Intention versus behaviour in parental sex preferences among the Mukogodo of Kenya

The relationship between parents' stated sex preferences for children and actual parental behaviour towards sons and daughters is examined among the Mukogodo, a group of traditional pastoralists in rural Kenya. Although their cultural values are male-centered and they tend to express a preference for sons, Mukogodo parents actually appear to be more solicitous of daughters, and the Mukogodo have a strongly female-biased childhood sex ratio. Studies of stated sex preferences should therefore be coupled with attempts to assess actual parental investment in sons and daughters.     

Low serum vitamin A mothers breastfeed daughters more often than sons in drought-ridden northern Kenya: a test of the Trivers–Willard hypothesis

The Trivers–Willard hypothesis predicts that natural selection should favor unequal parental investment between daughters and sons based upon maternal condition and offspring reproductive potential. Specifically, it predicts that mothers in good condition should increase investment toward sons, while mothers in poor condition should favor daughters. Previous tests of the hypothesis in human populations overwhelmingly focused on economic resources as maternal condition indicators. We test the Trivers–Willard hypothesis using maternal nutrition—energy and vitamin A status representing macro- and micronutrition, respectively—as the indicator for maternal condition, with breastfeeding frequency recalls serving as the indicator for parental investment. Data from exclusively breastfeeding mothers (n=83) in drought-ridden Ariaal agropastoral villages of northern Kenya were used to test the hypothesis that mothers in poor condition will breastfeed daughters more frequently than sons. Poor condition was defined as having a body mass index <18.5 or serum retinol (vitamin A) concentration <1.05 µmol/l. A linear regression model was applied using breastfeeding frequency as the dependent variable and respective maternal condition, infant's sex, and the maternal condition–infant's sex interaction as the predictors, controlling for covariates. Results supported the hypothesis only in the vitamin A model which predicts that low-vitamin-A mothers breastfeed daughters significantly more frequently than sons (11 vs. 6 times/day), while vitamin-A-sufficient mothers breastfeed daughters and sons equivalently (9 times). These results indicate that maternal nutritional status, particularly micronutrient status, can contribute to the investigation of the evolutionary hypothesis of sex-biased parental investment.     

The Trivers–Willard hypothesis of parental investment: No effect in the contemporary United States

The Trivers–Willard hypothesis (TWH) predicts that parents will bias their sex ratio toward sons when in good condition and toward daughters when in poor condition. Many human studies have tested the related hypothesis that parents' bias allocation of resources to existing sons and daughters according to the same principle. The present study used time diary and self-report data from the parents of 3200 children in the US to test the hypothesis that as status increases, parents will allocate more resources to sons vs. daughters. It finds no evidence that higher-status parents invest more in sons or that lower status parents invest more in daughters. This finding illustrates the specificity of situations in which the TWH effects should be expected. Only certain types of parental investment — such as protection and a bias in the sex ratio — may have been selected to vary according to parental condition. Optimal allocation of resources after the child is born, however, is achieved not by the simple bias predicted by the TWH, but by allocating resources among offspring in ways that yield the largest marginal inclusive fitness gains.     

No evidence for adjustment of sex allocation in relation to paternal ornamentation and paternity in barn swallows

Sex allocation theory predicts that parents should adjust investment in sons and daughters according to relative fitness of differently sexed offspring. In species with female preference for highly ornamented males, one advantage potentially accruing to parents from investing more in sons of the most ornamented males is that male offspring will inherit characters ensuring sexual attractiveness or high-quality genes, if ornaments honestly reveal male genetic quality. Furthermore, in species where extra-pair fertilizations occur, offspring sired by an extra-pair male are expected to more frequently be male than those of the legitimate male if the latter is of lower quality than the extra-pair male. We investigated adjustment of sex ratio of offspring in relation to ornamentation of the extra-pair and the social mate of females by direct manipulation of tails of male barn swallows Hirundo rustica . Molecular sexing of the offspring was performed using the W chromosome-linked avian chromo-helicase-DNA-binding protein (CHD) gene while paternity assessment was conducted by typing of hypervariable microsatellite loci. Extra-pair offspring sex ratio was not affected by ornamentation of their biological fathers relative to the experimental ornamentation of the parental male. Experimental ornamentation of the parental males did not affect the sex ratio of nestlings in their broods. Female barn swallows might be unable to bias offspring sex ratio at hatching according to the quality of the biological father. Alternatively, fitness benefits in terms of sexual attractiveness of sons might be balanced by the cost of compensating for little parental care provided by highly ornamented parental males, if sons are more costly to rear than daughters, or the advantage of producing more daughters, if males with large ornaments contribute differentially more to the viability of daughters than sons.     

In poor families, mothers' milk is richer for daughters than sons: A test of Trivers-Willard hypothesis in agropastoral settlements in Northern Kenya

The Trivers–Willard hypothesis predicts the unequal parental investment between daughters and sons, depending on maternal condition and offspring reproductive potential. Specifically, in polygynous populations where males have higher reproductive variance than females, it predicts that mothers in good condition will invest more in sons, whereas mothers in poor condition will invest more in daughters. Previous studies testing this hypothesis focused on behavioral investment, whereas few examined biological investment. This study investigates the Trivers–Willard hypothesis on both behavioral and biological parental investment by examining breastfeeding frequencies and breast milk fat concentrations. Data from exclusively breastfeeding mothers in Northern Kenya were used to test hypotheses: Economically sufficient mothers will breastfeed sons more frequently than daughters, whereas poor mothers will breastfeed daughters more frequently than sons, and economically sufficient mothers will produce breast milk with higher fat concentration for sons than daughters, whereas poor mothers will produce breast milk with higher fat concentration for daughters than sons. Linear regression models were applied, using breastfeeding frequency or log‐transformed milk fat as the dependent variable, and offspring's sex (son = 1/daughter = 0), socioeconomic status (higher = 1/lower = 0), and the sex‐wealth interaction as the predictors, controlling for covariates. Our results only supported the milk fat hypothesis: infant's sex and socioeconomic status interacted (P = 0.014, n = 72) in their relation with milk fat concentration. The model estimated that economically sufficient mothers produced richer milk for sons than daughters (2.8 vs. 0.6 gm/dl) while poor mothers produced richer milk for daughters than sons (2.6 vs. 2.3 gm/dl). Further research on milk constituents in relation to offspring's sex is warranted. Am J Phys Anthropol , 2012. © 2012 Wiley Periodicals, Inc.     

Pair bond duration influences paternal provisioning and the primary sex ratio of brown thornbill broods

Parents should vary their level of investment in sons and daughters in response to the fitness costs and benefits accrued through male and female offspring. I investigated brood sex ratio biases and parental provisioning behaviour in the brown thornbill, Acanthiza pusilla, a sexually dimorphic Australian passserine. Parents delivered more food to male-biased than female-biased broods. However, factors determining parental provisioning rates differed between the sexes. Female provisioning rates were related to brood sex ratio in both natural and experimental broods with manipulated sex ratios. In contrast, male provisioning rates were not affected by brood sex ratio in either natural or experimental broods. However, males in established pairs provisioned at a higher rate than males in new pairs. Data on the sex ratio of 109 broods suggest that female brown thornbills adjust their primary sex ratio in response to pair bond duration. Females in new pairs produced broods with significantly fewer sons than females in established pairs. This pattern would be beneficial to females if the costs of rearing sons were higher for females in new than established pairs. This may be the case since females in new pairs provisioned experimental all-male broods at elevated rates. The condition of nestlings also tended to decline more in these all-male broods than in other experimental broods. This will have additional fitness consequences because nestling mass influences recruitment in thornbills. Female thornbills may therefore obtain significant fitness benefits from adjusting their brood sex ratio in response to the status of their pair bond. Copyright 2002 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.     

Sex allocation in black-capped chickadees Poecile atricapilla

Optimal sex allocation for individuals can be predicted from a number of different hypotheses. Fisherian models of sex allocation predict equal investment in males and females up to the end of parental care and predict brood compositions based on the relative costs of producing males and females. The Trivers-Willard hypothesis predicts that individual females should alter the sex ratio of their broods based on their own condition if it has a differential impact on the lifetime reproductive success of their sons and daughters. The Charnov model of sex allocation predicts that females should alter sex allocation based on paternal attributes that may differentially benefit sons versus daughters. Because females are the heterogametic sex in birds, many recent studies have focussed on primary sex ratio biases. In black-capped chickadees Poecile atricapilla , males are larger than females suggesting they may be more costly to raise than females. Female condition affects competitive ability in contests for mates, and thus may be related to variance in fecundity. Females prefer high-ranking males as both social and extrapair partners. These observations suggest that females might vary the sex ratio of their broods based on the predictions of any of the above models. Here, we report on the results of PCR based sex determination of 1093 nestlings in 175 broods sampled from 1992 to 2001. Population-wide, we found a mean brood sex ratio of 0.525±0.016, with no significant deviation from a predicted binomial distribution. We found no effect of clutch size, female condition, hatch date, parental rank or paternity. Our results reject the idea that female black-capped chickadees systematically vary sex allocation in their broods.     

Waiting for Trivers and Willard: Do the rich really favor sons?

Parental investment theory has been put forward as a major evolutionary argument explaining male or female biased birth sex ratio, the Trivers-Willard (T-W) hypothesis, predicting that parents living in good circumstances will bias their investment to sons, whereas parents in poor circumstances will bias their investment toward daughters. Tests of the T-W hypothesis on human beings have shown limited evidence for parents appearing to differentiate their investment to sons or daughters according to the reproductive potential of each sex. The present study tests the T-W hypothesis among a large contemporary Polish sample using first birth interval and extent of breastfeeding as measures of parental investment, and economic status and level of parental education as measures of parental condition. The extents to which parental investment and markers of parental condition vary by sex of the child were examined using log-linear analysis. Weak support for the T-W effect is found among families where fathers were best educated, where a greater proportion of first-born boys are breastfed longer than girls, while the opposite trend is observed among families with fathers with lowest levels of education. Although the present study does not fully support the T-W hypothesis, it gives evidence of greater investment in female offspring at the lower extremes of income, and greater investment in males at higher levels of income.     

Engineers have more sons, nurses have more daughters: an evolutionary psychological extension of Baron-Cohen's extreme male brain theory of autism

In his extreme male brain theory of autism, Baron-Cohen postulates that having a typically male brain was adaptive for ancestral men and having a typically female brain was adaptive for ancestral women. He also suggests that brain types are substantially heritable. These postulates, combined with the insight from the Trivers-Willard hypothesis regarding parental ability to vary offspring sex ratio, lead to the prediction that people who have strong male brains should have more sons than daughters, and people who have strong female brains should have more daughters than sons. The analysis of the 1994 US General Social Survey data provides support for this prediction. Our results suggest potentially fruitful extensions of both Baron-Cohen's theory and the Trivers-Willard hypothesis.     

A sex allocation theory for vertebrates: combining local resource competition and condition-dependent allocation

Tests of sex allocation theory in vertebrates are usually based on verbal arguments. However, the operation of multiple selective forces can complicate verbal arguments, possibly making them misleading. We construct an inclusive fitness model for the evolution of condition-dependent brood sex ratio adjustment in response to two leading explanations for sex ratio evolution in vertebrates: the effect of maternal quality on the fitness of male and female offspring (the Trivers-Willard hypothesis [TWH]) and local resource competition (LRC) between females. We show (1) the population sex ratio can be either unbiased or biased in either direction (toward either males or females); (2) brood sex ratio adjustment can be biased in either direction, with high-quality females biasing reproductive investment toward production of sons (as predicted by the TWH) or production of daughters (opposite to predictions of the TWH); and (3) selection can favor gradual sex ratio adjustment, with both sons and daughters being produced by both high- and low-quality mothers. Despite these complications, clear a priori predictions can be made for how the population sex ratio and the conditional sex ratio adjustment of broods should vary across populations or species, and within populations, across individuals of different quality.     

Marriage patterns in a Mesoamerican peasant community are biologically adaptive

Differential investment in offspring by parental and progeny gender has been discussed and periodically analyzed for the past 80 years as an evolutionary adaptive strategy. Parental investment theory suggests that parents in poor condition have offspring in poor condition. Conversely, parents in good condition give rise to offspring in good condition. As formalized in the Trivers-Willard hypothesis (TWH), investment in daughters will be greater under poor conditions while sons receive greater parental investment under good conditions. Condition is ultimately equated to offspring reproductive fitness, with parents apparently using a strategy to maximize their genetic contribution to future generations. Analyses of sex ratio have been used to support parental investment theory and in many instances, though not all, results provide support for TWH. In the present investigation, economic strategies were analyzed in the context of offspring sex ratio and survival to reproductive age in a Zapotec-speaking community in the Valley of Oaxaca, southern Mexico. Growth status of children, adult stature, and agricultural resources were analyzed as proxies for parental and progeny condition in present and prior generations. Traditional marriage practice in Mesoamerican peasant communities is patrilocal postnuptial residence with investments largely favoring sons. The alternative, practiced by ～25% of parents, is matrilocal postnuptial residence which is an investment favoring daughters. Results indicated that sex ratio of offspring survival to reproductive age was related to economic strategy and differed significantly between the patrilocal and matrilocal strategies. Variance in sex ratio was affected by condition of parents and significant differences in survival to reproductive age were strongly associated with economic strategy. While the results strongly support TWH, further studies in traditional anthropological populations are needed.     

Provisioning in western bluebirds is not related to offspring sex

Parents should invest more in one sex of offspring if the fitnessreturn per unit investment is higher for that sex. Sex-biasedprovisioning may occur when sons and daughters differ in theirneeds or when there is local resource competition or enhancement.We used feeding observations and sex-ratio manipulations todetermine if sex-biased provisioning occurs in western bluebirds(Sialia mexicana). The sex ratio of the brood had no effecton feeding rates by adults at unmanipulated nests over a 6-yearperiod. Adult males and females also did not differ in the numberof feedings made to sons and daughters in 13 videotaped nests.Likewise, adult feeding rates to nests experimentally biasedtoward sons or daughters did not differ significantly. Nesdingsthat were closest to the nest hole and that reached highestwere the most likely to be fed. Sons and daughters did not differin the begging behaviors most likely to result in a feeding.We conclude that sex-biased provisioning does not occur in thispopulation of western bluebirds and diat nesding behavior maybe a more important determinant of feeding.     

Brothers and sisters

Data from the Kipsigis of Kenya are used to test two models for how parents invest in offspring, the Trivers-Willard and local resource competition/enhancement hypotheses. Investment is measured as age-specific survival, educational success, marital arrangements, and some components of property inheritance, permitting an evaluation of how biases persist or alter over the period of dependence. Changes through time in such biases are also examined. Despite stronger effects of wealth on the reproductive success of men than women, the survival of sons and daughters is not related to parental wealth. However, a Trivers-Willard effect characterizes educational investment: poor families show a greater concern for daughters’ (vis-à-vis sons’) schooling than do rich families, a trend that has increased over time. In regard to models of local resource competition and enhancement, men’s reproductive success decreases with number of brothers and increases with number of sisters; this pattern of competition with same-sex sibs and cooperation with opposite-sex sibs is not found among women. As predicted from these observations, parents show reduced investment in sons with a large number of brothers, and increased investment in sons with a large number of sisters. By contrast, investment in daughters is entirely unaffected by number of sisters and is influenced only in subtle ways by number of brothers. Levels of investment in relation to sibship size (irrespective of siblings’ sex) are highest for younger children of large sib sets. Discussion of the results in relation to those from other studies leads to three conclusions. First, predictive models for how investment biases vary across societies must consider a broad range of socioecological factors constraining parental options and payoffs. Second, the timing of investment biases within societies will be affected by the value of children and the costs of parental investment. Third, measures of investment appropriate for between-sex and between-class comparisons need careful attention. Each of these issues is brought to bear on the question of why, in contrast to so many other parts of the world, sex preferences are so muted in Africa.     

Offspring sex ratio allocation in the parasitic jaeger: selection for pale females and melanic males?

The maintenance of plumage color polymorphism in the parasiticjaeger (Stercorarius parasiticus) is still not well understood.Earlier studies indicated that selection may favor pale femalesand melanic males. If so, females would maximize their fitness,producing pale female and melanic male offspring. We thereforepredicted that females might bias their offspring sex ratiotoward daughters in pale pairs and toward sons in melanic pairs.Females might also choose to mate assortatively in relationto plumage color, thereby maximizing the probability of producingeither pale or melanic offspring. Because females are largerthan males, differential rearing costs may affect the offspringsex ratio independent of parental plumage color. We examinedoffspring sex ratio allocation, breeding variables indicativeof parental quality, and mating pattern in relation to plumagecolor in a colony of parasitic jaegers in northern Norway. Jaegerstended to mate assortatively in relation to plumage color. Thereproductive performance declined with season, and matched pairsappeared to be of lower quality than mixed pairs. The proportionof male offspring increased with hatching date in matched paleand mixed pairs, whereas the situation was reversed in matchedmelanic pairs. Matched pale pairs produced an overall surplusof favorable pale but costly daughters despite their lower quality,while melanic pairs produced a surplus of favorable melanicsons. However, differential offspring rearing costs and parentalrearing capacity may have additionally affected the realizedoffspring sex ratio. Mixed pairs producing an overall surplusof pale and melanic daughters allocated their resources accordingto differential rearing costs and parental quality only. Wesuggest that both strategies of sex ratio allocation togetherwith differences in reproductive success in matched versus mixedpairs may have a balancing effect on the mating pattern betweenplumage morphs and may contribute to the maintenance of thecolor polymorphism in this species.     

Sex ratio adjustments in common terns: influence of mate condition and maternal experience

Adaptive sex allocation has frequently been studied in sexually size dimorphic species, but far less is known about patterns of sex allocation in species without pronounced sexual size dimorphism. Parental optimal investment can be predicted under circumstances in which sons and daughters differ in costs and/or fitness returns. In common terns Sterna hirundo, previous studies suggest that sons are the more costly sex to produce and rear. We investigated whether hatching and fledging sex ratio and sex‐specific chick mortality correlated with the ecological environment (laying date, clutch size, hatching order and year quality) and parental traits (condition, arrival date, experience and breeding success), over seven consecutive years. Population‐wide sex ratios and sex‐specific mortality did not differ from parity, but clutch size, mass of the father, maternal breeding experience and to some extent year quality correlated with hatching sex ratio. The proportion of sons tended to increase in productive years and when the father was heavier, suggesting the possibility that females invest more in sons when the environmental and the partner conditions are good. The proportion of daughters increased with clutch size and maternal breeding experience, suggesting a decline in breeding performance or a resources balance solved by producing more of the cheaper sex. No clear patterns of sex‐specific mortality were found, neither global nor related to parental traits. Our results suggest lines for future studies on adaptive sex allocation in sexually nearly monomorphic species, where adjustment of sex ratio related to parental factors and differential allocation between the offspring may also occur.     

Estimating relative parental investment in sons versus daughters

Fisher proposed that natural selection would adjust the population sex ratio so that parental expenditure on sons equals expenditure on daughters. Thus if two daughters can be produced for every son, the Fisherian equilibrium is ? sons and ? daughters. The relative cost of a son versus a daughter is necessarily manifested in the trade-off between family size and sex ratio, and we offer a method to estimate this trade-off from data on family compositions. Simulation studies indicate that the method works well in some cases but not others. Application of the method to data on a polychaete suggests that sons are much costlier than daughters; the observed sex ratio in fact significantly favored daughters, but not to the extreme predicted by our measure of differential cost.     

Male-biased sex ratio in litters of Alpine marmots supports the helper repayment hypothesis

In a French population of Alpine marmots (Marmota marmota),the sex ratio at weaning was biased in favor of males. Thisbias also seemed to exist at birth. Under Fisher's equal allocationprinciple, this means that daughters should be more costlyto produce than sons. Because the Alpine marmot can be considereda cooperative breeding species, we investigated whether thedifferential cost between sons and daughters may be explainedby the helper repayment hypothesis. The Alpine marmot usessocial thermoregulation during hibernation, allowing juvenilesto better survive over winter. In the study population, juvenilesurvival during winter increased with group size. More precisely,juvenile survival during winter increased with the number andwith the proportion of subordinate males in the hibernatinggroup, but juvenile survival did not depend on the number of subordinate females. As our results did not support alternativehypotheses to explain the observed bias in sex ratio amongoffspring at emergence, we conclude that the helper repaymenthypothesis is the best candidate to explain the observed offspringsex ratio bias in Alpine marmots. By participating in socialthermoregulation, subordinate males may repay part of the investment they received from their parents and thus become less costlyto produce. We suggest that only subordinate males helped becausethey may gain direct fitness benefits, whereas subordinatefemales may only expect indirect fitness benefits from helping.Finally, the offspring sex ratio per individual parent wasmale biased, but mothers adjusted the size and the sex compositionof their litters according to their phenotypic condition asexpected from the Trivers-Willard hypothesis.     

No evidence for sex biases in milk macronutrients,energy, or breastfeeding frequency in a sample of filipino mothers

Maternal reproductive investment includes both the energetic costs of gestation and lactation. For most humans, the metabolic costs of lactation will exceed those of gestation. Mothers must balance reproductive investment in any single offspring against future reproductive potential. Among mammals broadly, mothers may differentially invest in offspring based on sex and maternal condition provided such differences investment influence future offspring reproductive success. For humans, there has been considerable debate if there are physiological differences in maternal investment by offspring sex. Two recent studies have suggested that milk composition differs by infant sex, with male infants receiving milk containing higher fat and energy; prior human studies have not reported sex‐based differences in milk composition. This study investigates offspring sex‐based differences in milk macronutrients, milk energy, and nursing frequency (per 24 h) in a sample of 103 Filipino mothers nursing infants less than 18 months of age. We found no differences in milk composition by infant sex. There were no significant differences in milk composition of mothers nursing first‐born versus later‐born sons or daughters or between high‐ and low‐income mothers nursing daughters or sons. Nursing frequency also showed no significant differences by offspring sex, sex by birth order, or sex by maternal economic status. In the Cebu sample, there is no support for sex‐based differences in reproductive investment during lactation as indexed by milk composition or nursing frequency. Further investigation in other populations is necessary to evaluate the potential for sex‐based differences in milk composition among humans. Am J Phys Anthropol 152:209–216, 2013. © 2013 Wiley Periodicals, Inc.     

Sex Allocation in a Colobine Monkey

In polygynous, sexual dimorphic species with higher variance in male reproductive success compared with females, females are expected to invest more heavily in sons than daughters within the constraints imposed by their physical condition (Science 1973; 179:90). Mothers in good condition, usually those of high rank, should produce more sons than females in poor condition or of low rank. We investigated sex allocation and sex‐biased maternal investment in a population of wild Hanuman langurs using rank and group size as approximations of female physical condition. Our results show that reproductive costs of sons were higher with both significantly longer interbirth intervals following male births and longer lactational periods for sons. Not in all groups did analyses of rank‐dependent sex allocation reveal the expected pattern of high‐ranking mothers producing more sons. However, sex ratio was significantly influenced by group size, with females from larger groups, i.e., in worse physical condition, producing a daughter‐biased sex ratio. In fact, only females of population‐wide superior physical condition can be expected to produce sons, because in Hanuman langurs males disperse and compete population‐wide. Thus, our results support the Trivers–Willard model and may explain the mixed evidence accruing from studies of single groups. We present a graphical model of how group size and dominance‐related differences in energy gain may influence sex allocation under different competitive regimes relative to overall resource availability. Tests of adaptive sex allocation models should consider whether reproductive competition of the preferred sex takes place primarily within a group or within the population.     

PATERNAL CONDITION DRIVES PROGENY SEX‐RATIO BIAS IN A LIZARD THAT LACKS PARENTAL CARE

Sex‐allocation theory predicts that females in good condition should preferentially produce offspring of the sex that benefits the most from an increase in maternal investment. However, it is generally assumed that the condition of the sire has little effect on progeny sex ratio, particularly in species that lack parental care. We used a controlled breeding experiment and molecular paternity analyses to examine the effects of both maternal and paternal condition on progeny sex ratio and progeny fitness in the brown anole (Anolis sagrei), a polygynous lizard that lacks parental care. Contrary to the predictions of sex‐allocation theory, we found no relationship between maternal condition and progeny sex ratio. By contrast, progeny sex ratio shifted dramatically from female‐biased to male‐biased as paternal condition increased. This pattern was driven entirely by an increase in the production of sons as paternal condition improved. Despite strong natural selection favoring large size and high condition in both sons and daughters, we found no evidence that progeny survival was related to paternal condition. Our results emphasize the importance of considering the paternal phenotype in studies of sex allocation and highlight the need for further research into the pathways that link paternal condition to progeny fitness.