Behavioural correlates of spatial distribution in Wagner's gerbilGerbillus dasyurus (Rodentia, Gerbillinae)

by V.S. GROMOV1, B.R. KRASNOV2 and G.I. SHENBROT2

1Severtsov Institute of Ecology and Evolution, Russian Academy of Sciences,Leninsky prosp., 33, Moscow 117071, Russia

2Ramon Science Center, Jacob Blaustein Institute for Desert Research,Ben-Gurion University of the Negev, P.O. Box 194, Mizpe Ramon 80600, IsraelCorresponding author: Dr. Boris Krasnov, Ramon Science Center P.O. Box 194

Mizpe Ramon 80600, Israel.e-mail: krasnov@bgumail.bgu.ac.il

Summary. - Spatial organization of Gerbillus dasyurus in the Central Negev (Israel) ischaracterized by isolated aggregations of several individuals within a homogeneous habitat. Westudied the behavioural interactions between individuals from the same and from different aggre-gations in male-male and female-female dyadic encounters on neutral arena. We tested thehypothesis that between-individual relationships, in G. dasyurus populations are in accordancewith the « dear enemy » phenomenon (individuals show more aggressive behaviour upon detect-ing strangers than when sensing neighbours). The number of cohesive interactions between ger-bils was much higher than that of agonistic behaviours. Male opponents in neighbour dyadsdemonstrated both agonistic and cohesive interactions significantly less frequently than oppo-nents in non-neighbour dyads. No significant difference either in manifestation or in frequencyof behavioural items were found between neighbour and non-neighbour dyads of females. Maleencounters were more asymmetric than female encounters in terms of defensive behaviour, butnot in terms of offensive behaviour. No difference in behavioural asymmetry between neighbourand non-neighbour encounters of both sexes was found. It is noteworthy that the defensive asym-metry was significantly higher than offensive asymmetry in male encounters.

Mammalia, t. 65, n° 2, 2001 : 111-120.

Résumé. - L'organisation spatiale de Gerbillus dasyurus dans le Negev central (Israël) estcaractérisée par des groupements de plusieurs individus isolés dans un habitat homogène. Nousavons étudié les interactions comportementales entre individus d'un même groupement et de dif-férents groupements, dans des rencontres mâle-mâle et femelle-femelle dans une enceinte neutre.Nous avons testé l'hypothèse que les relations entre individus dans les populations de G. dasyu-rus sont en accord avec le phénomène « cher ennemi » (les individus montrent un comportementplus agressif envers des étrangers que des voisins). Le nombre d'interactions de cohésion entregerbilles a été beaucoup plus élevé que celui de comportements agonistiques. Les mâles dans lesdyades de voisins montrent à la fois des interactions agonistiques et cohésives nettement moinssouvent que ceux des dyades de non-voisins.

On n'a pas trouvé de différence significative ni dans la manifestation ni dans la fréquenceentre les dyades de femelles voisines ou non. Les rencontres de mâles ont été plus asymétriquesque celles des femelles pour ce qui est du comportement défensif, mais pas en termes de com-

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portement offensif. Aucune différence de comportement asymétrique n'a été constatée à l'occa-sion de rencontres de voisins ou non-voisins des deux sexes. Il faut noter que l'asymétrie défen-sive a été significativement plus élevée que l'asymétrie offensive dans les rencontres de mâles.

KEY WORDS : Gerbillus dasyurus, behaviour, spatial organisation, dyadic encounters.

INTRODUCTION

The spatial organization of individuals within a population is determined by twomain groups of factors, environmental and behavioural. Environmental factors includethe abundance and pattern of distribution of food resources and shelters, the level ofpredation pressure and so on, representing an extrinsic component of spatial organiza-tion. Behavioural factors (such as aggression or avoidance) maintain a species-specificlevel of between-individual distances, thus, representing an intrinsic component of spa-tial structure. This component is determined by a trade-off between the environmentcarrying capacity and the abilities of a species to detect conspecific individuals, all elsebeing equal. In other words, an individual should both be guaranteed of having enoughfood to exist and sufficient shelters to avoid predators, and maintaining a certain levelof conspecific contacts (for example, to allow encounters of potential mates).

It is clear, that the extrinsic component of spatial organization is variable bothspatially (i.e., among habitats) and/or temporally (i.e., seasonally or annually), whereasthe intrinsic component is supposedly more conservative (Lawes and Henzi 1995). Therelative role of these two components varies among species depending of their socialsystems. It is commonly thought that the spatial distribution of individuals in solitaryspecies is mainly influenced by environmental factors whereas that of « social » speciesrelies on behavioural characteristics (e.g. Shilov 1991; Ganslober 1996). However, ithas been repeatedly shown that environmental factors do have a profound effect onspatial organization of « social » species (e.g. Taber and Macdonald 1992; Travis andSlobodchikoff 1993 ; Travis et al. 1995), and the behavioural basis of spatial distribu-tion of individuals in solitary species populations is less known.

Populations of gerbilline species demonstrate different levels of sociality as wellas different types of spatial distribution of individuals from evenly-spaced solitaries(e.g. Gerbillus henleyi, Shenbrot et al. 1994) to complex families (e.g. Rhombomysopimus, Goltsman et al. 1977). In addition, the spatial distribution of individuals insome species that were considered to be solitary appeared actually to be represented byseveral aggregations of individuals with broadly-overlapping home ranges rather thanevenly-spaced individuals (Daly and Daly 1975 ; Rogovin et al. 1994; Gromov andTchabovsky 1995 ; Matrosov et al. 1996 ; Krasnov et al. 1996).

We studied supposedly solitary Wagner's gerbils (Gerbillus dasyurus Wagner,1842) in the Central Negev (Israel) and found that the spatial organization of the popu-lation within homogenous habitat is represented by several spatially-isolated aggrega-tions of 6-8 individuals (Gromov et al. 2000). We hypothesized that between-individ-ual relationships in G. dasyurus populations are in accordance with the « dear enemy »phenomenon. The latter means that individuals show more aggressive behaviour upondetecting strangers than when sensing neighbours (Fisher 1954). « Dear enemy » rela-tionships have been shown to be related to territoriality in different taxa (e.g. birds -Godard 1993 ; ants - Heinze et al. 1996 ; frogs - Owen and Perrill 1998). These rela-tionships could be mutually beneficial to neighbours, as individuals who recognize

SPATIAL ORGANIZATION OF GERBILLUS DASYURUS 113

neighbours and reduce their responses to these neighbours would require less time andenergy for territorial defense (Jaeger 1981; Godard 1993 ; Randall 1995). If so, theinteractions between individuals from the same aggregation would be less aggressiveand/or more amicable than between individuals from different aggregations. To test thisprediction we studied behavioural interactions between neighbour and non-neighbourG. dasyurus during dyadic encounters on neutral arena.

MATERIAL AND METHODS

Study site and gerbil trapping

Gerbils were captured from 3 study sites located on the northern rim of theRamon erosion cirque, Negev Highlands, Israel (30° 35' N, 34° 45' E). The main land-scape of the study area is a complex of hills with a deep loess layer (up to 1 m) andwide dry riverbeds with vegetation dominated by Anabasis articulata, Atriplex halimus,Artemisia sieberi, Salsola schweinfurthii and Noaea mucronata. The region is charac-terized by hot, dry summers (mean daily air temperature in July is 34 °C) and relativelycool winters (mean daily temperature in January is 12.5 °C). Data were collected dur-ing October and November when mean daily temperature ranged between 14 °C and29 °C. Mean annual rainfall in the area is approximately 100 mm.

Animals were trapped using Sherman live-traps baited with millet seeds. The trapswere checked every morning between 07:00 and 07:30. A total of 82 adult G. dasyuruswere captured. Each captured gerbil was sexed, checked for reproductive status,weighed (to ± 0.1 g ; Pesola spring scale) and marked by toe-clipping and by fur color-ing with green wool dye (Brilliant Green) in a recognizable pattern. Reproductive statewas recorded as pregnancy, lactation, perforation of the vaginal opening or the pres-ence of vaginal plug for females, and scrotal testes for males. We determined the rela-tive age based on the size and reproductive status of each animal. Gerbils with bodymass less than 15 g were classified as juveniles; gerbils with body mass of 16-18 gwere classified as subadults; gerbils with body mass of 19-21 g were classified assubadults if they were sexually immature or as adults if they were in reproductive con-dition ; and gerbils with body mass greater than 21 g were regarded as adults.

The density of G. dasyurus on study sites ranged from 20 to 25 individuals per1 ha. Continuous captures-recaptures were conducted on one of the sites, whereasrodents were censuses three times during 2 months on two other sites that were locatedat the distance of 700 and 1 100 m from the main site. Three clusters of gerbil capturepoints were recorded on the main study site. These clusters indicated the occurrence ofspatial aggregations of individuals whose home ranges broadly overlapped with oneanother and were isolated from home ranges of individuals belonging to other aggrega-tions. Detailed information on the determination of home ranges, home range size andcriteria to delimit a given spatial aggregation can be found elsewhere (Gromov et al.2000).

Every morning eight to 10 randomly-selected adult individuals were transported tothe laboratory where they were maintained in small cages until late evening. Animalswere fed millet seed and leaves of Atriplex halimus ad libitum. Dyadic encounters wereconducted after sunset. Four to five encounters were conducted per evening. After eachseries of encounters, all gerbils were transported to the study site and each individualwas released in its capture location exactly.

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Experimental animals

Opponents in dyads were adult gerbils of the same sex and were arranged into twocategories, namely neighbour and non-neighbour. Individuals were considered to beneighbours if their home ranges were within the same spatial aggregation. Individualswere considered to be non-neighbours if (1) they belonged to different aggregationswithin the same site and their home ranges were isolated or (2) their home ranges werelocated at different sites. A total of 41 encounters were conducted. These were27 male-male (11 neighbour and 16 non-neighbour) and 14 female-female (6 neighbourand 8 non-neighbour) encounters. Among individuals that were used in the encounters,33 of 54 males possessed an active ventral gland and had scrotal testes and were con-sidered to be reproductively active. Proportion of reproductively active females waslower (six of 28 females). Other males and females were reproductively inactive adults.

Dyadic encounters

Behavioural encounters were conducted in a glass arena (55 by 30 by 40 cm)located in a darkened room, with a dim red light shining into the arena from the front.A plywood partition, dividing the arena into two equal parts, kept animals separatedprior to each encounter.

At the onset of each encounter, each of two individuals to be encountered wasplaced on either side of the partition and allowed to become familiar with the arena.During this period, animals generally explored their surroundings for several minutesand then settled into a corner of the arena. After 10 min, the partition was removed,and interactions between the animals were recorded continuously during 10 min. Thequantity and duration of the intervals were empirically determined during a few prelim-inary encounters. Between encounters, we thoroughly wiped the arena with 70 %ethanol and dried it.

We recorded the following behavioral items that were classified into four groupsaccording to apparent motivational similarity ; (1) Introductory acts (nosing, investiga-tion, approach and ano-genital sniff), (2) cohesive behaviour (grooming, presentationfor grooming, crawling-under and climbing-on), (3) agonistic offensive behaviour(sideways postures, aggressive grooming, boxing, attack, chase, locked fight) and (4)agonistic defensive behaviours (defensive postures and flee). The detailed descriptionsof these patterns can be found in Grant and Mackintosh (1963), Swanson (1974),Goltsman et al. (1977) and Agren et al. (1989). In addition, we measured the durationof a period when opponents sit one by another with close tactile contact.

Behavioural asymmetry (i.e. behavioural difference between partners) within adyad was evaluated by asymmetry index,

where A and B are numbers of behavioural items displayed by one and another oppo-nent during the encounter. Indices of asymmetry were calculated for offensive agonistic(sideways postures, aggressive grooming, boxing, attack, chase) and defensive agonis-tic (defensive postures, flee) behaviours separately. I = 0 when both animals con-tributed to the same amount to a given behaviour, and I = 1 when one animal only dis-played the total amount of a given behaviour.

Data from each encounter were expressed either as the number of behaviouralitems of a particular type per individual (for most comparisons) or as a value perencounter (for the comparison of asymmetry indices). The distributions of the behav-ioural variables did not conform to the assumptions of parametric tests ans transforma-

SPATIAL ORGANIZATION OF GERBILLUS DASYURUS 115

tion of data did not lead to normality (Shapiro-Wilk's tests). Thus, non-parametricanalyses, namely Mann-Whitney U test, Wilcoxon paired-sample test and Spearmanrank order correlation were used. Data are presented as means ± SE. We avoided aninflated Type I error by performing Bonferroni adjustments of a (Rice 1989). Signifi-cance is only recorded at this adjusted level.

RESULTS

In general, encounters of G. dasyurus were characterized by low level of agonismand significantly higher level of cohesive behaviour (Wilcoxon paired-sample test, Z =3.81, p < 0.001). The number of occurrences of some behavioural items in dyads ofneighbour males differed from those of non-neighbour males. Opponents in neighbourdyads demonstrated offensive agonistic interactions significantly less frequently thanopponents in non-neighbour dyads (Fig. 1) (e.g., sideways postures, Mann-WhitneyU = 258.5 ; aggressive grooming, Mann-Whitney U = 292.0 and attacks, Mann-Whit-

Fig. 1. Mean number (± S.E.) of behavioural items initiated by neighbour (white columns) andnon-neighbour (black columns) individuals during 10 min encounter. AP - approach, GR -grooming, PG - presentation for grooming, CU - crawling-under, CO - climbing-on, SP -sideways posture, AG - aggressive grooming, BX - boxing, DP - defensive posture, AT -attack, FL - flee. « * » denotes significant differences (p < 0.05).

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ney U = 140.5 ; p < 0.01 for all comparisons). At the same time, some introductoryand cohesive behaviours were observed in non-neighbour dyads significantly more fre-quently than in neighbour dyads (e.g. approach, Mann-Whitney U = 291.5 ; grooming,Mann-Whitney U = 322.0; and climbing-on, Mann-Whitney U = 255.0 ;p < 0.02 forall comparisons) (Fig. 1). On the other hand, neighbour partners spent significantlymore time sitting one by another with close tactile contact than non-neighbour partners(55.67 ± 20.14 s versus 10.07 ± 4.39 s, Mann-Whitney U = 218.0, p < 0.01). No sig-nificant differences either in manifestation or in frequency of behavioural items werefound between neighbour and non-neighbour dyads of females (Fig. 1).

To study the effect of reproductive status on gerbil behaviour, we pooled data forall reproductively active and all reproductively inactive males and females. Reproduc-tively active males initiated significantly more sideways postures, aggressive groom-ings and attacks and manifested fewer defensive postures than reproductively inactivemales (Mann-Whitney U = 303.0, U = 213.0, U = 314.0 and U = 208.5, respectively, p< 0.01; Fig. 2). Reproductively active females differed from reproductively inactivefemales in the manifestation of grooming only (Mann-Whitney U = 33.5, p < 0.03 ;Fig. 2).

Fig. 2. Mean number (± S.E.) of behavioural items initiated by reproductively active (whitecolumns) and reproductively inactive (black columns) individuals during 10 min encounter.Abbreviations of behavioural items are the same as in Fig. 1. « * » denotes significant dif-ferences (p < 0.05).

SPATIAL ORGANIZATION OF GERBILLUS DASYURUS 117

Indices of behavioural asymmetry between opponents are presented in Table 1. Ingeneral, male encounters were more asymmetric than female encounters in terms ofdefensive behaviour (Mann-Whitney U = 99.0, p > 0.01) but not in terms of offensivebehaviour (Mann-Whitney U - 193.0, p > 0.2). No difference in behavioural asymme-try between neighbour and non-neighbour male encounters was found for both offen-sive and defensive behaviour (Mann-Whitney U = 95.0, p > 0.3 and U = 100.5, p >0.5, respectively). The same was true for female encounters (Mann-Whitney U = 23.5,p > 0.3 and U = 13.0, p > 0.5, respectively). It is noteworthy that the defensive asym-metry was significantly higher than offensive asymmetry in male encounters (Wilcoxonpaired-sample test, Z = 3.79, p < 0.01).

No correlations between male body size and the manifestation of defensive andoffensive agonistic behaviour were found (Spearman R = - 0.05, t = - 0.41, p = 0.9 andR = 0.22, t = 1.66, p = 0.12, respectively). The same was true for females (Spearman R= 0.55, t = 0.85, p = 0.4 for defensive behaviour and R = - 0.09, t = - 0.53, p = 0.6 foroffensive behaviour).

TABLE 1. - Indices of behavioural asymmetry during the same-sex encounters of G. dasyurus

Sex

Male

Female

Neighbour versus non-

neighbour

Neighbour (n= 11)

Non-neighbour (n=16)

Neighbour (n=6)

Non-neighbour (n=8)

Defensive

asymmetry

0.85±0.07

0.7110.13

0.08±0.05

0.22±0.11

Offensive

asymmetry

0.29±0.06

0.36±0.06

0.11±0.05

0.23±0.07

DISCUSSION

Our prediction appeared to be only partly true. The frequency of agonistic interac-tions between male neighbours was lower than between male non-neighbours, a patternconsistent the « dear enemy » model. Males are less aggressive towards neighboursthan towards non-neighbours and, thus, can reduce the cost of contesting resources.The revealed differences between neighbour and non-neighbour male dyads in the fre-quency of agonistic behaviours suggest that males of G. dasyurus were able to distin-guish between familiar and unfamiliar individuals. Home ranges of G. dasyurus withinspatial associations broadly overlap (Gromov et al. 2000). This pattern of spatial distri-bution increases the probability of a male to encounter a potential mate, but the expec-tation of encountering a competitor is increased too. Low agonism among neighbourmales can be considered as a mechanism regulating their spatial relationships. Thiswould allow individuals to respond to a potential competitor (for example, by avoidingit) without real energy-consuming contact.

However, contrary to our expectation, introductory and cohesive behaviours wereless pronounced in neighbour encounters than in non-neighbour encounters, whereas,

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according with the "dear enemy" phenomenon, individuals should be more tolerant andamicable towards neighbours than towards non-neighbours (e.g., Godard 1993). Theexplanation can be that the main motivation for non-neighbour males of G. dasyuruswhen they meet each other is to become acquainted and, thereafter, they begin to ascer-tain their relationship. Thus, the possible scenario of an encounter between two malescan be as follows. At first, they would attempt to recognize whether they are familiaror not (neighbours or non-neighbours). Familiar individuals with previously establishedrelationships are not especially concerned with each other and drift apart, whereas non-familiar individuals get involved into interactions during which they establish theirrelationships. These interactions are characterized by low level of aggression, and theperiod of establishment of a stable behavioural asymmetry seems to be relatively short.After this, either they drift apart or the « surrender » individual gives up the struggleand leaves the "battle-field".

Studies of some gerbil species (Meriones unguiculatus, M. meridianus) demon-strated that larger males were usually found to be more aggressive and hence were usu-ally winners in dyadic encounters (Norris and Adams 1972; Agren et al. 1989 ; Gro-mov and Vorobieva 1995). Surprisingly, in G. dasyurus male body size appeared not tobe correlated with the manifestation of offensive agonistic behaviour. At the same time,reproductively active males demonstrated more offensive behaviour than non-breedingmales. A possible explanation for these results can be that physiological characteristics(e.g. hormonal status) may be more important than the morphological parameters (bodysize) in the determination of the behaviour and, consequently, a potential behaviouralrole of a male (e.g. Zielinski and Vandenbergh 1993). Thus, behavioural differencesbetween reproductively active and reproductively inactive males suggest that the repro-ductive status of a male can be one of the main factors determining its response toopponent in a neutral territory.

Both the repertoire and the frequency of behavioural acts did not differ signifi-cantly between dyads of neighbour versus non-neighbour females. The cause of theabsence of attempts to establish an asymmetry between females can be their behav-ioural stereotype to avoid one another regardless of the degree of their familiarity,which is similar to those described by Verevkin (1985) for Meriones meridianus andby Agrell (1995) for Microtus agrestis. It seems that this mutual avoidance allowsestablishing of such spatial-behavioural relationship that are even less costly than thoseof « dear enemies ».

An asymmetry of behavioural interactions between individuals in usually per-formed on the basis of directionality of specific, typically aggressive, behavioural acts(e.g. Jones and Nowell 1989; Goltsman and Volodin 1997; Gerlach 1998). However,offensive agonistic behaviour was found to be an unreliable indicator of behaviouralasymmetry in G. dasyurus. Behavioural asymmetry was based on defensive rather thanon offensive acts. These results fit well with our assumption that avoidance can be oneof the mechanisms maintaining spatial distribution of home ranges in G. dasyurus.Nevertheless, higher offensive agonistic interactions between non-neighbour males thanbetween neighbour males suggest that aggressive behaviour also play a role in thisprocess.

It should be noted, however, that between-individual relationships in G. dasyurusinfluence spatial distribution on a very local scale, while environmental factors are cer-tainly more critical at a broader scale (Shenbrot et al. 1997). In addition, this kind ofspatial structure seems to be supported if and only if the level of per capital/resourcesis relatively high, for example when the density is not extremely high. Indeed, the den-sity of G. dasyurus during the study period is considered to be moderate (in average,

SPATIAL ORGANIZATION OF GERBILLUS DASYURUS 119

20.8 resident animals per 1 ha) in comparison with densities on these study sites inother periods. Maximal observed density of resident individuals attained on these siteswas in average 32.3 animals per 1 ha (Shenbrot et al. 1997).

ACKNOWLEDGEMENTS

David Ward (Ben-Gurion University of the Negev,Israel) and Andrei Tchabovsky (Instituteof Evolution and Ecology, Russia) read the earlier version of the manuscript and made helpfulcomments. We thank two anonymous referees for their most helpful comments on the manu-script. The study was supported by the Blaustein Center for Scientific Cooperation at the JacobBlaustein Institute for Desert Research, Ben-Gurion University of the Negev, Israel (for VSG),Israel Ministry of Science, Israel Ministry of New Immigrant Absorption and Local Council ofMizpe Ramon (for BRK and GIS). This is publication no. 82 of Ramon Science Center.

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