A unified-models analysis of the development of sexual size dimorphism in Damaraland mole-rats, Fukomys damarensis

AbstractIndividual variation in growth rates often generates variation in fitness. However, the ability to draw meaningful inferences from growth data depends on the use of growth models that allow for direct comparisons of growth between the sexes, between populations, and between species. Unlike traditional sigmoid functions, a recently parameterized family of unified growth models provides a reliable basis for comparisons since each parameter affects a single curve characteristic and parameters are directly comparable across the unified family. Here, we use the unified-models approach to examine the development of sexual size dimorphism in Damaraland mole-rats (Fukomys damarensis), where breeding males are larger than breeding females. Using skeletal measurements, we show here that the larger size of male Damaraland mole-rats arises from an increased growth rate across the entire period of development, rather than through sex differences in the duration or timing of growth. Male-biased skeletal size dimorphism is not unusual among rodents, and our measures of sex differences in size in captive mole-rats are close to sexual size differences in the wild, where size dimorphism = 1.04 (male:female). We hope our study will encourage the wide use of unified growth models by mammalogists.</jats:p

Validating methods for estimating endocranial volume in individual red deer (Cervus elaphus).

Comparing brain sizes is a key method in comparative cognition and evolution. Brain sizes are commonly validated by interspecific comparisons involving animals of varying size, which does not provide a realistic index of their accuracy for intraspecific comparisons. Intraspecific validation of methods for measuring brain size should include animals of the same age and sex to ensure that individual differences can be detected in animals of similar size. In this study we compare three methods of measuring the endocranial volume of 33 red deer skulls to investigate the accuracy of each method. Methods for estimating endocranial volume included scanning each skull using computerised tomography (CT) and quantifying the volume with OsiriX software, filling the cranium with glass beads and measuring the bead volume, and linear measurements (length, width, and height) of the cranium using callipers. CT scan volumes were highly correlated with results from the bead method, but only moderately correlated with the linear method. This study illustrates the importance of validating intraspecies measurement methods, which allows for the accurate interpretation of results.This is the author accepted manuscript. The final version is available from Elsevier via http://dx.doi.org/10.1016/j.beproc.2012.10.01

Pregnancy is detected via odour in a wild cooperative breeder

Among mammals, scent has long been known to encode oestrus, however in many species detecting pregnancy may also be important in terms of both competition and mate-choice. Here we show, through odour presentation experiments, that pregnancy is discernible via scent by both sexes in the cooperatively breeding banded mongoose. Males spent more time investigating and were more likely to scent mark the odours of non-pregnant females, compared to pregnant females. Females showed increased levels of scent marking when odours were of the same reproductive state as themselves. These results present the first direct demonstration that pregnancy is detectable via scent in wild cooperative breeders. Detecting pregnancy may be particularly important in cooperative breeders as, in addition to the competition between males for receptive mates, there is also intense competition between females for access to alloparental care. Consequently, dominant females benefit from targeting reproductive suppression towards subordinates that represent direct threats, such as pregnant females

No apparent benefits of allonursing for recipient offspring and mothers in the cooperatively breeding meerkat.

Cooperative behaviours by definition are those that provide some benefit to another individual. Allonursing, the nursing of non-descendent young, is often considered a cooperative behaviour and is assumed to provide benefits to recipient offspring in terms of growth and survival, and to their mothers, by enabling them to share the lactation load. However, these proposed benefits are not well understood, in part because maternal and litter traits and other ecological and social variables are not independent of one another, making patterns hard to discern using standard univariate analyses. Here, we investigate the potential benefits of allonursing in the cooperatively breeding Kalahari meerkat, where socially subordinate females allonurse the young of a dominant pair without having young of their own. We use structural equation modelling to allow us to account for the interdependence of maternal traits, litter traits and environmental factors. We find no evidence that allonursing provides benefits to pups or mothers. Pups that received allonursing were not heavier at emergence and did not have a higher survival rate than pups that did not receive allonursing. Mothers whose litters were allonursed were not in better physical condition, did not reconceive faster and did not reduce their own nursing investment compared to mothers who nursed their litters alone. These patterns were not significantly influenced by whether mothers were in relatively good, or poor, condition. We suggest that allonursing may persist in this species because the costs to allonurses may be low. Alternatively, allonursing may confer other, more cryptic, benefits to pups or allonurses, such as immunological or social benefits.KJM was supported by a research grant from the Cambridge Philosophical Society. KEM was supported by a US National Science Foundation grant to Alison Bell and KEM (NSF IOS 1121980).This is the accepted manuscript. The final version is available at http://onlinelibrary.wiley.com/doi/10.1111/1365-2656.12343/abstract

Maternal effects and early-life performance are associated with parasite resistance across life in free-living Soay sheep

Maternal effects occur when the maternal phenotype influences that of the offspring in addition to the effects of maternal genes, and may have a considerable influence on offspring parasite resistance. These effects, and the effects of early levels of reproduction and parasite resistance, may persist into later life and even influence ageing rates. Here we analyse a 20-year longitudinal data set collected on a free-living population of Soay sheep, to investigate the associations between a suite of maternal phenotypic traits and early-life performance on measures of parasite resistance across life. Our results show that maternal effects are important in determining offspring parasite resistance, since lambs born as twins and those born to the youngest and oldest mothers show higher parasite burdens. We show that the association between parasite resistance and natal litter size persists into adulthood. We also show that age-specific changes in parasite resistance in males are associated with natal litter size, and that age-specific changes in females are influenced by early-life levels of reproduction and parasite infection. These results add to the growing evidence that conditions experienced by individuals during development can have a profound influence on immediate and late-life performance and may even influence ageing

Low costs of allonursing in meerkats : mitigation by behavioral change?

Allonursing, the nursing of another female’s offspring, is assumed to impose a substantial energetic cost given the high cost of lactation to mothers. However, these costs have not been quantified. In cooperatively breeding mammals where helpers contribute to lactation, they might be expected to modify their behavior to mitigate these potential costs. Here, we show that overnight weight loss during lactation did not differ between allonurses and controls. However, meerkat helpers that allonursed do not gain weight over a reproductive bout as non-allonursing subordinate females did, suggesting that allonurses may incur some cost. Allonurses may mitigate the costs by increasing foraging effort during lactation. Allonurses do not, as expected, reduce investment in other cooperative behaviors during lactation. We suggest that the increase in cooperative behavior, including allonursing, may serve a social function, but further work is needed to confirm this hypothesis.Cambridge Home and European Scholarship Scheme and the Cambridge Philosophical Society.http://beheco.oxfordjournals.org2016-05-31hb201

No evidence for adaptive sex ratio variation in the cooperatively breeding meerkat, Suricata suricatta

Where a maternal trait influences the fitness of sons or daughters, mothers would be expected to bias the sex ratio towards the sex whose fitness they are more able to increment. In many polygynous species, maternal characteristics affect the fitness of sons more than that of daughters, but, in meerkats, variance in female reproductive success exceeds variance in male reproductive success and maternal rank affects the success of daughters more than sons. Dominant females would therefore be expected to produce an excess of daughters, a reversal of the hypothesis’ usual predictions. In a long-term data set, despite a strong effect of maternal rank on daughters’ success, we found no indication that dominant females produce female-biased litters. Offspring sex ratios did not deviate significantly from equality, and were also unaffected by maternal mass, age or number of previous litters produced in the same season. We suggest that potential advantages to both mother and offspring of producing and developing in mixed litters may result in the adaptive maintenance of an equal offspring sex ratio.http://www.elsevier.com/locate/anbehavhb201