Local Resource Competition Hypothesis Research Articles

PARENTAL CORRELATES OF OFFSPRING SEX RATIO IN EURASIAN OYSTERCATCHERS

Abstract We investigated hatchling and fledgling sex ratios in Eurasian Oystercatchers (Haematopus ostralegus) using random amplified polymorphic DNA markers. The overall hatchling (53% males, n = 374 hatchlings from 177 broods) and fledgling (49% males, n = 51) sex ratio did not differ significantly from unity. Hatchling and fledgling sex ratios were not correlated with laying date, clutch size, brood size, egg-laying sequence, territory quality, male age, or male breeding experience, but hatchling sex ratio was positively correlated with age and breeding experience of females (0.05 < P < 0.075, n = 71). Older females produced more sons irrespective of the position of the offspring in the egg-laying sequence. Fledging mass was not correlated with female age, so the Trivers and Willard (1973) hypothesis is unlikely to explain our results. Sons dispersed less than daughters, so the local resource competition hypothesis of Clark (1978) might apply. The adaptive significance of a male-biased sex ratio in clu...

Contradictory findings in studies of sex ratio variation in roe deer ( Capreolus capreolus )

Patterns of sex ratio variation and maternal investment reported in the literature are often inconsistent. This could be due to intra- and inter-specific variation in social systems, but may also be a result of the a posteriori nature of much of this type of analysis or the testing of models which are inappropriate. Two recent papers reported directly opposed results concerning variation in offspring sex ratios in relation to maternal condition in roe deer, interpreting the results as support for the Trivers and Willard model and for the local resource competition hypothesis, respectively. In this paper, we present data on offspring sex ratios and early juvenile body weight from two long-term studies of this species to test predictions arising from these two models concerning sex biases in litter composition and maternal care. First, we observed no consistent pattern of sex differences in an index of weaning weight or body weight at 1 month old in either population, indicating a lack of sex bias in maternal care. However, in one population, higher maternal body weight was associated with higher juvenile body weight of daughters, but not of sons. Secondly, we found a negative, but not statistically significant, relationship between maternal body weight and litter sex ratio such that heavier females tended to produce more daughters and lighter females to produce more sons. These results indicate that roe females which have additional investment potential available do not invest it in sons, as predicted by the Trivers and Willard model. Our results may provide some support that roe deer are subject to local resource competition acting at the level of the individual mother; however, the fact that particular trends in sex ratio data can be explained in functional terms provides no indication that they are actually adaptive.

Natal Philopatry and the Local Resource Competition Hypothesis: Data from the Common Goldeneye

Sex-biased local resource competition (LRC) between parents and offspring has been suggested to exert selection pressure on progeny sex ratio variation in birds. In a recent article Weatherhead and Montgomerie (1995) questioned that the assumptions underlying the LRC hypothesis are applicable to birds. We here study the basic assumptions of the LRC hypothesis with data from the Common Goldeneye Bucephala clangula. First, we found strong female-biased natal philopatry, as expected in Anseriformes. Second, of all the females breeding in the study area, individuals born there comprised a major proportion each year. Third, we document a considerable overlap between home ranges of breeding mothers and daughters. Our results, combined with results from other Common Goldeneye populations and other Bucephala species, give strong support for the assumptions of the LRC hypothesis to be applicable to birds, at least to some species.

The socio-ecology of sex ratio variation in primates: evolutionary deduction and empirical evidence

Fisher (1930, The genetical theory of natural selection. Clarendon Press, Oxford) has shown that evolution on the basis of individual selection must yield population sex ratios of 1.0 (50% males and 50% females), provided that the costs of producing a male or a female are equal, and that there is panmixis. Evolutionary deduction predicts that mechanisms for adaptive sex ratio manipulation could evolve when the above conditions do not hold. Empirical evidence for such adaptive manipulation exists especially for haplo-diploid insects. Here the mechanism by which the manipulation is achieved is evident: selective admission by the female of stored sperm to the maturing ova. For “our kind of animals”, the mammals, both the theoretical models and the empirical evidence are considered much more ambiguous. The primates offer an illustrative case. On the one hand there is the ‘male quality hypothesis’. It predicts that females will shift the sex ratio of their offspring depending on their social position and their condition, provided that status and condition infleunce the competitive potential of their sons. this should apply when the mating system is one of male contest competition, allowing dominant males to monpolize access to fertile females. In this case dominant females should shift the sex ratio towards more sons. There is another hypothesis, a modified version of the ‘local resource competition hypothesis’. It predicts that in female-bonded species, where males tend to emigrate and where natal female kin form within-group power blocks to raise their inclusive fitness, dominant females should shift towards producing more daughters and subordinate females towards more sons. However, empirical data are confilicting: biases towards either side have beenr eported for one and the same species. This has cast doubts whether adaptive sex ratio manipulation exists and all in these and other tax. Forone thing, so-called significant deviations from a 50-50 ratio could be the seletively reported, accidental extremes, from a population with a random 50-50 distribution. Recently Van Schaik and Hrdy (1991) have tried to integrate the earlier ‘male quality’ (MQ) and ‘local resource competition’ (LRC) hypotheses. Their model supposes that the strength of the local resource competition, experienced in a population, is the crucial factor. It predicts that dominant females will choose the ‘male quality’ option when LRC is low. They will choose to ‘strengthen the female power block’ option when LRC is high. The model indeed appears to reconcile the seemingly contradictory results that have been obtained from some cercopithecoid primates. Similarly, recent studies on the influence of social variables in human females, such as dominance status and being socially challenged, on the sex ratio of their offspring seemed contradictory. Here also the van Schaik and Hrdy model offiers a possiblity to accomodate these results. Even though the mechanism(s) by which psychological and conditional factos might affect the sex of mammalian offspring remain largely a matter of speculation, there is increasing evidence that adaptive sex ratio manipulation is a real phenomenon alsoin mammals, that can be accounted for in evolutionary terms.

Asymmetries in grooming interactions between juvenile and adult female wedge-capped capuchin monkeys

Abstract. Motivation for participation in social grooming differs for juvenile and adult primates in female-bonded social systems; juveniles attempt to develop social relationships as a means of integration into the group, whereas adults attempt to maintain relationships with other adults. Juveniles should therefore seek grooming relationships with adults but adults should be relatively uninterested. The local resource competition hypothesis predicts that grooming relationships between adults and juveniles should occur primarily within matrilines and should be negatively correlated with aggressive interactions. Alternatively, grooming relationships may depend on the ability of juveniles to provide additional services, such as assistance in aggressive interactions and allomaternal care. Grooming interactions of juvenile and adult female wedge-capped capuchin monkeys, Cebus olivaceus, were predictably asymmetric. Juveniles approached and groomed more than adults, whereas adults terminated grooming sessions more and often failed to reciprocate grooming. Juvenile-adult grooming was inconsistent with patterns predicted by the local resource competition model. Dyads of closely related females did not groom more than unrelated dyads and there was no relationship between rates of grooming and aggression. Frequency and duration of grooming was significantly correlated with frequency of allomaternal care given by juveniles. Frequency of grooming also correlated with rates of alliance formation by adults and juveniles. Juveniles provided agonistic support during alliances twice as often as adults supported juveniles. The structure of grooming indicated that juveniles were the primary beneficiaries of grooming relationships with adults. Juveniles provided grooming services to adults and provided additional benefits of allomaternal care and agonistic support during aggression. In return, juveniles were integrated into the adult social structure in the group.

Differential Dispersal, Local Resource Competition, and Sex Ratio Variation in Birds

Local resource competition arising through differential natal dispersal of daughters and sons may be an overlooked selection pressure on progeny sex ratio variation in birds. Passerines have predominantly daughter-biased natal dispersal; anseriforms have predominantly son-biased natal dispersal. Therefore, the local resource competition hypothesis predicts that passerines will have daughter-biased sex ratios and anseriforms will have son-biased sex ratios. These predictions are supported by observations of between-order variation in progeny sex ratios. Across latitude and within season, variation in progeny sex ratios of given species may also reflect selection through local resource competition arising in association with variation in relative son and daughter philopatry.

Secondary sex ratio and maternal dominance rank among wild yellow baboons (Papio cynocephalus) of Mikumi National Park, Tanzania.

The Trivers and Willard model predicts that offspring of dominant mothers will be biased toward males and offspring of subordinate mothers towards females, whereas a local-resource-competition hypothesis predicts the reverse. Available data bearing upon these alternative predictions are inconsistent. It has been suggested that the local-resource-competition hypothesis will predominate when resources are scarce, and the Trivers-Willard hypothesis when resources are abundant. The relationship between maternal dominance and secondary sex ratio for 214 offspring of 61 females was examined using four troops of wild yellow baboons living in Mikumi National Park, where the population was increasing in a resource-rich habitat. For all troops combined, no significant relationship was found between offspring sex ratio and maternal rank. The four troops separately showed inconsistent trends, and in no case did the relationship reach conventional levels of statistical significance. In contrast, the local resource-competition hypothesis was supported by a study of yellow baboons done with one troop and 80 offspring in Amboseli National Park, where a massive population decline had occurred. The contrasting Amboseli and Mikumi results may be due to differences in resource-competition at the two study sites, or to stochastic variation. © 1992 Wiley-Liss, Inc.

Fragile Sons or Harassed Daughters?

In most mammals, juvenile males tend to be more vulnerable to starvation than females and consequently experience a higher mortality. This has been attributed to selection on high male growth rates in response to strong intrasexual competition. Among primates, by contrast, it has been noted that females tend to be less viable as juveniles. This has been attributed to a greater ability of juvenile males to contest for food. An alternative explanation is that there is local resource competition, and adults of the resident sex (usually females) attempt to limit the recruitment of unrelated immatures of the same sex by harassing them. These ideas are not mutually exclusive. A set of predictions from these three hypotheses was derived for two social systems and two levels of food supply. They were tested using estimates of juvenile mortality and juvenile sex ratios of non-human primates based on over 40 data sets drawn from the literature. The results indicate that the local resource competition hypothesis provides the best explanation for the observed patterns in differential juvenile mortality in primates. The contrast between the findings for primates and those for other taxa is attributed to the low growth rate of immatures and the widespread occurrence of conditions conducive to local resource competition in primates.

Sex Ratio in Primates: A Test of the Local Resource Competition Hypothesis

Sex Ratio in Primates: A Test of the Local Resource Competition Hypothesis

Social rank and sex ratio of captive long‐tailed macaque females (Macaca fascicularis)

Variation in birth sex ratios in primates can be accounted for by two hypotheses: the local resource competition hypothesis [Silk: American Naturalist 121:56-66, 1983] and the hypothesis of Trivers & Willard [Science 179:90-92, 1973] concerning the maternal effect on the quality of a male. We examined the effects of female dominance rank on aspects of reproduction in three well-established captive groups of long-tailed macaques (Macaca fascicularis). High-ranking females produced a higher proportion of sons than low-ranking females, and factors other than rank did not have significant effects on birth sex ratios. Interbirth intervals following daughters were longer than those following sons, but they were independent of the mother's rank. The sons of high-ranking mothers had better survival prospects than sons of low-ranking mothers in some of the groups; no such difference was found for daughters. Overall, there was no sex difference in survival up to 5 years of age. These results support the Trivers-Willard hypothesis rather than the local resource competition hypothesis. An analysis of interbirth intervals suggested that the deviation in birth sex ratio is already established at conception.