Mortality patterns in a protected population of isards (Rupicapra pyrenaica)
We considered variation in mortality over 8 years in a protected population of isards (Pyrenean chamois; Rupicapra pyrenaica) within the Pyrenean National Park using monthly systematic skull collection. After testing the stationary-type pattern of estimates of mortality of the living population, we (i) analysed the composition of a sample of dead animals in terms of age and sex; (ii) investigated temporal patterns of mortality and their relationship with winter severity; (iii) tested for sexual differences in mortality rates in this weakly sexually dimorphic species. Of 588 animals found dead from natural causes, 114 were kids (19.4%). No statistical differences existed in the composition of the sample according to sex (the ratio of males to females was 1/1.13). Age at death did not differ significantly between the sexes or among years. A positive relationship was found between winter severity (expressed as cumulative snowfall) and mortality rates for both kids and adults. During mild winters, the mortality rate was low (a mean of 38.4 carcasses per year versus 124 per year in severe winters) and was concentrated on the oldest animals, mainly males. Based on 303 skulls more than 1 year old, annual survivorship curves revealed a period of low mortality (prime age) extending to 9 and 10 years of age for males and females, respectively, followed by a period of increasing mortality (senescence). Our results are commented on in light of recent publications on the demography of populations of large herbivores.