Long‐term studies of bighorn sheep and mountain goats reveal fitness costs of reproduction

Abstract

Fitness costs of reproduction are expected when resources are limited. Costs drive the evolution of life‐history strategies and can affect population dynamics if females change their allocation of resources to reproduction. We studied fitness costs of reproduction in mountain ungulates in Alberta, Canada. We monitored two populations of bighorn sheep (Ovis canadensis) for 44 and 30 years, and one of mountain goats (Oreamnos americanus) for 30 years. Both species are highly iteroparous. Heterogeneity in individual reproductive potential makes fitness costs of reproduction difficult to detect and quantify without manipulations. In capital breeders, individual differences can be partly accounted for by considering body mass and other correlates of reproductive potential. Long‐term monitoring can reveal costs that only manifest under stressful conditions such as disease or resource scarcity. Despite individual differences in reproductive potential, we detected fitness costs of reproduction in females. Costs, in terms of mass gain and survival, are almost entirely born by subsequent offspring, as mothers prioritize their own maintenance and survival. Costs are greater for primiparous females, decrease with increasing body mass and increase as resource availability declines, and sons are costlier than daughters. Costs may increase for senescent females that appear to reduce allocation to reproduction. In bighorn sheep, costs mostly involve reduced mass gain and lower survival of subsequent offspring. In mountain goats, costs include reductions in mass gain, subsequent fecundity and juvenile survival. In males, fitness costs derive mostly from attempts to reproduce rather than from siring success and likely depend upon individual competitiveness. In the absence of selective harvests, dominant males may enjoy high fitness and possibly lower costs compared to subordinates. The conservative reproductive tactic of mountain ungulate females likely explains why density dependence mostly involves later primiparity and lower recruitment, but rarely affects adult survival. Future research will seek to better account for heterogeneity in reproductive potential, assess cumulative reproductive costs and investigate the potential effects of fathers on maternal allocation tactics.

Trade‐offs between age‐related breeding improvement and survival senescence in highly polygynous elephant seals: Dominant males always do better

“…We provide valuable insight into male southern elephant seal demography, and polygynous male life history in general, which has received much less attention than females (Pistorius, Bester, & Kirkman, ; Pistorius, Bester, & Taylor, ). Our findings support recent studies of other polygynous systems that early male recruits do not pay a cost of attending breeding events (Markussen et al, ), and that breeding males pay a cost of attempting to be successful rather than actually being successful (Festa‐Bianchet, Côté, Hamel, & Pelletier, ).…”

“…According to the hazard function, mortality increased at the oldest age classes, substantiating evidence for actuarial senescenceexp(− (b x age) c − (b x age) 1 c − (d x age))Costs to breeder survival likely resulted from attempting to obtain dominance rather than from the consequences of breeding successfully (i.e. 'the cost of trying';Festa-Bianchet, 2012;Festa-Bianchet et al, 2019), as we failed to find evidence of higher breeding costs in dominant…”

“…Costs to breeder survival likely resulted from attempting to obtain dominance rather than from the consequences of breeding successfully (i.e. ‘the cost of trying’; Festa‐Bianchet, ; Festa‐Bianchet et al, ), as we failed to find evidence of higher breeding costs in dominant breeders compared to subordinate breeders. Despite substantial energetic allocation to breeding through sexual competition, several studies have found that successful breeding does not incur a cost to survival in polygynous males (Festa‐Bianchet, ).…”

Multi‐event capture‐recapture analysis in Alpine chamois reveals contrasting responses to interspecific competition, within and between populations

“…Loison, Jullien, et al, 1999, on chamois; Coulson et al., 2001, on Soay sheep Ovis aries ; Hansen et al., 2019, on reindeer Rangifer tarandus ). In contrast, according to the demographic buffering hypothesis (Hilde et al., 2020; Morris & Doak, 2004), survival of the prime‐aged class (2–12 years old) that has the highest impact on population growth rate of long‐lived species is expected to be partly canalized/buffered against environmental changes (Gaillard & Yoccoz, 2003) through an adjustment of the reproductive effort (Festa‐Bianchet, Côté, Hamel, & Pelletier, 2019). As we found a negative effect of interspecific competition on old and young ages and not on survival of prime‐aged females, it is likely that the impact of red deer population size on chamois population growth rate is limited.…”

Trade-off between offspring mass and number: the lightest offspring bear the costs

“…maternal size, offspring sex, litter sex ratio and population density index [12,25], were included as fixed effects. To account for the potential masking effect of female size on life-history trade-offs [6,33], we included an interaction between maternal size and litter size. Random effects included year of capture and litter identity nested in maternal identity.…”