Sex Ratio Is Synergistically Modulated by Local Resource Competition and Enhancement: the Case of a Primitively Eusocial Wasp Under Female Philopatry

BackgroundIn animals, the offspring sex ratio is modulated by kin conflict and cooperation, and determining the ratio is a main concern in evolutionary biology. Male competition for access to local mates is predictive of a female-biased sex ratio in the offspring (local mate competition; LMC). Conversely, female competition for access to local resources is predictive of a male-biased sex ratio in the offspring (local resource competition; LRC). However, several factors other than competition should synergistically operate in real-world populations. In the Australian paper wasp Ropalidia plebeiana, LRC and local resource enhancement (LRE) may operate simultaneously. To determine whether this is the case, we evaluated colony sex ratios and examined whether competition and/or enhancement operates at the population level in this species. ResultsIn spring, many foundress queens started their colonies by comb-cutting, in which nest combs from the previous season were divided into several combs to be reused. Genetic relatedness among foundresses did not differ before and after comb-cutting. Relatedness among foundresses was 0.339, whereas relatedness among new foundresses was 0.589, revealing nearly functional monogyny. The global FST value calculated with mtDNA markers was higher than that calculated with microsatellite markers, even after we corrected for differences in effective population sizes between sexes. This finding indicates female philopatry, which was also confirmed by mark–release–recapture before and after the hibernation of new foundresses. The colony sex ratio of reproductives became slightly biased toward males in larger colonies. In addition, both the number of foundresses and number of workers were positively associated with the number of reproductives, which indicates that LRE was also operating.ConclusionsOur results suggest that although the population structure seems to meet the requirements of LRC, the sex ratio is not modulated solely by LRC. Instead, the availability of female helpers at the founding stage likely mitigates the sex ratio predicted by LRC through LRE. Thus, LRC at the founding stage and LRE at the reproductive stage synergistically modulate the colony sex ratio in R. plebeiana.

Female philopatry and limited male-biased dispersal in the Ussuri tube-nosed bat, Murina ussuriensis

Male-biased dispersal and female philopatry are common traits among social mammals, often leading to elevated relatedness within social groups. However, exceptions do occur, with documented cases of female-biased dispersal, dispersal by both sexes, and philopatry of both sexes. In this study, we examined levels of dispersal and relatedness based on analyses of the mitochondrial D-loop and 12 nuclear microsatellite markers in the woodland specialist Ussuri tube-nosed bat ( Murina ussuriensis ), a relatively widespread, yet locally rare species that is thought to be threatened by loss of its forest habitats across its range. A total of 85 individuals were captured in lowland temperate forest on the island of Yakushima and 28 individuals in montane forest in Hokkaido, Japan. In the former, haplotypes showed extreme spatial clustering among females consistent with strong philopatry, but spatial mixing among males suggesting dispersal over short distances. These findings were broadly supported by microsatellite analyses, which indicated considerable genealogical structure within sampling locations but a lack of spatial structure, again indicating that some gene flow does occur in one or both sexes. The Hokkaido data, although limited, did not show these patterns and instead suggest that differences in environmental and behavioral variables may influence movement ecology of individuals at these 2 sites and consequently fine-scale genetic structure within this species.

Evidence of male-biased dispersal in eastern grey kangaroos (Macropus giganteus)

Dispersal reduces the likelihood of inbreeding and maintains gene flow among populations. Many polygynous mammals exhibit male-biased dispersal with female philopatry. Previous observational studies of eastern grey kangaroos (Macropus giganteus) suggested female philopatry while genetic studies showed weak structuring. We tested for sex-biased dispersal using two Queensland populations of kangaroos: one in Sundown National Park and the second at Elanda Point, Australia. Samples from 25 females and 23 males were collected from Sundown National Park, and analysed for partial mtDNA control region sequences (n = 47) and genotypes based on 12 microsatellite loci (n = 41). Samples from 18 males and 22 females from Elanda Point were genotyped at 8 loci and a subset sequenced for mtDNA (n = 19). Analyses showed higher mtDNA haplotype and nucleotide diversity in males than females within both populations, genetic relatedness based on microsatellite data was significantly higher among females, and microsatellite allelic richness was higher in males, suggesting that females are more likely to be philopatric and males more likely to disperse. These findings reinforce the value of including multiple types of genetic markers in dispersal analyses as mtDNA results showed higher male diversity (suggesting male dispersal) but males also contributed microsatellite alleles to the local population, masking differentiation between the sexes and confounding analyses.