Reproductive senescence in free-ranging North American elk Cervus elaphus Cervidae

Abstract

Successful production of calves is necessary for growth of North American elk (Cervus elaphus Linnaeus 1758) populations, but few studies have evaluated age-related effects on both the conception and survival of a calf to weaning in multiple free-ranging populations. Conception and survival of calves to weaning were both affected by maternal age, with old (age 9 and older) females showing reproductive senescence as compared to prime-aged (ages 2–8) females despite achieving similar or greater size and condition. Reproductive senescence in our free-ranging populations ultimately resulted in old females weaning fewer calves (0.42 calves/female) than did prime-aged females (0.64 calves/female). Other factors, especially maternal size, also influenced conception and survival to weaning, and these interacted with age in a consistent manner, i.e. larger females or females in better condition were more likely to conceive and successfully wean calves within each age class. Female age structure receives less consideration in ungulate management than does male age structure, despite demonstrated impacts on population productivity of multiple species because of reproductive senescence. Because of the large proportion of individuals in senesced age classes in elk populations, low productivity in populations may simply reflect female age structure, rather than other frequently hypothesized factors.

Incorporating vital rates and harvest into stochastic population models to forecast elk population dynamics

“…Young animals frequently improve reproductive performance with age because of a combination of increased experience with successive reproductive events and the continued somatic investment after age at first reproduction, a pattern consistent with increasing pregnancy rates with age for young elk documented here (Forslund and Pärt 1995, Blas et al 2009. Similarly, the decline in pregnancy rates for older animals estimated here is consistent with senescence-related processes diminishing the ability to meet and recover from the demands of annual reproduction (Lemaître andGaillard 2017, Bender andPiasecke 2019). In contrast, survival is expected to decrease with age from the age at first reproduction as residual reproductive value declines with age and a trade-off between survival and reproduction reduces the energetic reserves dedicated to somatic maintenance each year (Kirkwood andRose 1991, Nussey et al 2013).…”

“…Understanding age-related variation in the vital rates of iteroparous, long-lived vertebrates such as elk has received considerable attention from life-history theory and empirical studies, and our estimates of age-specific probabilities of pregnancy and survival are largely confirmatory (Gaillard et al 1998, Festa-Bianchet et al 2017, Lemaître and Gaillard 2017, Bender and Piasecke 2019. Young animals frequently improve reproductive performance with age because of a combination of increased experience with successive reproductive events and the continued somatic investment after age at first reproduction, a pattern consistent with increasing pregnancy rates with age for young elk documented here (Forslund and Pärt 1995, Blas et al 2009.…”

“…An unaddressed question in elk population dynamics is if there are consequences of harvest to the population trajectories of elk beyond the reduction in adult survival if some fraction of the harvest is additive. There is ample evidence that pregnancy rates of elk demonstrate age-related variation characterized by lower pregnancy rates for younger animals, higher rates for middle-aged animals, and senescence-related lower rates for older animals (Raithel et al 2007, Proffitt et al 2014, Bender and Piasecke 2019. Such variation can set the stage for impacts to population-level recruitment rates from a shifted age structure, although the significance of the effects on population dynamics are not well understood (Langvatn and Loison 1999, Coulson et al 2001, 2004, Shelton et al 2015, Bender and Piasecke 2019.…”

Understanding Reproductive Aging in Wildlife to Improve Animal Conservation and Human Reproductive Health

“…Interestingly, this hormonal decline is not observed in aging female cheetahs (Acinonyx jubatus) after the age of 9, which is even older than the average life expectancy (Crosier et al, 2011). Surprisingly, aging red deer (Cervus elaphus) stags show a faster decline in annual breeding success than females that went through successive gestations and lactations (Nussey et al, 2009;Bender and Piasecke, 2019). In wild red wolves (Canis rufus), reproductive aging in males also is observed while there is no evidence for reproductive aging in wild female wolves (Sparkman et al, 2017).…”

“…Surprisingly, aging red deer ( Cervus elaphus ) stags show a faster decline in annual breeding success than females that went through successive gestations and lactations ( Nussey et al, 2009 ; Bender and Piasecke, 2019 ). In wild red wolves ( Canis rufus ), reproductive aging in males also is observed while there is no evidence for reproductive aging in wild female wolves ( Sparkman et al, 2017 ).…”