Even mortality patterns of the two sexes in a polygynous, near‐monomorphic species: is there a flaw?

Abstract

The chamois Rupicapra rupicapra has been termed a highly polygynous species, with a great male competition for mating. If so, a lower survival should be expected for the male sex. From 1986 to 2000, 1801 carcasses of chamois were collected in the Maritime Alps Regional Park, Italy, where a protected, healthy, stable population of chamois occurred (c. 12 individuals 100 ha À1 ). Each year, population structure from carcasses was consistent with that from the count carried out on the preceding year on live individuals. Demographic features (assessed from mortality data, as well as from live counts) showed a balanced age structure and a good adult survival (10% individuals older than 11 years). Mortality peaks showed a cyclic pattern of 3-4 years. Winter severity and local density affected survival, with no significant difference between sexes. The number of carcasses was dependent on the combination of snow depth and mean temperature, in winter. Both sexes showed nearly the same survivorship curves, with a quite similar life expectancy in the first year (males = 6.8 years, females =7.0 years), and the same maximum age at death (16 years), as it may be expected in a monomorphic, monogamous species. This is, however, a rare event among polygynous species, with a high male competition for females and male juvenile dispersion, which normally affect male survival. The similar adult survival of the two sexes could be explained by comparable energetic costs and risks for reproduction, or through greater fat reserves put on by males, before the rut, which may lower their winter mortality.

Effects of an exceptionally snowy winter on chamois survival

“…The very low survival of PNGP males was likely partly due to their advanced age, but it was much lower than the 75-80% survival reported for old male Pyrenean chamois (Loison et al 1999). Overall, sex-and age-specific survival data for PNAM and PNM confirm earlier reports that adult chamois mortality does not differ substantially according to sex (Loison et al 1999;Bocci et al 2010).…”

“…We examined its consequences for the survival of adult (2-9 years old) and senescent (10 years and older) chamois in three populations. We expected that the harsh winter would substantially lower the survival of senescent but not of prime-aged females and that males would not suffer much higher mortality than females, as previously reported for this species (Loison et al 1999;Bocci et al 2010). …”

“…Consequently, males should be less capable than females to buffer yearly variation in environmental conditions. For chamois (Rupicapra rupicapra), however, there appears to be very little sexual difference in adult survival, despite moderate size dimorphism and possibly strong polygyny (Loison et al 1999;Bocci et al 2010).…”

Unbiased sex-specific survival in Alpine chamois

“…We monitored 35 animals older than 15 years, and 20 animals older than 18 years, suggesting that chamois are an unusually long-lived ungulate. The high maximum longevity of individuals within our population (21 years for males, 22 years for females), compared to previously recorded figures of 17 years in both sexes (Bocci et al 2010) seems unlikely to be due to methodological reasons, as in Table 4 Mean estimate, standard error and 95% CI of sex-and age-specific survival rates calculated from models #4 [˚(sex + time) p(age · sex)] and #7 [˚(age + time) p(age · sex)] for 116 Alpine chamois over 13 years (1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008) both cases age was estimated by counting growth rings. Likewise, Loison et al (1994) found high survival in female chamois older than 3.5 years (0.95, 95% CI = 0.93-0.99), suggesting that survival senescence in chamois may be weaker than in other ungulates.…”

“…6% in January, Garel et al 2009; 4% in spring, Rughetti and Festa-Bianchet 2011). To date only two studies, based on life tables analysis, investigated the sex-specific survival of Alpine chamois (Schröder 1971;Bocci et al 2010), producing conflicting results.…”