Local Resource Competition Hypothesis Research Articles

Sex Investment Ratio and Split Sex Ratio in the Fission-Performing Ant Cataglyphis tartessica

Inclusive fitness theory predicts that the sex investment ratio should be female biased in social insects with haplodiploidy-generated relatedness asymmetry (between females and males and also among females). However, this ratio should become male biased if related females are competing with each other for resources, as it is predicted by local resource competition hypothesis. This specific situation is expected to occur in ant species that reproduce by fission, where young queens leave their mother colonies on foot to found daughter colonies. We tested this idea using the ant Cataglyphis tartessica in Donana National Park (Spain). This species is monogynous (colonies headed by a single queen), monandrous (queen mate with only one male), and reproduces by fission. Given the similarity between foraging and colony fission distances in this species, a strong local competition for resources should be expected between mother and daughter colonies. In this study, we considered investment in females the sum of the investment in gynes (virgin queens) plus the investment in the worker force that leaves with them to found a new colony. Our results show a male-biased sex investment ratio in C. tartessica of 1:8 (numerical ratio gynes/males 1:56), suggesting queen control over sexual production. In addition, we found that, during its reproductive period, this species exhibits split sex (gyne-male) production as well as split male-worker production, without any evidence of variation in relatedness asymmetry between its colonies, at the population level. An interaction between local resources competition and colony size could explain our results.

Different maternal investment strategies for male and female calves in a polygynous mammal.

Maternal effects occur when the phenotype of the mother influences that of the young to the detriment of her survival, growth or fitness. The investment of the mother can be affected by maternal body condition and/or experience. Trivers–Willard Hypothesis (TWH) and Local Resource Competition Hypothesis (LRCH) are the main hypotheses used to explain bias in birth sex-ratios in mammals, as well as for sex-biased maternal investment. Both hypotheses suggest that a different amount of investment must be expected according to the sex of the young. However, recent studies suggest that these differences are not in quantity but in the strategies: mechanisms and objectives may differ for each sex. We studied how maternal characteristics (age, body mass, body condition, and dominance status) influence relevant aspects of the birth and early growth of the calf (birth date, birth body mass, body mass at weaning, and body condition at weaning) separately for each sex; and how that investment is mediated by milk production and composition (lactose, fat, and protein). One hundred eighty-eight newborns from 75 captive red deer hinds aged from 2 to 19 years were analyzed. The main differential investment observed was related to birth date: when producing a female, hinds give birth earlier in the season only if they have a good body condition; however, when gestating a male it is the older hinds those which deliver earlier. Subsequently, milk production and composition are correlated with birth body mass in female calves, but to weaning body mass in males. Thus, only hind body mass affects the weaning body mass of female calves, compared with age and hind body mass in males. These results suggest that while TWH fits the maternal investment strategy found for male calves, it is LRCH which correlates with the maternal investment patterns observed for females.

Sex allocation and secondary sex ratio in Cuban boa (Chilabothrus angulifer): mother’s body size affects the ratio between sons and daughters

Secondary sex ratios of animals with genetically determined sex may considerably deviate from equality. These deviations may be attributed to several proximate and ultimate factors. Sex ratio theory explains some of them as strategic decisions of mothers improving their fitness by selective investment in sons or daughters, e.g. local resource competition hypothesis (LRC) suggests that philopatric females tend to produce litters with male-biased sex ratios to avoid future competition with their daughters. Until now, only little attention has been paid to examine predictions of sex ratio theory in snakes possessing genetic sex determination and exhibiting large variance in allocation of maternal investment. Cuban boa is an endemic viviparous snake producing large-bodied newborns (∼200g). Extremely high maternal investment in each offspring increases importance of sex allocation. In a captive colony, we collected breeding records of 42 mothers, 62 litters and 306 newborns and examined secondary sex ratios (SR) and sexual size dimorphism (SSD) of newborns. None of the examined morphometric traits of neonates appeared sexually dimorphic. The sex ratio was slightly male biased (174 males versus 132 females) and litter sex ratio significantly decreased with female snout-vent length. We interpret this relationship as an additional support for LRC as competition between mothers and daughters increases with similarity of body sizes between competing snakes.

Costs of Rearing the Wrong Sex: Cross-Fostering to Manipulate Offspring Sex in Tammar Wallabies.

Sex allocation theory assumes that offspring sex (son vs. daughter) has consequences for maternal fitness. The most compelling experiment to test this theory would involve manipulating offspring sex and measuring the fitness consequences of having the “wrong” sex. Unfortunately, the logistical challenges of such an experiment limit its application. In tammar wallabies (Macropus eugenii), previous evidence suggests that mothers in good body condition are more likely to produce sons compared to mothers in poor condition, in support of the Trivers-Willard Hypothesis (TW) of condition-dependent sex allocation. More recently, we have found in our population of tammar wallabies that females with seemingly poor access to resources (based on condition loss over the dry summer) are more likely to produce sons, consistent with predictions from the Local Resource Competition (LRC) hypothesis, which proposes that production of sons or daughters is driven by the level of potential competition between mothers and philopatric daughters. We conducted a cross-fostering experiment in free-ranging tammar wallabies to disassociate the effects of rearing and birthing offspring of each sex. This allowed us to test the prediction of the LRC hypothesis that rearing daughters reduces the future direct fitness of mothers post-weaning and the prediction of the TW hypothesis that rearing sons requires more energy during lactation. Overall, we found limited costs to the mother of rearing the “wrong” sex, with switching of offspring sex only reducing the likelihood of a mother having a pouch young the following year. Thus, we found some support for both hypotheses in that rearing an unexpected son or an unexpected daughter both lead to reduced future maternal fitness. The study suggests that there may be context-specific costs associated with rearing the “wrong” sex.

Sex ratio variation in a eusocial mammal, the Damaraland mole‐rat, Fukomys damarensis

AbstractSex allocation theory predicts that mothers benefit from adjusting the sex ratio of their offspring in relation to their offspring's future reproductive success. In cooperative breeders, parents are expected to bias the sex ratio in relation to their current need for help and the benefit received from helpers of each sex as proposed by the local resource enhancement (LRE) and helper repayment hypotheses (HR). Consequently, as group size increases, sex ratios are expected to be biased towards the sex that is more likely to disperse to avoid competition as proposed in the local resource competition hypothesis (LRC). The current study aimed to investigate helper effects on breeder fecundity and offspring sex ratio adjustments in a eusocial mammal the Damaraland mole‐rat Fukomys damarensis. Both sexes equally contribute to helping in this species, but breeding dispersal is male biased. We found no evidence for helper effects on maternal body mass and litter size. Offspring sex ratio was not affected by maternal mass or litter size. However, the probability of male pups to be born increased with the number of female helpers and decreased with the number of male helpers. In addition, the number of male pups in a litter increased significantly with the number of female but not male helpers, while no helper effects were apparent for the number of female pups born. Hence, our data suggest that the mechanisms underlying the LRE, HR and LRC may operate simultaneously in the study species.

Facultative Sex Allocation and Sex‐Specific Offspring Survival in Barrow's Goldeneyes

AbstractSex allocation theory predicts that females should bias their reproductive investment towards the sex generating the greatest fitness returns. The fitness of male offspring is often more dependent upon maternal investment, and therefore, high‐quality mothers should invest in sons. However, the local resource competition hypothesis postulates that when offspring quality is determined by maternal quality or when nest site and maternal quality are related, high‐quality females should invest in the philopatric sex. Waterfowl – showing male‐biased size dimorphism but female‐biased philopatry – are ideal for differentiating between these alternatives. We utilized molecular sexing methods and high‐resolution maternity tests to study the occurrence and fitness consequences of facultative sex allocation in Barrow's goldeneyes (Bucephala islandica). We determined how female structural size, body condition, nest‐site safety and timing of reproduction affected sex allocation and offspring survival. We found that the overall sex ratio was unbiased, but in line with the local resource competition hypothesis, larger females produced female‐biased broods and their broods survived better than those of smaller females. This bias occurred despite male offspring being larger and tending to have lower post‐hatching survival. The species shows strong female breeding territoriality, so the benefit of inheriting maternal quality by philopatric daughters may exceed the potential mating benefit for sons of high‐quality females.

Influence of social dominance on the secondary sex ratio and factors affecting hierarchy in Holstein dairy cows

In several studies of different species, the proportions of male and female offspring did not match the statistical assumption of 50:50. Aside from environmental and genetic factors, biological parameters were found to influence the sex ratio. Three hypotheses (Trivers-Willard, advantaged daughter, and the local resource competition hypothesis) have been proposed, stating that the sex of progeny might be influenced by sex-biased maternal investment in the raising of offspring. The results of existing studies are still conflicting, even within studies on the same species. The aim of the present study was to investigate whether the dominance status of cows influences the secondary sex ratio. A herd of lactating cows was observed directly for 126h, and a dominance index (DI) was established for 71 individuals (aged between 2.6 and 9.8 yr) for which at least 8 encounters had been observed to categorize individuals as subordinate, intermediate, or dominant. Furthermore, the number and sex of calves were assessed, and further individual characteristics of the animals (e.g., age, milk yield, body condition, and lameness scores) were analyzed for interrelationships with dominance status as well as the influence of the DI on the secondary sex ratio. The results indicated that the DI was independent of age, but a negative correlation was found between DI and lameness, whereas a positive correlation was shown for DI and body condition score. In general, more dominant animals had lower lameness scores and better body conditions. No sex ratio bias was found for primi- or for pluriparous animals and no seasonal influences were detected. The proportion of male offspring was significantly correlated with the DI of lactating cows, where animals with a higher social rank produced a lower proportion of male progeny. The pairwise comparison between dominant and subordinate lactating cows revealed a significant difference in the sex ratios of progeny, with subordinate animals having more male calves. These results indicate that the maternal dominance rank was associated with the secondary sex ratio in cattle. These observations are generally in agreement with the advantaged daughter hypothesis.

Cooperatively breeding carrion crows adjust offspring sex ratio according to group composition

In cooperatively breeding species, parents should bias offspring sex ratio towards the philopatric sex to obtain new helpers (helper repayment hypothesis). However, philopatric offspring might increase within-group competition for resources (local resource competition hypothesis), diluting the benefits of helper acquisition. Furthermore, benefits of offspring sex bias on parents’ fitness may depend on different costs of production and/or different breeding opportunities of sons and daughters. Because of these counteracting factors, strategies of offspring sex allocation in cooperative species are often difficult to investigate. In carrion crows Corvus corone corone in northern Spain, sons are more philopatric and more helpful at the nest than daughters, which disperse earlier and have higher chances to find a breeding vacancy. Consistent with the helper repayment hypothesis, we found that crows fledged more sons in groups short of subordinate males than in groups with sufficient helper contingent, where daughters were preferred. Crow females also proved able to bias primary sex ratio, allocating offspring sex along the hatching sequence in a way that provided the highest fledging probability to sons in the first breeding attempt and to daughters in the following ones. The higher cost of producing male offspring may explain this pattern, with breeding females shifting to the cheapest sex (female) as a response to the costs generated by previous reproductive attempts. Our results suggest complex adjustments of offspring sex ratio that allowed crows to maximize the value of daughters and sons.

Alternatives to the Grandmother Hypothesis

We conducted a meta-analysis of 17 studies that tested for an association between grandparental survival and grandchild survival in patrilineal populations. Using two different methodologies, we found that the survival of the maternal grandmother and grandfather, but not the paternal grandmother and grandfather, was associated with decreased grandoffspring mortality. These results are consistent with the findings of psychological studies in developed countries (Coall and Hertwig Behavioral and Brain Sciences 33:1-59, 2010). When tested against the predictions of five hypotheses (confidence of paternity; grandmothering, kin proximity, grandparental senescence, and local resource competition), our meta-analysis results are most in line with the local resource competition hypothesis. In patrilineal and predominantly patrilocal societies, the grandparents who are most likely to live with the grandchildren have a less beneficial association than those who do not. We consider the extent to which these results may be influenced by the methodological limitations of the source studies, including the use of retrospective designs and inadequate controls for confounding variables such as wealth.

Female philopatry and male-biased dispersal in a direct-developing salamander, Plethodon cinereus

The local resource competition hypothesis and the local mate competition hypothesis were developed based on avian and mammalian systems to explain sex-biased dispersal. Most avian species show a female bias in dispersal, ostensibly due to resource defence, and most mammals show a male bias, ostensibly due to male-male competition. These findings confound phylogeny with mating strategy; little is known about sex-biased dispersal in other taxa. Resource defence and male-male competition are both intense in Plethodon cinereus, a direct-developing salamander, so we tested whether sex-biased dispersal in this amphibian is consistent with the local resource competition hypothesis (female-biased) or the local mate competition hypothesis (male-biased). Using fine-scale genetic spatial autocorrelation analyses, we found that females were philopatric, showing significant positive genetic structure in the shortest distance classes, with stronger patterns apparent when only territorial females were tested. Males showed no spatial genetic structure over the shortest distances. Mark-recapture observations of P. cinereus over 5 years were consistent with the genetic data: males dispersed farther than females during natal dispersal and 44% of females were recaptured within 1 m of their juvenile locations. We conclude that, in this population of a direct-developing amphibian, females are philopatric and dispersal is male-biased, consistent with the local mate competition hypothesis.

Sex Ratio at Birth and Age-Reversed Dominance among Female Varecia

Analysis of 283 offspring born at the Duke University Primate Center show that young female ruffed lemurs, Varecia variegata, produce significantly more daughters, whereas old mothers produce more sons than expected. Data are compared to 3 hypotheses for sex ratio bias: the Trivers-Willard hypothesis which predicts that dominant females produce more males, the local resource competition (LRC) hypothesis which predicts that subordinate females produce more males, and the local resource enhancement (LRE) extension of the LRC hypothesis that females produce the sex that provides later cooperative benefits. In the case of Varecia, this may include alloparenting or cooperative breeding. The results are more consistent with the LRC model. However, observations of 8 mother-daughter relationships show that female dominance rank in free-ranging Varecia groups is age reversed, with daughters aggressively outranking their mothers. Daughters born into the group were not beneficial as future supporters in within-group intermatriline competition as assumed by the LRE model, or as subordinate alloparents, cooperative breeders or aids in territorial defense. Both sex ratio and ranking observations are consistent, however, with the hypothesis that mothers produce daughters when they are young and able to invade elsewhere and sons as they age and are less able to do so. This is supported by a single observation of a mother dispersing which resulted in her daughter inheriting the natal territory. These data are supportive of the LRE model with female alliances and cooperative breeding among dispersed females.

Local resource competition and sex ratio in the ant Cataglyphis cursor

The local resource competition (LRC) hypothesis predicts that wherever philopatric offspring compete for resources with their mothers, offspring sex ratios should be biased in favor of the dispersing sex. In ants, LRC is typically found in polygynous (multiple queen) species where foundation of new nests occurs by budding, which results in a strong population structure and a male-biased population-wide sex ratio. However, under polygyny, the effect of LRC on sex allocation is often blurred by the effect of lowered relatedness asymmetries among colony members. Moreover, environmental factors, such as the availability of resources, have also been shown to deeply influence sex ratio in ants. We investigated sex allocation in the monogynous (single queen) ant Cataglyphis cursor, a species where colonies reproduce by budding and both male and female sexuals are produced through parthenogenesis, so that between-colony variations in relatedness asymmetries should be reduced. Our results show that sex allocation in C. cursor is highly male biased both at the colony and population levels. Genetic analyses indicate a significant isolation-by-distance in the study population, consistent with limited dispersal of females. As expected from asexual reproduction, only weak variations in relatedness asymmetry of workers toward sexual offspring occur across colonies, and they are not associated with colony sex ratio. Inconsistent with the predictions of the resource availability hypothesis, the male bias significantly increases with colony size, and investment in males, but not in females, is positively correlated with total investment in sexuals. Overall, our results are consistent with the predictions of the LRC hypothesis to account for sex ratio variation in this species. Copyright 2006.

Adaptive sex allocation in relation to life‐history in the common brushtail possum, Trichosurus vulpecula

Summary Maternal control of offspring sex remains a contentious topic in the evolutionary and behavioural ecology of vertebrates. Two models of adaptive sex allocation − the Trivers–Willard (TW) and local resource competition (LRC) hypotheses − have been supported in some studies, but in many other cases the forces shaping offspring sex ratios are unclear. One reason that past studies may not have provided clear support for either model is that the two processes interact, producing complex patterns of sex allocation in relation to the life histories of individual mothers. The TW hypothesis predicts that in dimorphic, polygynous mammals, females in good condition should be more likely to produce male offspring. The LRC hypothesis predicts an independent effect of age on the offspring sex ratio; females should be more likely to produce sons early in their reproductive lives and daughters later, in order to minimize the duration and reproductive cost of mother–daughter competition. We tested these two predictions in a study of sex allocation in the common brushtail possum, Trichosurus vulpecula. Offspring sex was strongly dependent on maternal age: females breeding for the first time were likely to produce sons, and at subsequent breeding attempts the sex ratio of offspring was slightly biased towards females. After the first breeding season, sex of offspring was influenced more strongly by body condition than by age, and females in good condition were more likely to have sons. Individual females that showed a between‐year gain in condition were also more likely to produce a male in the subsequent year. Body mass of male offspring at 5 months was positively correlated with maternal condition. No relationship was found between maternal condition and mass of female offspring. We conclude that patterns of change in sex allocation in relation to individual life histories of female common brushtail possums can be explained by the interaction of TW and LRC effects.

A TEST OF SEX-RATIO BIASING IN THE WHITE-THROATED MAGPIE-JAY, A COOPERATIVE BREEDER WITH FEMALE HELPERS

Abstract Recent studies suggest that many bird species are able to vary the sex ratio of their young. Among cooperative breeders, in which adult helpers aid the genetic parents in the rearing of young, two models have been invoked to explain this variation. According to the local resource competition hypothesis, parents should bias offspring sex ratios toward the dispersing sex in order to minimize competition for local resources. In contrast, the local resource enhancement (or “production of helpers”) hypothesis states that parents should overproduce the nondispersing sex if the presence of relatives enhances reproductive success. I examined these models in a Costa Rican population of White-throated Magpie-Jays (Calocitta formosa), a cooperatively breeding corvid with female helpers. Using DNA microsatellite analysis, I sexed 135 offspring from 38 broods and 14 groups over 3 years. I tested for variation in offspring sex ratio at the population level and as a function of social group, helper number, bree...

Recruitment sex ratios in gray-tailed voles (Microtus canicaudus) in response to density, sex ratio, and season

We tested predictions associated with three widely used hypotheses for facultative sex-ratio adjustment of vertebrates using eight enclosed populations of gray-tailed voles, Microtus canicaudus. These were (i) the population sex ratio hypothesis, which predicts that recruitment sex ratios should oppose adult sex-ratio skews, (ii) the local resource competition hypothesis, which predicts female-biased recruitment at low adult population density and male-biased recruitment at high population density, and (iii) the first cohort advantage hypothesis, which predicts that recruitment sex ratios should be female biased in the spring and male biased in the autumn. We monitored naturally increasing population densities with approximately equal adult sex ratios through the spring and summer and manipulated adult sex ratios in the autumn and measured subsequent sex ratios of recruits. We did not observe any significant sex-ratio adjustment in response to adult sex ratio or high population density; we did detect an influence of time within the breeding season, with more female offspring observed in the spring and more male offspring observed in the autumn. Significant seasonal increases in recruitment sex ratios indicate the capacity of female gray-tailed voles to manipulate their offspring sex ratios and suggest seasonal variation in the relative reproductive value of male and female offspring to be a regular phenomenon.

Male-biased sex ratios in New Zealand fur seal pups relative to environmental variation

A number of models have been proposed to provide adaptive explanations of sex-ratio variation in mammals. Two models have been applied commonly to primates and ungulates with varying success—the Trivers-Willard (TW) hypothesis, and the local resource competition (LRC) hypothesis. For polygynous, sexually dimorphic mammals, where males are larger and disperse more readily, these models predict opposite outcomes of sex-ratio adjustment within the same environmental context (high-resource years: TW—more sons; LRC—more daughters). However, many of the predictions of these two models can vary depending on factors influencing resource availability, such as environmental stochasticity, resource predictability, and population density. The New Zealand fur seal (Arctocephalus forsteri) is a polygynous mammal showing marked sexual dimorphism (larger males), with higher variation in male reproductive success expected. We provide clear evidence of male-biased sex ratios from a large sample of A. forsteri pups captured around South Island, New Zealand during 1996/1998, even after accounting for a sex bias in capture probability. The extent of the bias depended upon year and, in 1998, strong climatic perturbations (El Nino/Southern Oscillation, ENSO) probably reduced food availability. Significant male-biased sex ratios were found in all years; however, there was a significant decline in the male bias in 1998. There was no relationship between sex ratio and population density. We suggest that the sex-ratio bias resulted from the production of relatively more male pups. Under the density-independent scenario, the strong male bias in A. forsteri sex ratios is support for the TW model within an environment of high resource predictability. We suggest that some plasticity in the determination of pup sex among years is a mechanism by which A. forsteri females in New Zealand, and perhaps other otariid seals, can maximise fitness benefits when living in regions of high, yet apparently predictable, environmental variability. We also suggest that much of the inconsistency in the reported sex ratios for otariid seals results from the complex interaction of population density and environmental stochasticity influencing relative food availability over time.

OFFSPRING SEX RATIO VARIATION IN THE EUROPEAN BADGER, MELES MELES

The sex ratio of post-emergence offspring of the European badger Meles meles in Wytham Woods, Oxfordshire, UK did not differ significantly from 50% between 1987 and 2001. Insofar as male offspring are more costly to rear successfully, mothers optimized cub productivity with regard to the level of resources that they could invest. When females had a relatively high index of body condition, they implanted early; when implantation was early, the cub sex ratio was male biased. Additionally, years with a male-biased cub sex ratio coincided with a significantly larger cub cohort. These results do not support the local resource competition hypothesis, which proposes that in years when female body condition is poor, females should lower competition for local resources by producing the dispersing sex, i.e., males. The potential mechanisms by which offspring sex ratios may be skewed are discussed. Overall, it appears that bias in the cub sex ratio may result from a combination of adaptive parental strategies, differential infanticide by individuals other than the mother, and differential fetal mortality between the sexes. Corresponding Editor: J. M. Fryxell.

Costs and benefits of female-biased natal philopatry in the common goldeneye

Sex-biased natal dispersal in long-lived species may result in interactions between parents and mature young of the philopatric sex. To investigate the evolutionary basis of natal philopatry in a noncooperative species, the common goldeneye Bucephala clangula, we studied possible costs and benefits of simultaneous breeding of females and philopatric daughters. We did not find any fitness consequences of a daughter's breeding on their mother's breeding in terms of nest-site selection, body weight, clutch size, hatching date, or hatching success. Our results, therefore, did not support the assumption of the local resource competition hypothesis, that the natally philopatric sex should be more costly to a breeding parent. As possible benefits for daughters returning to their natal area, we tested inheritance of nest sites from mothers and explored whether daughters utilize the presence of their mother by parasitically sneaking into her mother's nest. Daughters' nest-site selection was not associated with the presence of their mothers. A comparison between daughters and control females revealed that daughters chose their nest site closer to their natal nest than expected by nest-site availability alone. Daughters could not expect to inherit a nest site from their mother, and we did not find other indications of cooperation between relatives either. The mother's clutch size did not increase in the year breeding with the daughter, indicating daughters do not parasitize their mother's nest. We suggest that benefits such as decreased nest predation risk associated with nesting close to the natal nest site may be important in the natal philopatric behavior of the species.

Progeny sex ratio variation in ungulates: maternal age meets environmental perturbation of demograpgy

Many adaptive theories have been proposed for explaining variation in progeny sex ratio (PSR) of vertebrates (Hardy 1997). Of these, two opposing hypotheses are commonly used for ungulates: the Trivers and Willard hypothesis (TWH, Trivers and Willard 1973) and the local resource competition hypothesis (LRC, Silk 1983, Caley and Nudds 1987). Trivers and Willard (1973) proposed that when adaptive variance in reproductive success differs between the sexes, mothers should invest more in the sex with the higher variance and, therefore, opt to produce this sex when conditions enable the extra investment. Because most ungulates are polygynous with reproductive success varying considerably between males, this hypothesis is invoked if a male-biased PSR is associated with favorable conditions (e.g. Clutton-Brock et al. 1988 and others mentioned in Table 1). LRC predicts that the gender less apt to disperse imposes overall greater competition on the mother. To minimize this competition, mothers should produce the non-dispersing gender when conditions are favorable. Because in ungulates sons tend to disperse earlier than daughters, the LRC hypothesis is invoked if favorable conditions are associated with a female biased PSR (e.g. Hewison and Gaillard 1996 and others mentioned in Table 1). A separate adaptive theory for each of two opposing situations makes any results explainable on an ad hoc basis. This has caused several critics to question the findings (Williams 1979, Festa-Bianchet 1996, and others) and attribute them to random occurrences. However, as long as the pattern remains consistent within a species, the findings cannot be challenged and could be explained by differences in the biology of the various species (Kojola 1998). Nonetheless, I documented nine cases where the pattern was inconsistent within a species (Table 1). As pointed out by Hewison and Gaillard (1999), the abundance of intraspecific inconsistencies requires some explanation. While Type-I-error (Festa-Bianchet 1996) or heterogeneity within species (Hewison et al. 1999) are possible explanations for these inconsistencies, they appear improbable to me. It is my opinion that in many cases PSR is determined by the age of the mother and if the age of the mother is unaccounted for spurious correlations may be produced between PSR and other factors that have an age-dependent impact on the population. I demonstrate this effect using a Leslie matrix model (Appendix I) based on the dynamics of a reintroduced population of Asiatic wild ass exhibiting PSR that is dependent on maternal age (Saltz and Rubenstein 1995).

Parental Correlates of Offspring Sex Ratio in Eurasian Oystercatchers

AbstractWe 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 clutches produced by older females is speculative because the costs and benefits of dispersing versus philopatric offspring to parents and offspring are largely unknown.