delayed hormonal responses to aggressive en-counters. Anim. Behav. 52, 1045–1055.

Knapp, R., Moore, M.C., 1997. Male morphs in tree lizards have differenttestosterone responses to elevated levels of corticosterone. Gen. Comp.Endocrinol. 107, 273–279.

Leshner, A.I., 1979. Hormonal control of submissiveness in mice: irrele-vance of the androgens and relevance of the pituitary–adrenal hor-mones. Physiol. Behav. 22, 531–534.

Leshner, A.I., 1980. The interaction of experience and neuroendocinefactors in determining behavioral adaptations to aggression. Prog.Brain Res. 53, 427–438.

Maher, C.R., 2000. Quantitative variation in ecological and hormonalvariables correlates with spatial organization of pronghorn (Antiloca-pra americana) males. Behav. Ecol. Sociobiol. 47, 327–338.

Marler, C.A., Moore, M.C., 1988. Evolutionary costs of aggression re-vealed by testosterone manipulations in free-living male lizards. Behav.Ecol. Sociobiol. 23, 21–26.

Martin, R.F., 1977. Variation in reproductive productivity of range margintree lizards (Urosaurus ornatus). Copeia 1977, 83–92.

Mathias, L.J., Jacobson, N.A., Rhees, R.W., Lephart, E.D., 1999. Brainaromatase in control versus castrated Norway Brown, Sprague–Dawleyand Wistar adult rats. Proc. Soc. Exp. Biol. Med. 221, 126–130.

M’Closkey, R.T., Deslippe, R.J., Szpak, C.P., Baia, K.A., 1990. Tree lizarddistribution and mating system: the influence of habitat and foodresources. Can. J. Zool. 68, 2083–2089.

Meaney, M.J., Viau, V., Shatnagar, S., Betito, K., Iny, L.J., O’Donnell, D.,Mitchell, J.B., 1991. Cellular mechanisms underlying the developmentand expression of individual differences in the hypothalamic–pituitary–adrenal stress response. J. Steroid Biochem. Mol. Biol. 39, 265–274.

Moore, M.C., 1988. Testosterone control of territorial behavior: tonic-release implants fully restore seasonal and short-term aggressive re-sponses in free-living castrated lizards. Gen. Comp. Endocrinol. 70,450–459.

Moore, M.C., 1991. Application of organization-activation theory to alter-native male reproductive strategies: a review. Horm. Behav. 25, 154–179.

Moore, M.C., Thompson, C.W., 1990. Field endocrinology in reptiles:hormonal control of alternative male reproductive tactics, in: Epple, A.,Scanes, G.C., Stetson, M.H. (Eds.), Progress in Comparative Endocri-nology. Wiley–Liss, New York, pp. 685–690.

Moore, M.C., Hews, D.K., Knapp, R., 1998. Hormonal control and evo-lution of alternative males phenotypes: generalizations of models forsexual differentiation. Am. Zool. 38, 133–151.

Moore, M.C., Thompson, C.W., Marler, C.A., 1991. Reciprocal changes incorticosterone and testosterone levels following acute and chronichandling stress in the tree lizard, Urosaurus ornatus. Gen. Comp.Endocrinol. 81, 217–226.

Orr, T.E., Mann, D.R., 1992. Role of glucocorticoids in the stress-inducedsuppression of testicular steroidogenesis in adult male rats. Horm.Behav. 26, 350–363.

Parker, W.S. and Pianka, E.R., 1975. Comparative ecology of populationsof the lizard Uta stansburiana. Copeia 1975, 615–632.

Reynolds, J.D., Gross, M.R., Coombs, M.J., 1993. Environmental condi-tions and male morphology determine alternative mating behavior inTrinidadian guppies. Anim. Behav. 45, 145–152.

Robinson, B.W., Schluter, D., 2000. Natural selection and the evolution ofadaptive genetic variation in northern freshwater fishes, in: Mousseau,T.M., Sinervo, B., Endler, J. (Eds.), Adaptive Genetic Variation in theWild. Oxford Univ. Press, New York, pp. 65–94.

Roff, D.A., Stirling, G., Fairbairn, D.J., 1997. The evolution of thresholdtraits: a quantitative genetic analysis of the physiological and life-history correlates of wing dimorphism in the sand cricket. Evolution51, 1910–1919.

Romero, L.M., Wikelski, M., 2001. Corticosterone levels predict survivalprobabilities of Galapagos marine iguanas during El Nino events. Proc.Natl. Acad. Sci. USA 98, 7366–7370.

Sandi, C., Venero, C., Guaza, C., 1996. Novelty-related rapid locomotoreffects of corticosterone in rats. Eur. J. Neurosci. 8, 794–800.

Sapolsky, R.M., 1989. Styles of dominance and their endocrine correlatesamong wild olive baboons (Papio anubis). Am. J. Primatol. 18, 1–13.

Sapolsky, R.M., 1993. Endocrinology alfresco: psychoendocrine studies ofwild baboons. Recent Prog. Horm. Res 48, 437–468.

Schwabl, H., 1995. Individual variation of the acute adrenocortical re-sponse to stress in the white-throated sparrow. Zoology (Jena) 99,113–120.

91R. Knapp et al. / Hormones and Behavior 43 (2003) 83–92

Shelly, T.E., Greenfield, M.D., 1989. Satellites and transients: ecologicalconstraints on alternative mating tactics in male grasshoppers. Behav-iour 109, 200–221.

Shire, J.G.M., 1979 (Ed.). Genetic Variation in Hormone Systems, Vol. 1.CRC Press, Boca Raton, FL.

Sinervo, B., Svensson, E., 1998. Mechanistic and selective causes of lifehistory trade-offs and plasticity. Oikos 83, 432–442.

Sinervo, B., Miles, D.B., Frankino, W.A., Klukowski, M., DeNardo, D.F.,2000. Testosterone, endurance, and Darwinian fitness: natural andsexual selection on the physiological bases of alternative male behav-iors in side-blotched lizards. Horm. Behav. 38, 222–233.

Skulason, S., Smith, T.B., 1995. Resource polymorphism in vertebrates.Trends Ecol. Evol. 10, 366–70.

Sullivan, J.P., Jassim, O., Fahrbach, S.E., Robinson, G.E., 2000. Juvenilehormone paces behavioral development in the adult worker honey bee.Horm. Behav. 37, 1–14.

Svare, B.B., 1983 (Ed.). Hormones and Aggressive Behavior. Plenum,New York.

Taborsky, M., 1994. Sneakers, satellites, and helpers: parasitic and coop-erative behavior in fish reproduction. Adv. Study Behav. 23, 1–100.

Thompson, C.W., Moore, M.C., 1991a. Syntopic occurrence of multipledewlap color morphs in male tree lizards, Urosaurus ornatus. Copeia1991, 493–503.

Thompson, C.W., Moore, M.C., 1991b. Throat color reliably signals statusin male tree lizards, Urosaurus ornatus. Anim. Behav. 42, 745–753.

Thompson, C.W., Moore, M.C., 1992. Behavioral and hormonal correlatesof alternative reproductive strategies in a polygynous lizard: tests of therelative plasticity and challenge hypothesis. Horm. Behav. 26, 568–585.

Thompson, C.W., Moore, I.T., Moore, M.C., 1993. Social, environmentaland genetic factors in the ontogeny of phenotypic differentiation in alizard with alternative male reproductive strategies. Behav. Ecol. So-ciobiol. 33, 137–146.

Thornhill, R., Alcock, J., 1983. The evolution of insect mating systems.Harvard Univ. Press, Cambridge, MA.

Tinkle, D.W. and Dunham, A.E., 1983. Demography of the tree lizard,Urosaurus ornatus, in central Arizona. Copeia 1983, 585–598.

Tokarz, R.R., 1995. Importance of androgens in male territorial acquisitionin the lizard Anolis sagrei: an experimental test. Anim. Behav. 49,661–669.

Vitt, L.J., Van Loben Sels, R.C., Ohmart, R.D., 1978. Lizard reproduction:annual variation and environmental correlates in the iguanid lizardUrosaurus graciosus. Herpetologica 34, 241–253.

Waltz, E.C., Wolf, L.L., 1984. By Jove! Why do alternative mating tacticsassume so many different forms? Am. Zool. 24, 333–343.

West-Eberhard, M.J., 1989. Phenotypic plasticity and the origins of diver-sity. Annu. Rev. Ecol. Syst. 20, 249–278.

Wingfield, J.C., Hegner, R.E., Dufty Jr., A.M., Ball, G.F., 1990. The“challenge hypothesis:” theoretical implications for patterns of testos-terone secretion, mating system, and breeding strategies. Am. Nat. 136,829–846.

Wingfield, J.C., Maney, D.L., Breuner, C.W., Jacobs, J.D., Lynn, S.,Ramenofsky, M., Richardson, R.D., 1998. Ecological bases of hor-mone–behavior interactions: the “emergency life history stage.” Am.Zool. 38, 191–206.

Zera, A.J., Huang, Y., 1999. Evolutionary endocrinology of juvenile hor-mone esterase: functional relationship with wing polymorphism in thecricket. Evolution 53, 837–847.

92 R. Knapp et al. / Hormones and Behavior 43 (2003) 83–92