Generation time: a reliable metric to measure life-history variation among mammalian populations

Oli and Dobson proposed that the ratio between the magnitude and the onset of reproduction (F/ alpha ratio) allows one to predict the relative importance of vital rates on population growth rate in mammalian populations and provides a reliable measure of the ranking of mammalian species on the slow-fast continuum of life-history tactics. We show that the choice of the ratio F/ alpha is arbitrary and is not grounded in demographic theory. We estimate the position on the slow-fast continuum using the first axis of a principal components analysis of all life-history variables studied by Oli and Dobson and show that most individual vital rates perform as well as the F/ alpha ratio. Finally, we find, in agreement with previous studies, that the age of first reproduction is a reliable predictor of the ranking of mammalian populations along the slow-fast continuum and that both body mass and phylogeny markedly influence the generation time of mammalian species. We conclude that arbitrary ratios such as F/ alpha correlate with life-history types in mammals simply because life-history variables are highly correlated in response to allometric, phylogenetic, and environmental influences. We suggest that generation time is a reliable metric to measure life-history variation among mammalian populations and should be preferred to any arbitrary combination between vital rates.     

Sex ratio variation in the cooperatively breeding alpine marmot Marmota marmota

In cooperatively breeding vertebrate species, a clear theoreticalprediction about the direction of sex ratio adjustment can bemade: mothers should bias the sex ratio of their offspring towardsthe helping sex when helpers are absent. A consistent trendin the direction predicted by theory exists in cooperative birds,but theory is still poorly tested in cooperative mammals. Here,multivariate analyses are applied to a long-term data set totest this prediction in two ways in the alpine marmot: (1) acrossfemales in a population and (2) in individual females acrossmultiple years. It was shown that in the alpine marmot offspringsex ratio was biased towards the helping sex (males) when helperswere absent, whereas helped mothers produced unbiased sex ratio.Unhelped mothers did not adjust the litter size but producedmore sons and fewer daughters than helped mothers. These resultssupport the theoretical prediction and explain well the malebias observed among juvenile alpine marmots at the populationlevel. The occurrence of possible sex ratio manipulations incooperatively breeding vertebrates is also discussed.     

Does social complexity lead to sex-biased dispersal in polygynous mammals? A test on ground-dwelling sciurids

Mating systems are well known to influence the dispersing sex,but the magnitude of the sex-biased dispersal has not actuallybeen measured, whereas many theoretical predictions have beenmade. In this study, we tested a new prediction about the coevolutionbetween natal dispersal and sociality from a recent evolutionarilystable strategy (ESS) approach. From a comparative approach,we showed that, in agreement with the model, the male-biaseddispersal increases with increasing level of sociality in polygynousground-dwelling sciurids. In addition, the increase in male-biaseddispersal with increasing sociality results from an increasein male dispersal rates, whereas female dispersal rates remainconstant, contrary to what is expected from the ESS model. Althoughthe mating system through the level of polygyny could act asa confounding factor, our results strengthen previous work thatstates that inbreeding avoidance plays a major role in the evolutionof dispersal for the most social mammalian species.     

Litter size and fetal sex ratio adjustment in a highly polytocous species: the wild boar

For species in which reproductive success is more variable inone sex than the other, the Trivers and Willard model (TWM)predicts that females are able to adjust their offspring sexratio. High-quality mothers should provide greater investmentto one sex than the other. Previous tests of the TWM have beeninconsistent, and whether the TWM applies to species with severaloffspring per litter is unclear due to possible trade-offs betweensize, number, and sex of the offspring. Williams' model (WM)accounts for confounding effects of these trade-offs on sexratio variation. Lastly, the "extrinsic modification hypothesis"predicts changes in offspring sex ratio in relation to climaticconditions and population density. Using wild boar as a model,we tested 1) whether the WM fitted observed sex ratio variationand 2) whether sex ratio variations were related to maternalattributes (test of the TWM) and/or to resource availability(test of the extrinsic modification hypothesis). Females adjustedtheir litter size rather than their litter composition, so thatthe WM was not supported. Likewise, changes in resource availabilitydid not influence the fetal sex ratio, so that the extrinsicmodification hypothesis was not supported. The fetal sex ratiowas negatively related to increasing litter size, providingsome support for the TWM. Sex ratio was male biased for littersizes up to 6 and then became female biased in larger litters.Our results provide the first case study showing marked changesin sex ratio in relation to litter size in a large mammal.     

Variation in growth form and precocity at birth in eutherian mammals.

Using the flexible Chapman-Richards model for describing the growth curves from birth to adulthood of 69 species of eutherian mammals, we demonstrate that growth form differs among eutherian mammals. Thereby the commonly used Gompertz model can no longer be considered as the general model for describing mammalian growth. Precocial mammals have their peak growth rate earlier in the growth process than altricial mammals. However, the position on the altricial-precocial continuum accounts for most growth-form differences only between mammalian lineages. Within mammalian genera differences in growth form are not related to precocity at birth. This indicates that growth form may have been associated with precocity at birth early in mammalian evolution, when broad patterns of body development radiated. We discuss four non-exclusive interpretations to account for the role of precocity at birth on the observed variation in growth form among mammals. Precocial and altricial mammals could differ according to (i) the distribution of energy output by the mother, (ii) the ability of the young to assimilate the milk yield, (iii) the allocation of energy by the young between competing functions and (iv) the position of birth between conception and attainment of physical maturity.     

Socio-spatial Organization and Activity Distribution of the Alpine Marmot Marmota marmota: Preliminary Results

The behavioural ecology of the Alpine marmot Marmota marmota (Linné, 1758) was studied Jun.—Sep. 1990 in the Vanoise National Park (French Alps). We describe the socio-spatial structure of a high-altitude population, to give additional information on the formerly unclear marmot social organization. The social unit was the family group, with a common home range between 0.9 and 2.8 ha; a slight overlapping occurred (9 to 12.5%). Each home range had a central area of main burrows, where the hibernaculum was located, and peripheral areas. Space utilization and distribution of activities during the season were analysed. Some activities took place exclusively in the centre area while peripheral parts were used for foraging. This latter increased in Aug. and decreased in Sep., marmots tending to remain more and more at the centre area as the hibernation period approached.     

Is there an optimal number of helpers in Alpine marmot family groups?

The consequence of helping behavior on breeders fitness is stillcontroversial. We used multivariate analyses to investigatefor the effects of male and female subordinates on breeders'components of fitness in the Alpine marmot, Marmota marmota.We found that male and female subordinates, respectively, increasedand decreased juvenile survival during winter. Thus, we giveevidence that male subordinates should be considered as helpers,and that helpers provided breeders with immediate reproductivesuccess gains, whereas subordinates females were costly. Helpershad no positive effects on female body condition, on persistence(future survival) of dominants, and on future reproduction (occurrenceand size of a litter). Helpers thus did not provide breederswith delayed fitness benefits, and therfore, the load-lighteninghypothesis was not supported. On the contrary, helpers had delayedfitness cost for dominant males and, consequently, for dominantfemales. Immediate benefits counterbalanced by delayed costssuggested an optimal number of helpers in the family group bothfrom male and female perspectives. An optimality model wellpredicted the observed mean number of helpers in Alpine marmotfamily groups. Optimal numbers of helpers were slightly differentfor males and females, suggesting a potential conflict of interestbetween dominants. We finally discuss the possible mechanismsof helping that may explain the observed pattern in the Alpinemarmot.     

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.