AGE-SPECIFIC SURVIVAL IN FIVE POPULATIONS OF UNGULATES: EVIDENCE OF SENESCENCE

Methodological problems in describing patterns of senescence(definition) in wild populations have until recently impeded progress in understanding the evolution of a process that decreases individual fitness. We investigated age- and sex-specific survival in five populations of three species of ungulates (roe deer, Capreolus capreolus; bighorn sheep, Ovis canadensis; and isard, Rupicapra pyrenaica), using recent statistical developments of capture–mark–recapture models and long-term (12 to 22 yr) data on marked individuals. The yearly survival of females aged 2–7 yr was remarkably similar and very high (92–95%) in all five populations. Survival of adult males varied among species and populations. Survival decreased from 8 yr onward for both sexes in all populations, suggesting that senescence was a common phenomenon. Male survival was lower than female survival, and the gender difference increased with age. The extent of sex differences in survival was related neither to sexual dimorphism in mass nor to the level of polygyny, suggesting that species differences in social behavior, particularly mating system and the level of male–male aggression, may be more important than simply the level of polygyny in explaining sexual differences in survival. Our results underline the advantages of long-term monitoring of marked individuals for the study of evolutionary ecology.