Experimental evidence of cost of lactation in a low risk environment for a long‐lived mammal

Abstract

In a previous experiment we have documented that organisms adopt a risk‐sensitive reproductive allocation when summer reproductive investment competes with survival in the coming winter (Bårdsen et al. 2008). This tradeoff is present through autumn female body mass, which acts as an insurance against unpredictable winter environmental conditions. We tested this hypothesis experimentally on female reindeer experiencing stable and benign winter feeding conditions. Additional supplementary feeding and removal of newborns represented two sets of experimental manipulations. Females in the supplementary feeding group increased more in winter body mass relative to control females. This manipulation, however, did not have any effect on summer body mass development for neither females nor offspring, but we found a positive effect of feeding on offspring birth mass for smaller females. In contrast, offspring removal did have a positive effect on summer body mass development as females in this group were larger in the autumn relative to control females. In essence, we documented two immediate effects as: (1) supplementary feeding did have a positive effect on spring body mass for smaller females; and (2) offspring removal did increase the female summer somatic growth as this had a positive effect on female autumn body mass. Additionally, we tested for lagged effects, but we could not document any biologically significant effects of neither manipulation in the coming spring. The fact that we only found rather weak effects of both manipulations was as expected for risk sensitive individuals experiencing benign environmental conditions over many years.

Observational evidence of risk-sensitive reproductive allocation in a long-lived mammal

“…In a recent study from the same study region, we found that age and body mass were highly correlated for young (·4 year) reindeer, but not for prime-aged individuals (5-13 year: Bårdsen et al 2009). We did not include females older than 13 years old so that decreased performance due to senescence was not likely to occur (see Supplement S1).…”

“…First, Tveraa et al (2003) found that especially harsh winter conditions can greatly reduce adult survival and reproductive success the following spring and summer. Second, when late winter feeding conditions are improved adult females increase their late winter body mass relative to early winter body mass (Fauchald et al 2004;Bårdsen et al 2009). This gain in body mass is, however, rapidly lost during the calving season as the diVerence in body mass is not present in the summer (Bårdsen et al 2008(Bårdsen et al , 2009Fauchald et al 2004).…”

“…Second, when late winter feeding conditions are improved adult females increase their late winter body mass relative to early winter body mass (Fauchald et al 2004;Bårdsen et al 2009). This gain in body mass is, however, rapidly lost during the calving season as the diVerence in body mass is not present in the summer (Bårdsen et al 2008(Bårdsen et al , 2009Fauchald et al 2004). It has, thus, been concluded that female reindeer regulate their body mass down to some minimum threshold during spring in order to take care of their newborns when the risk of starvation is low (Bårdsen et al 2008;Fauchald et al 2004).…”

Combined effects of long‐term feeding, population density and vegetation green‐up on reindeer demography

“…Our results are nonetheless in line with previous findings showing no difference in body condition (i.e., body fat) between supplementary fed and free-ranging elk in winter (Bailey 1999). Similarly, in reindeer, positive effects of winter supplementary feeding were reported on spring but not summer or autumn body masses (Bå rdsen et al 2009). …”

“…Thus, body mass plays a central role for reindeer life histories and population dynamics. The effects of density and environmental fluctuations on reindeer are mostly food mediated, and supplementary feeding has been used to improve late winter condition (Bå rdsen et al 2008(Bå rdsen et al , 2009). In fact, supplementary feeding of ungulates is a common management practice used to buffer the effect of environmental stochasticity on body mass by protecting animals from winter starvation while maintaining high population densities (Fauchald et al 2004a, Putman andStaines 2004).…”