Comparative analyses of survival senescence by using life tables have identified generalizations including the observation that mammals senesce faster than similar-sized birds. These generalizations have been challenged because of limitations of life-table approaches and the growing appreciation that senescence is more than an increasing probability of death. Without using life tables, we examine senescence rates in annual individual fitness using 20 individual-based data sets of terrestrial vertebrates with contrasting life histories and body size. We find that senescence is widespread in the wild and equally likely to occur in survival and reproduction. Additionally, mammals senesce faster than birds because they have a faster life history for a given body size. By allowing us to disentangle the effects of two major fitness components our methods allow an assessment of the robustness of the prevalent life-table approach. Focusing on one aspect of life history - survival or recruitment - can provide reliable information on overall senescence.

In most social ungulate species, males are larger than females and the sexes live in separate groups outside the breeding season. It is important for our understanding of the evolution of sociality to find out why sexual segregation is so widespread not only in ungulates but also in other mammals. Sexual body size dimorphism was proposed as a central factor in the evolution of sexual segregation in ungulates. We tested three hypotheses put forward to explain sexual segregation : the predation-risk, the forage-selection, and the activity budget hypothesis. We included in our analyses ungulate species ranging from non-dimorphic to extremely dimorphic in body size. We observed oryx, zebra, bighorn sheep and ibex in the field and relied on literature data for 31 additional species. The predation-risk hypothesis predicts that females will use relatively predator-safe habitats, while males are predicted to use habitats with higher predation risk but better food quality. Out of 24 studies on different species of ungulates, females and their offspring chose poorer quality but safer habitat in only eight cases. The forage-selection hypothesis predicts that females would select habitat based on food quality, while males should prefer high forage biomass. In fact, females selected higher quality food in only six out of 18 studies where males and females segregated, in eight studies there was no difference in forage quality and in four studies males were in better quality habitat. The activity budget hypothesis predicts that with increasing dimorphism in body size males and females will increasingly differ in the time spent in different activities. Differences in activity budgets would make it difficult for males and females to stay in mixed-sex groups due to increased costs of synchrony to maintain group cohesion. The predictions of the activity budget hypothesis were confirmed in most cases (22 out of 23 studies). The heavier males were compared to females, the more time females spent foraging compared to males. The bigger the dimorphism in body mass, the more males spent time walking compared to females. Lactating females spent more time foraging than did non-lactating females or males. Whether species were mainly bulk or intermediate feeders did not affect sexual differences in time spent foraging. We conclude that sexual differences in activity budgets are most likely driving sexual segregation and that sexual differences in predation risk or forage selection are additive factors.

In many mammals, males and females live solitarily or in separate groups outside the breeding season. Sexual segregation is widespread in ungulates, but also occurs in whales, seals, monkeys, macropods, elephants, sh and bird species. What causes segregation by sex is still poorly understood, despite intense research done mainly on different ungulate species. In most species studied, males were clearly larger than females. The evolution of sexual dimorphism in body size has largely been attributed to sexual selection and mating strategies. While the consequences of body-size differences on energy requirements and metabolic rates received most attention, studies on consequences of sexual body-size differences on behavior are lacking. This review emphasizes the importance to study a wide range of social mammals with a different or no degree of sexual segregation and sexual dimorphism in body size; something which has not been done and has greatly limited our ability to test alternative hypotheses. More emphasis has to be put on the study of activity budgets, sociality and habitat choice of non-dimorphic species to explain the evolution of permanent territoriality, of long-term bonds of male-female pairs and of the occurrence of mixed-sex groups (adults), as well as its absence in dimorphic species. We review ve hypotheses proposed to explain sexual segregation, discuss alternative outcomes and predictions for each hypothesis, suggest alternative explanations for the evolution of sexual segregation and mating systems, and indicate new and important directions for research. We conclude that a phylogenetic comparison of behavior of a wide range of ungulates and other mammals will be needed to solve the enigma of sexual segregation.

The Trivers-Willard theory proposes that the sex ratio of offspring should vary with maternal condition when it has sex-specific influences on offspring fitness. In particular, mothers in good condition in polygynous and dimorphic species are predicted to produce an excess of sons, whereas mothers in poor condition should do the opposite. Despite the elegance of the theory, support for it has been limited. Here we extend and generalize the Trivers-Willard theory to explain the disparity between predictions and observations of offspring sex ratio. In polygynous species, males typically have higher mortality rates, different age-specific reproductive schedules and more risk-prone life history tactics than females; however, these differences are not currently incorporated into the Trivers-Willard theory. Using two-sex models parameterized with data from free-living mammal populations with contrasting levels of sex differences in demography, we demonstrate how sex differences in life history traits over the entire lifespan can lead to a wide range of sex allocation tactics, and show that correlations between maternal condition and offspring sex ratio alone are insufficient to conclude that mothers adaptively adjust offspring sex ratio.

Multiple mating by females is common in many mammalian species, often resulting in mixed paternity litters. In such mating systems, mating order, male age, and male body mass frequently play an important role in determining male reproductive success. We tested for these effects on male reproductive success in Columbian ground squirrels (Urocitellus columbianus). The mating activity of estrous females was observed, and the occurrence of sperm precedence was tested using microsatellites to determine paternity in a total of 147 litters (434 offspring), including 110 litters (334 offspring) where the mating position of individual males was determined. Females mated with up to 8 males per litter, whereas paternity analyses revealed that only the first 5 males to mate actually sired offspring. The number of offspring sired significantly decreased with position in the mating sequence, showing a strong first male advantage. The extent of this advantage diminished with an increasing number of male mating partners, indicating that sperm competition plays an important role. A male's position in the females' mating sequences was not consistent within and across seasons, suggesting that individual males did not follow distinct reproductive strategies. Rather, males of intermediate age were more successful than young and old males, when corrected for age effects; heavier males were more likely to mate first. We conclude that males gain the largest part of their seasonal reproductive output from mating first with a female due to a pronounced first male advantage but gain considerable additional fitness from mating with additional, already mated females.