Senescence impacts reproduction and maternal investment in bottlenose dolphins

Abstract

Reproductive senescence is evident across many mammalian species. An emerging perspective considers components of reproductive senescence as evolutionarily distinct phenomena: fertility senescence and maternal-effect senescence. While fertility senescence is regarded as the ageing of reproductive physiology, maternal-effect senescence pertains to the declining capacity to provision and rear surviving offspring due to age. Both contribute to reproductive failure making it difficult to differentiate between the two prenatally in the wild. We investigated both components in a long-lived mammal with prolonged maternal care through three parameters: calf survival, interbirth interval (IBI) and lactation period. We provide clear evidence for reproductive senescence in a wild population of bottlenose dolphins () using 34+ years of longitudinal data on 229 adult females and 562 calves. Calf survival decreased with maternal age, and calves with older mothers had lower survival than predicted by birth order, suggesting maternal-effect senescence. Both lactation period and IBIs increased with maternal age, and IBIs increased regardless of calf mortality, indicating interactions between fertility and maternal-effect senescence. Of calves that survived to weaning, last-born calves weaned later than earlier-born calves, evidence of terminal investment, a mitigating strategy given reduced reproductive value caused by either components of reproductive senescence.

Quality and quantity of genetic relatedness data affect the analysis of social structure

“…These models included data from a subset of 176 individuals out of the 272 total, 92 females and 84 males, who had at least 35 sightings post‐weaning. Individuals in the population are typically weaned at around four years of age, but with a range of 2–8 years (Karniski et al, ; Mann et al, ). We selected an inclusion threshold of 35 sightings in order to accurately capture the degree of spatial overlap between individuals (see Supporting Information for details), as well as each individual's position in the social network (Stanton & Mann, ).…”

Every scar has a story: age and sex-specific conflict rates in wild bottlenose dolphins

“…The age of a dolphin is determined by known or estimated birthdates, which are established based on sightings as a calf, physical and behavioral characteristics (Mann and Smuts 1999), and/or through ventral speckling (Krzyszczyk and Mann 2012). Calves are defined as still nursing; the average weaning age is 4 years (Mann et al 2000;Karniski et al 2018). The juvenile period is defined post-weaning to age 10, the earliest age at first birth (Karniski et al 2018).…”

“…Calves are defined as still nursing; the average weaning age is 4 years (Mann et al 2000;Karniski et al 2018). The juvenile period is defined post-weaning to age 10, the earliest age at first birth (Karniski et al 2018). The average age of first birth is 13 years (Mann 2019).…”

“…The reasons behind this may be multifold. Sexual coercion may be higher towards younger females because they have higher calving success than older females (Karniski et al 2018). Alternatively, or in addition, older females may have learned how to respond to, counter, or avoid sexual coercion, and thus experience fewer instances of aggression .…”

“…We also predict that juvenile females would have more tooth rake scars than adult females from their encounters with juvenile males. Although adult females receive significantly more tooth rake scars when cycling than other reproductive states (Scott et al 2005), cycling periods are relatively infrequent given the long lactation period (Mann et al 2000;Karniski et al 2018).…”