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Sex-linked Strategiesof Human Reproductive Behavior

Klaus Jaffe, Debora Urribarri, Grace C. Chacon, Gerardo Diaz,Alfredo Torres, ancl Gabriele Herzog

Departalllfl1l0 de lJiologia de Organismos. Universidad .'limon Bolivar. Apdo. 89000. Cara­cas 10SO. l'('//cwela; Mcvenea. Ca/'{/eas, VellcZ1Iela; Celltl'o de IlIvestigaciones Politicas, Ca­meas. VeneZ/lela; ,\1ax Plallck Illstit/lt f/ll' Verhaltemforsch/lllg, Seell'iese11, Ger11lany

ABSTH,\CT: \Ve present data on fertility characteristícs in the Venezuelan populatíon for each sexseparately. aJ10wing a detailed comparative analysis of the variance in fertility between males[111(1 females. \Ve show that the fertility distributíOl1 for both sexes is discontinllOUs, that theaverage fe¡nale has a Inrger number of offspring per indiYidunl tha'n the average male, allu thathighly fertile males oulnumber highly fertile females so that the total number of offspringproduced by males and females is balnnced. Hesults indicate that a few males are responsible forn reJa!j\'e higher fertility of the average female ami that interactions between polyandric femaleswith rnonogamic nnd polyg)'nic males are comino!!. Among the Yanomami, ti relatÍ\'ely unac­culturélted hllnter-gatherer-horticulturist tribe, similar differences in fertility distribution of bothsexes are apparent. The dala sllggest that hllman populations contain statistically distínctsubpopulations, with different reproducti\'e strategies, sllggesting the existence of complexinteractiolls aIllong human poplllations which me not evident fron1 the stllcly of individuals 01'

groups.

Recent studies have shown that

many species traditionally thought ofas monogamolls in their pairing habitsare polygamous in their reprodllctiveItabits, making it necessary to rcstlldythe mating systems and reprocluctivestrategies of most specíes (Clutton­

Brock, 198?), In aclclition,the sex ratioof most species, even in polygamousones, is c10se to onc (Fisher, 1930). Asfecundily differs markedly between thesexes, oplimaJ reproduction (maximiz­ing direct <lnd inclusive fitness in thelong term) varíes oetwcen sexcs, l:mdeach sex has to use a different repro­ductive stra.legy. A result of such strat­egies can be seen in the presence ofsíze dimorphism bctween the sexes,i.e., spccies wit\¡ polygynous malesproduce males which are larger thanthcir females (Wilson, 1976).

61

1t is gene rally assumed that Homosapicns is essentially polygynous,where the varianee in reproductive suc­cess is greater among men than amongwomen (Daly and Wilson, 1983;Chag­non, 1980). The polygynous behavioraccordingly explains a slight sex dil11or­phism in human body size (Wilson,1976; Alexander et al., 1979; Daly andWilson, 1983; Eibl-Eibesfeldt, 1984).Polygyny is caused by differences inthe amount of paren tal investmentgiven by each sex for' the survival oftheir offspring. Femnles invest more inmetabolic costs and in time dedicated

to child rearing and thus Can bear tll1drear a limited number of offspring(Trivers, 1972; Trivers ai1d Willard,1973; Maynard-Smith, 1976). Extremepolygyny in bumans is Iimited by fe­male behavior. Hidden estrus, cOlitin-

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62 Jaffc el al. Social Blology

uous sexual receptivity, and syncl1roni­zatiol1 of menstrual cyc\es, and otherfactors, make it difficult for males to

control large harems (Daniels, 1979;Burley, 1979). These facts suggest thatthe optinml reproductive strategy forcach sex ShOllld be different among hu­mans (Daly and WiIson, 1983; Gram­mer, 1989: Buss, 1989).

Thc existence of clifferent combina­

tiol1s of optimal reprodllctive strategiesjmplies that fertility will be distribllteddifferently among individllals 01' eachscx in a given population. Unfortu­natel)', fe¡-¡ility data for hllmnn popu­lations, obtainecl through populationCeI1S11S,is availnble on]y for fel118les,ami the few publisbed data availabIe011male fertility (1'01'example, Howell,1979: Chagnon, 1980; Da]y and \Vi]­son, 1983: 13rown, 1988; Hewlett,

1988) stem froIn studies conducted011 population isolates and minorities\\hich do not permit the plotting 01'de­tailed, statistically meaningful, fertilitydistribution cUl·ves. However, all these

stlldies showed a greater variance formale fertility compared to that 1'01'fe­males, althollgh detai]ed ana]ysis ofthe source 01' this greater variability ismissing. On the other hand, data 011fe­maIe fertility distributions from 13an­gladesh (13ongaarts and Potter, 1983)and from U.S. cenSllses (David anclS~nderson, 1987) showed c1ear discon­tinuOlls distriblltions of fertility charac­tcristics between the populations, indi­cating complex reproductive patternsin human populations.

We had the opportllnity to partid­pate in a survey based onlarge regionalpopulations and to obtain clata on fer­tility for both males and females,thanks to the inclusion of questions onfcrtility in tlle political polls for the

Venczuelan presidential elections of1988. We studied the Venezuelan vot­

ing population (agecl 18 years andover) and estimated direct individualfitness, using the total number of off­spring (TO) ever boril for each individ­ual and the number of living offspring(LO) at the moment of the data col­

lection. We also studied fertility amongthe Yanomami, a relatively unaccul~turated tribe which inhabit the upperOrinoco River in Venezuela. Yano­mami are swidden horticulturists w110

supp]ement their diet with hunting andgathering (Lizot, 1988). These data al­lowed a comparative analysis of thevariance in fertility, between males andfemales.

MA TERIALS AND METHODS

Data from al1 Yanomami present inthe community wel'e collected throllghinformants and direct conversa tion byanthropologist Gabriele Herzog in1988 at Seroana, Tayariteri, Maheho­toteri, Nahetipe, and Tortitha, all ofwhich are located in the Territorio

Federal Amazonas. Only informationabout living children could be obtainedas Yanomami will 110t speak abouttheir dead. Age of subjects \Vas not

possib]e to obtain as Yanomami do notkeep track of their age in number ofyears. In addition, it \Vas not possiblefor LIS to estimate the end of the re­

productive life of sllbjects. Thus, allsubjects who were recognized byYanomami as "marriageable" were in­c1uded. Results of the 227 individuals

represent the total population at, thattime in the five communities sampled,which in turn have cultural character­

istics unique to these commllnities anddifferent from those of other Yano-

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Vol. 40, No. 1-2 Reproductive Behavior 63

mami. Thus, statistical analysis ofthese datil is meaningless.

Data frol11 6,500 subjects over 17yearsqf age (including 1,098 femaJesover 40 amI 985 males over 45 years),in 15 urban areas, as representative ofthe total voting population of Venezu­ela, were collected throllgh the strati­fIcd (by geograpbical area, economicstatus, age, and sex, based on the 1981national census) polls prepared bySímevcn, measuring voting in ten­tions for Carlos Andres Perez and byjvlevenca for other c1ients, as part ofthe presitlentiaI campaign of 1988 inVenezuela. (The reprcsentativcness ofthe sample was sho\Vn a posli?I'iori to beaccurate, with Simeven predicting themain reslllts with an accuracy of 99per cent). The following qllestions",cre given to the subjects (female or

mate) to answer confidenti31ly and in­diviclllally: 1. How many living chil­dren (legitimate or not) do you have?2. Ho\V many children did you bearduring )'our' life? 3. Age of children?4. Sex and age of informant? AII ínter­views were conducted confidentially,in Spanish, by trained local staff.

Although data collection tbrollghqllestionnaires could seem to be a du­biolls l1lethod for estimating male fer­tility, this stlldy shows that data pro­vided by males is as reliable as thatprovided by females, which is the stan­dard procedllre for collecting fertilitydata in hllman populations, since bolhsamples reported statisticaJly the sametotal nllmber of children (see results).

Few reliable methods for statistical

analysr.s of mixed samples exist, andamong those, the most simple and re­Ijable method available is the Bhatta­

char)'a plot (Everitt and Hand, 1981;Bhattacharya, 1967), which detects

discontinuities on a normal frequencydistribution through changes in theslope of the distribution curve. Thus,\Ve chose this method to analyze ourdata.

RESUL TS AND DISCUSSION

The fertiJity distríbution among thetotal adult population sampled, for theYanomami as well as for the rest of the

population studied, showed a differentdistribution of fertiJity characteristicsfor males and (emales. In Figure 1wepresent a snapshot of the total adultpopulatiol1. Here, the number of adultindividllals is plotted according to thenllmber of living children they re­ported (LO). In both samples \Ve findmore childless males compared to fe­males, but females olltnumbered males

in the lo\V fertility categories. Maleshaving very many offspring outnum­bered females.

This common pattern atnong totallydifferent cultures seems neither to be

an artifact of the sampling procedurenor a particular characteristic of thesamples, as similar sex-diffel'entiatedfertility distributions were found fol'cllltures which differ in many otheraspects. Among the adult Yanomami,males outnumbered females by a ra­tio of 1.25. Within a sample of 13Yanomami villages, Chagnon et al.(1979) found a similar average sex­ratio of 1.29. In contrast, among thepolled population, females outntun­bered males by a ratío of 1.05. AIso,higher child mortality among Yano­mami, a general featúre of societieslacking modern medieal facilities(Campbell and Wood, 1990) explainstheír compal'atively lower fertílity, asmeasured by LO. In spite of these

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64

1.1

Jaffc ct al. Social Biology

0.8

en

-o:::l 0.7T.I >

.--oc:4-

0.5

o \...

Q).0E0.3

:;¡z

0.1O

Ó

Ferlilily Colegory (LO)

12 14

FIG. 1. -Number of indh'jduals (females in continuous lines; males in discontinuous lines). inIhollsal)ds for the \"oting population (Ihíck lines) and in hundreds for the Yanomami (thin lines).in ench fertility category determined through LO (Iil'e offspring). (Chi-square between the male andfemale fertility distributions: x~ = 164. p < 0.001. df = 20)

dissimilarities, common patterns in thesex-linked fertility distribution be­tween both samples (Le., both cul­tures) are evident.

Differential fitness for males and fe­

males could be estimated by eliminat­ing sL!bjects from the analysis who hadnot completed their reproductive lifespan (only for the polled population).By comparing the age of the last childwith that of the informant, we ca\cu­

lated ¡he probability of bearing a childfor subjects over 40 years, which \Vas0.27 for males and 0.07 for females.

Su bjects agecl over 45 years had a prob­ability of child bearing of 0.08 formales and 0.02 for females, and for

subjccts over 50 years of age the prob-

ability was O for females and 0.07 for

males. Accordif!g to these data, \Ve de­fined the end of the reproductive lifespan at 45 years for males and 40 yearsfor females. We could thus estimate

TO and LO for any individual duringhis 01' her reproductive Iife span withan error smaller than 8 per cent. AI­though an analysisof age cohorts wasnot possible with the data (and thus thecontribution of individuals who died

earlier, was not taken into accounthere), the frequency distributions fremindividuals above 35 01' 55 yearsshowed qualitatively the same resultsas reported here (not showll). Thus,the distribution presented is a reliablerepresentation of a specific character of

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Yol. 40, No. 1-2

140

Reproductive llehavior 65

VI-o:J'D 80> ue.- 60

..•... o'-OJLl 40 -E :JZ

20

0-

O

2

4 6

Fertility

8 10 12

Colegory ( LO)

14 16 18 20 >20

[IG. 2. --Number of indiyiduals ([emales in continuous and males in discontinuous lines) perfertility category. l11easured through LO. for subjccts who completed their reproductive years. (Chi­squared test between sexes: Xc = 33.1) < 0.05. df = 20)

a given population (Le., distribution ofthe maximal probable individual fit­ness among the poputation of individ­uals) ancllhlls serves to analyze fertilityciiffercnces among sexes.

A plot of fertility distributions usingthese data (Fig. 2), showed a cIear sex­differenliated discontinuous dislribll­

(ion, confirming the findings givenabove. Here, the sex difference for

chilcl\ess individuals was greatly re­dllced, indicating lhat males deJay theiniliation of their reproductive lifelonger than do females. Again, agreater number of females had a highernumber of offspring than males, andonly among.individuals with more than12 children, did males outnumber fe-

males. Integrating the total outpllt ofoffspring (LO) for each sex renderedcomparable nllmbers. That is, in oursample, all the females prodllced a to­tal of 4,661 offspring, whereas ali themales together reported 4,690 children(a difference smaller than 0.62 percent). These results can only be ex­plained if we assume that a relativelysmall nllmber of males (those withmore than 12 children) were responsi­ble for the differential offspring pro­dllclion between the average femaleand lhe average maleo The equivalencein the total number of offspring pro­dllced by each sex is even more strik­ing, if \Ve remember that more females

than males were present in the sample

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20 >20181614e 10 12

Category (LO)

4 6

Ferlility

2

66 .Jaffe et al.Social Blology

600 500~ 400

o::J""O.->u 300c:'+-o

\...

200a.> .oE::JZ 100

f'10. 3. - TOlalnumber of li\'ing children (number of individllals) prodllced by females (continllOllslines) and by males (discontinllOlls lines) in each fertility category. who completed their reprodllcti\'eycars (LO x NI' of individuals gi\'en in Fig. 2; ,Y1 = 450. p < 0.001. df = 20)

dlle to differential mortality amongsexcs (se e also CowgilI and Hutchin­son. 1963; Teitelbaum, 1972) and thatthe duration of the reproductive lifewas different for each sexo

The formo of the distribution curve is

similar for' both sexes, although thevarious modes do not overIap (Fig. 2);but the variance in fertility differed sig­nificantly among the sexes. Variancefor LO of fem<\les was 10.92, whereasthat of males was 15.44 (p < 0.001).Clearly. males may have a higher m3X­il11ulll number of children than fe­males.

The differences in the pattern of thefertility distribution of each sex wereamplified ,in Fig. 3, where the propor­tional contriblltion of each rertility cat-

egory to the total number of offspringsproduced is shown. Here the principalmodes which have the major impact onpopulation growth are clearly distin­guishable, showing one for men with 5children and one for females with 8

children. Again, children produced bymales \Vith ayer 12 children were more

numerous than those produced by theequivalent females ..

Child l1lorfalil)'. - Data fl'Om thepolIs also permitted estimation of theeffect of child mortality on the fertilitydistribution curves. Although the mea­sure of LO may be overestimated, itindicates trends in child mortality ifcompared to TO. Figure 4 gives thefertility distribution for females andmales respectively, estimated through

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Vol. 40, No. 1-2

120

100

Ul

o.g 80.:::uc:

.•... 60o'­Q).oE::lZ

20

o

Reproductive Behavior 67

40 s:o..,o

20 _'<

o 2 4 6 8 10 12 14 16 18 20 > 20

140

Ferfilify Cofegory

120

Ul 100"O::l.~.:: 80uc:

'O 60•..Q).o~ 40z

20-

oO 2 4 6 8 10 12 14 16 lB 20

20 !:!.

10 .:;

Ferlilily Colegory

FIG. 4. - Total (TO, eontinuous Hnes) and Live (LO, diseontinuous Hnes) offspring born to fe­males (Fig. 4a) and males (Fig. 4b) who eompleted their reproductive years (those in Fig. 2). Thinline indieates the ehild mortality gh'cn as percentage of single children dying befote their parentslcae\¡ age 40 (female) or 45 (malc).

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68 Jnffe et al. Social Diology

1'0 ane! LO, ane! at the same time it

sho\Vs Ihe percentage of children dying .at each fertility category. Clearly, theeffect on the fertility distribution ofchild 11l0rtality is different for men ane!women. Chile! mortality affects the fer­tility distribution of the female popu­latíon much more than that of males

(Fig. 4). Women with more childrenreport more clead offspring than lessferti1e \\'omen. That is, the frequencyclistribution curve of LO for [emales isdoser to a Poisson distribution than

that of TO. partly due to the fact thatthe death rate mnong children in­creased linearly with fertility. [emalesscem to register TO 1110re accuratelythan do males, which could explainthe significantly (p < O.(JOI, Students[-test) higher child mortality dec1areclby [emales, more than twice that givenby males. This sex differenee in the re­

porting of deaths validates the use ofLO instead oí TO for inter-sex COI11­

parisons of fertility.A11 important factor that might bias

our results could be the fact that males

do no! know or do not recognize alltheir children, Le., they may report alower number of offspring. If so, ourresuIts shoulcl show a smaller number

of total children produced by malesthan that produced by females. To the

cOl~trary, our sample showed thatmales reported on the average aslightly higher number of chilclren thanfemales. The mean number of children

per female was 5.10, whereas that ofmales was S.41 children per individual.This figure may be a close approxima­tion of reality, as surviving males haveto compensate somehow for the COI11­paratively greater mortality rate oftheir sexo However, it may also reflectthe fact that males tend to overesti-

mate their numbers of offspring. lnde­pendent evidence suggests that malesoften recognize as their own (con­sciously or not) children who have adifferent biological father (Wiener,1943; Schacht and Gershowitz, 1963).Contrary to this expectation, our re­suIts shO\ved that the female fe rtilitydistribution peaked at a higher fertilitycategory than that of males (Figs. 2 and3), strongly suggesting that the averagefemule in the low-fertility categories(fewer than 12 children) has moremates (in time or space) than the av­erage male, whereas the inverse occursin the high feitility categories (morethan 12 children).

Although fertility data do not char­acterize reproductive strategies cIi­rectIy, here t!ley serve to estimate therelative importance of the various re­

productive strategies and to describethe sex-clependent variance in repro­dllctive success. The data are consis­

tent with a population consisting of alarge proportíon of polyandrous fe­males (possibly sequentially and not si­multaneously in time) and an averagemonogamic male with so me highly po­lygy110us males. The polygamous re­productive systems found !lere do notnecessarily correlate with polygamousmarriage syst~ms, although this couldbe checked, for example, by correlat­ing data on fertílity with ínformationon divorce.

Mixed population samples. -Noneof t!le fertility distributions presentedcouid be adjusted to a Poisson distri­bution (nor any normal distribution:p « 0.001; SPSS's goodness of fittests). 11lUS, a11important point whichemerges frol11 t!lis stucIy is that averagevalues do not accurately reflect thestrllcture of populations. There is more

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Vul. 40, Nu. 1-2 Reproductivc nehavior 69

than one modal point within a popula­tian. For a finer analysis of human so­cieties. populatiol1s should be studied

as being made up of individuals whouse different noncompatible, but fi­nite, strategies. Thus, techniques forl1lixed samples analysis (Bhattacharya,1967; Everítt and Hand, 1981) are use­fui for hUl1lan population studies.

In one sllch approach, two statisti­cally recognizable SUbpoplllations inthe United States have been det1nedas "birth controllers" and "non­

controllers" (David and Sanderson,1987). In our study, we recognizé atieast three subpopulations: infertile,wcakly fertile, and highly fcrtile indi­viduals, respectively, although a Dllat­tachar)'a plot, rebounding the madesof the curve (by plotting the derivativeof the curve, Le., the slope at eachpoin t of the curve), shows the presence

of an even more complex pattern (Fig.5). Here we see that females constitute

four statistically distinct subpopula­tions which on average have 3.8,6, 7.5,al' ] 2 living children, whereas malesconstitute at least four distinct sllbpop­lllations which on average have 2.5,4.5, 8.5, al' over 12 living childrell (Ta­ble 1). That is, the fertility of femaleSUbpoplliations does 110t match that ofmales, although the total productiol1 ofchildren is balanced. For the two sub­

populatíons with low fertility, femaleshave more children than males, but for

the subpopulations with high fertility.males have more chilclren than fe­

males. Thus, non e of the subpopula­tions are mainly monogamous. For lowfertility, poiyandry seems to prevail; athigh fertility, polygamous males are1110re cOl11l11on.

More individuals were found in the

two low-fertility categories (see Fig. 2),

indicating that most females are morefertile than males and a few males

compensate for tbis fertility differen­tia!. Although no appropriate data onmale fertility are available for otherpopulations, data exist on female fer­tility. Fertility data from Bangladeshfemates (Bongaarts and Potter, 1983,Figure 6), if analyzed with a Bhatta­charya plot, also show distinct subpop­lllations. where two subpopulatiol1s(those with a mean of 4 and 13 chil­dren) resemble those of the Venezue­tan population with a mean of 3.8 and12 children (Table 1).

Sexual selectioll through fe/1lalechoice. - Independent support to ourt1ndings (Le .• the existence of a highproportion of polyandrous femalesinhuman populations) comes from stud­ies on sexual selection of animal geni­talía (Eberhard, 1985). Morphologicalevidence indicates that sexual selection

of genitalía through female choice is atwork in most animal species, includingprimates. That is, Eberhard found thatmale genitalia are subjected to stron­gel' selection pressures than femalegenitalía, thanks to female choice. Sex­ual selection of genitalia through fe­male choice will occur (Eberhard,1985) when (1) females normally matewith more than one male, and (2) a sin­gle copulation" do es not always result infertilization of the female's entire com­

plement of eggs. The latter is certainlytrue for humans (Tietze, 1960), and theformer seems to be confirmed by outstudy.

In addition, data from British fe­

males support the hypothesis that fe­males promote sperm competition(Bellis and Baker, 1990), adding to theevidence that females tend to be poly­andric.

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70

0.31 ••.°0.2

0.1---

c:~

O

..--0.1+

c:§- -0.2(J1

0-0.3--0.4

-0.5O24

Jnffe ct nI.

6 8 10 12

n (LO)

14. 16

Social Blology

18 20

0.4

0.2~

c:'0.O

'-.-0.1..- +c:

"3 -0.3(J1

O-0.5

-0.7. -0.9

O2

4 6 8 10 12 14 16 18 20

n (LO)

FIG. 5. -Bhattachnrya plot for femnle (Fig. 5a) and male (Fig. 5b) fertility distributions with dntafwm Fig. 2. The abscissa shows the log of the slope at each point of the ctlrve in Fig. 2. The intercept",¡th log O (given in the ordinate as LO) gives the average fertility of each sub-population, whereasIhe angle al Ihe intercept is proportional to the standard deviation (Bha1l3charya 1967).

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Vol. 40, No. 1-2 Reproducti\'e Behavior

TABLE 1

11

STATISTICS OF TIlE REGRESSIONS USE!) TO CAlCl'lATE TIlE MODES OF TIlE BIlATTACHARYA PLOTS IN

FiGlTRES 5a (FEMAlES). 5b (MALES) AND 6 (FEMALES)

~fodcFigure (Mena Nr Df LO)

rig:. 5a (F) 3.86.07.5

12.0

Fig. 5b (1\'1) 2.5-1.5

8.512.5

.................... 4.0Flg. 6 (F) 13.0

:W.O

r1 dfP

0.912<0.05

0.982< 0.01

0.981< 0.1

0.681> 0.1

0.92

1> 0.10.79

3< 0.050.996

1< 0.0010.81

2< 0.1

0.82

2< 0.10.94

5< 0.0010.98

-1< 0.001

050

~

~000'-... ---1I><l.L.••.....•

-050(9 O-1

-100

o-150

2 4 6 8 10 12 14 16 18 20 22 24

n (LO)

FIG. 6.-Bhattacharya plol for female fertilily dístributioll with data froro Bangladesh (deducedfrorn Bongaarts and Potter 1983). Interprctation of the graph as in Fig. 5.

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72 Jarfe et al. Social Biology

CONCLUSION

.Our results eontraclict the prevaJentassllmption that humans are sJightJypolygynie and rather suggest tbat amore eompJex pattern exists, where in­teractions between poJyandric femaleswith monogamic and polygynic malesare eommon.

The anaJysis shows that humanpopulations are a mixture of subpopu­lations wilh c1ifferent fertiJity charae­teristics and that the fertility charaeter­istics are not continuously distributedamong a given populatíon. Indiviclualsseem to eongregate at dísercte fertility"optima." Tbese optima eould beplaeed on a j'-K gradient (Rushlon.1985) where caeh may represent aspeeífie adaptatíon, maximizing e¡lberpopulation growth 01' possibilities ofsUl'vival of a rather limited numbcr

of offspring. The faet that this r- K gra­dicnt is not eontinuous, as had been

supposed by Rushlon (1985), inclicatesthe existence of a compJex pattern ofadaplive strategies in hUl11an popuJa­tions whieh should be further investi­

gated. Eaeh bJend of reproductivestrategies, together witb the specificferlility characteristics of the popuJa­tion, wil! produce a specifie assemblage

of subpopulations based on fertilityeharacteristics which wil! in turn pro­duce specific fertiJity frequency distri­butions,\vhieh can be charaeterized, as

done here. This method alIows us to

compare populations with differenteultures, deteeting the various statisti­

cal subpopulations, estimating theirrelative proportions, and eventuaIlytelling us about the factors which alIowthese subpopulations to segrega te sta­tisticalIy.

It would be of great interest to as­sess an eventual effect of inereasing in­dustrializatiOI1 (and thus inereased so­

phisticatiol1 of Jarger populations) onbasic reproduetive behavior at the pop­1I1ution leve\. If the sex-related fertilitydifferences are at least partly biologi­cal!y deterl11in~d, similar proportionsof 1110nogamic, polygynic, and poly­undric individuals should exist in alI

human poplllations. 1'0 confinn this,we need to co\lect male fertility datathrough nutionwide' surveys world­wide, \vhich will help us to get a deeperinsighl into our reproductive behavior.With this knowJedge \Ve may eventu­alIy formulate more effective policiesfor popuJation control.

ACKNOWLEDGMENTS

Thanks are due to Carlos Andres Perezfor allowing access to his political machin­ery during his presidential campaign; to Dr.Richarc1Wall al Cambriclge, UK, for accessto bis Iibrary and for heIpful discussions;and to Drs. I. Eibl-EibesfeIdt, P. Wiessner,M. DaIy, P. E. Howse, and C. Bosql!e rOl'helpful comments on the manuscript.

REFERENCES

ALEXANflER. R. D. 1979. Evollltion and cllltllre.p. 59-70. /11 N. A. Chagnon anu W. 1rons(Cd5.). Evolutionary biology and hllJll3n so­cial bchador. Duxbury Press. North Scitu­ate. ¡'olA.

BELLls. 1'01.A .. and R. R. 8AKER. 1990. Do fe­

males promote sperm competition? Data forhuman5. Anim. Beha\'. 40:997-999.

BIIAnACHAR\'A. C. G. 1967. A simple method ofrcsollltion of a distribution ¡nlo Gaussian

components. 8iomelr. 23:115-135.BONG.\ARTS. J .• and R. G. POTTER. 1983. Fertil­

ity. biology and behavior. Academic Press,New York.

BROWN, D., ane! D. HOTRA. 1988. Are prescrip­I¡\,e monogamolls societies effectively mo-

JOBNAME: 7-34 dlh PAGE: 13 SESS: 4 OUTPUT: Thu Nov 18 18:15:18 1993/xycomp/ - main - /journals/socbio/springsum/jaffe

Vol. 40, No, 1-2 Reproductive llehavior 73

nogmnous?, p. 153-159. 11/ L. Betzig, M..BorgerhoffJ\Julder. and P. Turke (eds.), Hu­Illan reproducth'e behador. Call1bridge Uni­wrsity Pre~s, Call1bridge.

BURLEY, N. 1979. The evolution of concealedo\'ulation. Al11er. Nal. 114:835-858.

BIISS, D. 1989. Sex differences in human matepreferences: E,'olutionary hypotlteses testedin 37 cultures. Behav. Brain Sci. 12:1-49.

CA~IPnEU., K. L., anó J. W. WOOD. 1989. Fer­tility intraditional societies 11/ P. Diggory andS. Teper (eds.), Natural human fertility: So­cial and biolog.icalmechnnisms. Macll1illan,London. ~

CII.\GNON, N. A. 1980. Kin-selection theorv.

kinship, marriage. alld fitlless among tlíeYanomamo indiano, p. 545-57l. 11/ G. W.Bario\\' and l. Sih'erberg (eds.), Sociology:Beyond naturefnurture? Colorado.

CH,\GNON. N. A .. M. FUNN. and T. J\ln.·\NcoN.

1979. Sex ratio \'ariances among the 'Ya­nOll1ami.indians, p. 308-310. /1/ M. Chagnonanu W. 'Irons (eds.). E\'olutionary biologyand hUl11an social behavior, an anthropolog­ical.perspective. Duxbury Press. Mass.

CLUTTEN·BROCK, T,A. 1989. Mamalian matingsystell1s. Proc. Royal Soco London, Biol.236:339-372.

CO\\'GIU., U. M., and G. E. HUTCHINSON. 1963.Differential mortality among the sexes inchildhood and its possible significance in hu­rnan e\·oIlItion. Proc. Nat. Acad. Sci. 49:425­429.

DALY, M., and l\t WILSON. 1983, Sex, e\'olutionand behador. \Vads\Vorth.

DANIElS. D. 1983. The e\'01ut10n of concealed

oVlllation and self-deception. Ethol. Socio­biol. 4:69-87.

DAVID, P. A., and W. C. SANDERSON. 1987. The

emergence of a two-child norm among Amer­ican birth-controllers. Popul. Devel. Re\'.13:1-4l.

EDERH\RD, W, G. 1985. Sexual selection and an­

imal genitalia, Harvard University Press,Cambridge, MA.

EIDL-EIRESFELDT. 1. 1984. Die bíologie desmenschlichen verha!tens. Piper, 1\1unchen.

EVERITT. B. S .. and D. l. R\ND. 1981. Finitemixture -distributions. Chapman and Hall,LOl1(lon.

FISHER, R. A. 1930. The geneticaltheory of nat­ural selection. Clarendon Press, OxforQ.

GRAMMER, K. 1989, Human courtship be ha\'­iour: Biological basís and cognitive process­ing. 111A. Rasa, C. Voget, and E. Voland(eds.), The sociobiology of sexual and repro­ducti\'e strategíes. Chapman and Hall, NewYork.

HE\VlETT. B. 1988. In L. Betzig, M. BorgerhoffMulder. and P. Turke (eds.), Human repro­ductive behavior. Cambridge Univ. Press,Call1bridge.

HO\\'ELL N. 1979. Del110graphy of ¡he Dobe!Kullg. Acadel11ic Press, NY.

LIZOT, l, 1988. Los Yanomami, p.479-584. 111

Los aborígenes de Venezuela, Vol. 3. Fun­clacion La Salle/Monte A\'í1a, Caracas.

1\1,\\'N,\RD-S~lITH. J. 1976. The evotution of sex,

Cambridge Univ. Press, Cambridge, U.K.RUSIlTON, J. P. 1985. Dífferential K theory: The

sociobiology of individual and group differ­ences. Persono Individ. Diff. 6:441.

SCfL\CIIT, L. E .. and H. GERSHO\\'lTZ. 1963. Fre­quency of extramarital chíldren as deter­mined by btood groups, p, 894-997. Proc.Seco ¡nler. Congo Human gelletics, Vol. 2,Instituto G. Mendel, Rome,

TEITELBAUM,M. S, 1972, Factors associated withthe sex ratio in human populations, p. 91­109. 111 G. A. Harrison and A. J. Boyce(eds.), The structure of human poputations.Clarendon Press, Oxford.

TIETZE, C. 1960. Probability ofpregnancy reSult­ing from a single unprotectecl coitus. Fertil.Steríl. 11 :485.

TRlvERS, R. L. 1972. Parentat investment and

sexual selectioll, p. 136-179. lit B. Campbell(ed.), Sexual selection alld the descent ofman, 1871-1971. Atdine, London.

TRIVERS, R. L., and D. E. WILlARD. 1973. Not­

ural selection of paren tal abílíty to vory ¡hesex ratio of offspring. Science 179:90-92.

VINING, D. R. 1986. Social versus reproductivesuccess: The central theoretical problem ofhuman sociobiology. Behav. Brain Sci.9:167-215,

WIENER, T. 1943. Blood groups and transfusion.3rd ed. Thomas, Springfield, IL.

WILSON, E. O. 1976. Sociobiology. BelknapPress, Harvard.