Sex Ratio Adjustment Research Papers (original) (raw)

Based on evolutionary theory, Trivers & Willard (TW) predicted the existence of mechanisms that lead parents with high levels of resources to bias offspring sex composition to favor sons and parents with low levels of resources to favor daughters. This hypothesis has been tested in samples of wealthy individuals but with mixed results. Here, I argue that both sample selection due to a high number of missing cases and a lacking specification of the timing of wealth accumulation contribute to this equivocal pattern. This study improves on both issues: First, analyses are based on a data set of U.S. billionaires with near-complete information on the sex of offspring. Second, subgroups of billionaires are distinguished according to the timing when they acquired their wealth. Informed by recent insights on the timing of a potential TW effect in animal studies, I state two hypotheses. First, billionaires have a higher share of male offspring than the general population. Second, this effect is larger for heirs and heiresses who are wealthy at the time of conception of all of their children than for self-made billionaires who acquired their wealth during their adult lives, that is, after some or all of their children have already been conceived. Results do not support the first hypothesis for all subgroups of billionaires. But for males, results are weakly consistent with the second hypothesis: Heirs but not self-made billionaires have a higher share of male offspring than the U.S. population. Heiresses, on the other hand, have a much lower share of male offspring than the U.S. average. This hints to a possible interplay of at least two mechanisms affecting sex composition. Implications for future research that would allow disentangling the distinct mechanisms are discussed.

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- Business, Business Administration, Evolutionary Biology, Sociology

We thank the Population Association of America for providing funding to attend the PAA 2010 annual meeting where an early version of this paper was presented. We also thank Sunita Kishor, ICF International, for her valuable suggestions and the anonymous reviewers for their excellent feedback, which helped to improve this paper. However, none of them are responsible for any errors or for the opinions expressed in this paper.

- by Srinivas Goli
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- Sociology, Economic Sociology, Sociology of Religion, Demography

Impacts of sodium and potassium in diet to determine offspring gender in rabbits have investigated. Sex determinat ion has scientific basis for prevention of genetic diseases in addition to social backgrounds.20 healthy rabbits chosen from the second generation of the previous experimental rabbits, were divided in to two groups in the rabbits male to female 1:1. The first group was Na and K, the second group was control unit without Na and K. It found that the
delivered offsprings male to female ratio were 1.95:1and 1:1, for the first and second groups respectively. In addition, it found that rabbits fed with (Na, K) food yields maximum numbers of male offspring 65, while rabbits fed with normal food yields lowest numbers of male offspring 35.

Evolutionary theory predicts birds should adjust the sex ratio of their broods in response to external factors that differentially affect the reproductive value of each sex. We examined the brood sex ratio in the Lilac-crowned Parrot (Amazona finschi) in relation to climate, hatching date, and hatching order. We used polymerase chain reaction amplifications to identify the gender of 66 nestlings from 32 clutches spanning 7 years. There was a tendency to produce more female offspring in years of high nestling survival following high rainfall with a slight female-bias in third-hatched nestlings. We found no significant associations between brood sex ratio and rainfall, hatching date, or hatching order within clutches. Our results suggest the examined factors provide insufficient differential costs or benefits of offspring gender to promote sex ratio bias in this monomorphic species

1. The reproductive fitness of a parasitoid depends on its mating and ovipositing success. Virgin haplodiploid females can reproduce, but produce only males, and may diminish fitness by producing more male offspring than required. Therefore, females must decide on whether to mate or oviposit first.2. This study was conducted to assess the mating versus ovipositing decision and its impact on the reproductive fitness of Diaeretiella rapae (Hymenoptera: Aphididae), an endoparasitoid of the cabbage aphid Brevicoryne brassicae (Hemiptera: Aphididae).3. When newly emerged females were given a choice between mating and ovipositing, about 62% of D. rapae females preferred to mate before ovipositing. Those females who oviposited before mating parasitised only 10% of the available aphids. After mating, females superparasitised their hosts with fertilised eggs, which resulted in a highly female-biased sex ratio in the offspring.4. Mating success was very high (91%) in the presence of hosts (cabbage aphid nymphs) compared with that in the absence of aphids. However, mating success was not influenced by the quality (size) of the hosts present in the mating arena, despite a parasitoid preference for larger hosts during oviposition. The time between pairing and mating was also shorter in the presence of host aphids. The mean number of aphids parasitised and the parasitism rate were significantly greater after mating.

- by Rashmi Kant
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- Insect Mating Behavior, Sex Ratio Adjustment, Hymenoptera, Parasitism

We investigated sex allocation in the Neotropical ant Allomerus octoarticulatus var. demerarae. Because Allomerus is a plant symbiont, we could make geographically extensive collections of complete colonies and of foundresses in saplings, allowing us to estimate not only population- and colony-level sex allocation but also colony resource levels and the relatednesses of competing ant foundresses. This species exhibits a strongly split sex ratio, with 80% of mature colonies producing ≥90% of one sex or the other. Our genetic analyses (DNA microsatellites) reveal that Allomerus has a breeding system characterized by almost complete monogyny and a low frequency of polyandry. Contrary to theoretical explanations, we find no difference in worker relatedness asymmetries between female- and male-specialist colonies. Furthermore, no clear link was found between colony sex allocation and life history traits such as the number of mates per queen, or colony size, resource level, or fecundity. We also failed to find significant support for male production by workers, infection by Wolbachia, local resource competition, or local mate competition. We are left with the possibility that Allomerus exhibits split sex ratios because of the evolution of alternative biasing strategies in queens or workers, as recently proposed in the literature.

- by Douglas Yu
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- Symbiosis, Biodiversity, Evolutionary Ecology, Mutualisms and Parasitisms

Nests of the primitively eusocial orchid bee Euglossa viridissima are generally founded
by a solitary female but can be reactivated by female offspring, in the presence or
absence of the mother. The population therefore exists of solitary and social nests
that co‐occur in an area. A female can stay as a subordinate helper under a dominant
female or disperse to become a solitary foundress. Yet, the costs and benefits of the
different social phenotypes are so far little understood. Here, we compared solitary
and social nests of orchid bees. We used offspring of solitary and social nests to
calculate offspring sex ratio, and applied molecular markers to calculate intranidal
relatedness, infer maternity and test whether sociality may have a genetic component.
We found that social nests had on average more brood than solitary nests.
The overall sex ratio in the population did not differ from 1:1. However, social nests
tended to produce a split sex ratio with some nests producing mainly males and others
mainly female offspring. Regardless of social phenotype, the number of offspring
was correlated with the sex ratio, with smaller nests having a more female‐biased
offspring. In social nests, not all offspring resulted from a single‐mated mother, which
was also the case for some solitary nests. This suggests shared reproduction in social
nests and may be an indication that intraspecific parasitism and nest takeover are not
uncommon. Structure analyses did not reveal different genetic background of the
two social phenotypes. Our results suggest that there is no clear benefit that favours
one of the two social phenotypes over the other and that the population is kept at
balance in terms of sex ratio.

- by Michael Lattorff
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- Social insects, Animal Behaviour, Sex Ratio Adjustment, Social behavior

Offspring sex ratios in mammals vary in potentially adaptive yet unpredictable ways. An integrative approach that simultaneously examines proximate and ultimate explanations of mammalian sex ratios would greatly advance the field. We examined the importance of maternal glucose and stress hor- mones for offspring sex (male or female) as mechanisms associated with the Trivers–Willard and the local resource competition hypotheses of sex allocation. We tested this framework in a marsupial mammal, the tammar wallaby (Macropus eugenii). Mothers that were better able to maintain body condition over the driest part of the year, a presumptive proxy for local resource availability, were more likely to produce daughters (the philopatric sex), consistent with local resource competition. Maternal glucose was correlated with offspring sex, but in the opposite direction than we predict- ed—higher maternal glucose was associated with female pouch young. These patterns, however, were not consistent across the 2 years of our study. Maternal stress hormone metabolites measured from fecal samples did not predict glucose or offspring sex. A causative glucose mechanism may underlie an adaptive strategy for mothers with high local resources (high glucose) to produce philopatric daughters that will benefit from inheriting resource access. Examining species-specific relationships between glucose and offspring sex across mammals could provide crucial insight into the disparate ecological and selective pressures faced by mam- mals with respect to offspring sex ratio.

- by Kylie Robert and +1
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- Sex Allocation, Sex Ratio Adjustment, Sex ratio, Sex Ratios