Conservation biology: science, sex and the kakapo

Sex ratio and relatedness in the Griffon vulture (Gyps fulvus) population of Serbia

Background

Once a widespread species across the region of Southeast Europe, the Griffon vulture is now confined to small and isolated populations across the Balkan Peninsula. The population from Serbia represents its biggest and most viable population that can serve as an important reservoir of genetic diversity from which the birds can be used for the region's reintroduction programmes. The available genetic data for this valuable population are scarce and as a protected species that belongs to the highly endangered vulture group, it needs to be well described so that it can be properly managed and used as a restocking population. Considering the serious recent bottleneck event that the Griffon vulture population from Serbia experienced we estimated the overall relatedness among the birds from this population. Sex ratio, another important parameter that shows the vitality and strength of the population was evaluated as well.

Methods

During the annual monitoring that was performed in the period from 2013-2021, we collected blood samples from individual birds that were marked in the nests. In total, 169 samples were collected and each was used for molecular sexing while 58 presumably unrelated birds from different nests were used for inbreeding and relatedness analyses. The relatedness was estimated using both biparentally (10 microsatellite loci) and uniparentally (Cytb and D-loop I of mitochondrial DNA) inherited markers.

Results

The level of inbreeding was relatively high and on average it was 8.3% while the mean number of relatives for each bird was close to three. The sex ratio was close to 1:1 and for the analysed period of 9 years, it didn't demonstrate a statistically significant deviation from the expected ratio of 1:1, suggesting that this is a stable and healthy population. Our data suggest that, even though a relatively high level of inbreeding can be detected among the individual birds, the Griffon vulture population from Serbia can be used as a source population for restocking and reintroduction programmes in the region. These data combined with previously observed genetic differentiation between the populations from the Iberian and Balkan Peninsulas suggest that the introduction of foreign birds should be avoided and that local birds should be used instead.

Seeing-good-gene-based mate choice: From genes to behavioural preferences

Although vertebrates have been reported to gain higher reproductive outputs by choosing mates, few studies have been conducted on threatened species. However, species recovery should benefit if natural mate choice could improve reproductive output (i.e. pair performance related to offspring number, such as increased clutch size, numbers of fertilized egg and fledglings). We assessed the evidence for major histocompatibility complex (MHC)‐based mate preference in the endangered crested ibis (Nipponia nippon) and quantified the impacts of such choice on reproductive output.

We tested the hypothesis that crested ibis advertise “good genes” through external traits, by testing whether nuptial plumage characteristics and body morphology mediate mate choice for underlying genetic MHC variation.

We found differences between males and females in preferred MHC genotypes, external traits used in mate choice and contributions to reproductive outputs. Females preferred MHC‐heterozygous males, which had darker [i.e. lower total reflectance and ultraviolet (UV) reflectance] nuptial plumage. Males preferred females lacking the DAB*d allele at the MHC class II DAB locus, which had higher average body mass. DAB*d‐free females yielded heavier eggs and more fledglings, while MHC‐heterozygous males contributed to more fertilized eggs and fledglings. Fledging rate was highest when both parents had the preferred MHC genotypes (i.e. MHC‐heterozygous father and DAB*d‐free mother). Comparisons showed that free‐mating wild and semi‐natural pairs yielded more fertilized eggs and more fledglings, with a higher fledging rate, than captive pairs matched artificially based on pedigree.

Conservation programmes seldom apply modern research results to population management, which could hinder recovery of threatened species. Our results show that mate choice can play an important role in improving reproductive output, with an example in which an endangered bird selects mates using UV visual capability. Despite the undoubted importance of pedigree‐based matching of mates in conservation programmes, we show that free mating can be a better alternative strategy.

Maternal diet and other factors affecting offspring sex ratio: a review

Mammals usually produce approximately equal numbers of sons and daughters, but there are exceptions to this general rule, as has been observed in ruminant ungulate species, where the sex-allocation hypothesis of Trivers and Willard has provided a rational evolutionary underpinning to adaptive changes in sex ratio. Here, we review circumstances whereby ruminants and other mammalian species, especially rodents and primates, appear able to skew the sex ratio of their offspring. We also discuss some of the factors, both nutritional and nonnutritional, that potentially promote such skewing. Work from our laboratory, performed on mice, suggests that age of the mother and maternal diet, rather than the maternal body condition per se, play directive roles in controlling sex ratio. In particular, a diet high in saturated fats but low in carbohydrate leads to the birth of significantly more male than female offspring in mature laboratory mice, whereas when calories are supplied mainly in the form of carbohydrate rather than fat, daughters predominate. As the diets fed to the mice in these experiments were nutritionally complete and because litter sizes did not differ between treatments, dietary inadequacy seems not to be the cause for sex-ratio distortion. A number of mechanisms, all of which are testable, are discussed to provide an explanation for the phenomenon. We conclude the review by discussing potential implications of these observations to human medicine and agriculture.