Effects of Senescence on Reproduction and Behavior in Bottlenose Dolphins

Abstract

Senescence is the degenerative change in function of all organ systems with age and is accompanied by precipitous declines in survival and fertility. In a few species, reproductive senescence culminates in menopause, the complete cessation of reproduction and a significant post-reproductive lifespan. Adaptive hypotheses for the evolution of menopause have gained recent attention. Investigating whether pathways essential to these hypotheses are also found in non-menopausal species will reveal whether these processes exclusively drive the evolution of menopause. Using a 35-year study of the bottlenose dolphins of Shark Bay, Australia, I investigate how aging affects reproduction and behavior in the context of these evolutionary hypotheses.

In Chapter 1, I examine two components of reproductive senescence: fertility and maternal-effect senescence. With increasing maternal age, calf survival decreased, while lactation period increased. Interbirth intervals increased regardless of calf mortality, indicating interactions between fertility and maternal-effect senescence. Of calves that survived to weaning, last-born calves weaned latest, evidence of terminal investment.

In Chapter 2, I investigate whether senescence impacts behavior in late adulthood. While time spent cycling increases in late adulthood, time spent with adult males does not change, nor do activity budgets, time spent alone, or average group sizes. Time spent socializing decreases in late adulthood when females have a dependent calf, suggesting that aging mothers regulate energy budgets in late adulthood.

In Chapter 3, I explore the nature of intergenerational relationships in the context of the “grandmother” and “reproductive conflict” hypotheses for the evolution of menopause. Females do not affect survival or weaning of grandoffspring or reproductive success of daughters. However, they accelerate their daughters’ maturation, with an earlier age of first birth, dependent on grandoffspring survival. Second-generation offspring born into reproductive conflict exhibit reduced survival. These results indicate that matrilineal investment and reproductive conflict occur simultaneously in a non-menopausal species, suggesting that these processes alone do not drive the evolution of menopause. This work demonstrates how aging impacts mammalian reproduction and behavior and contributes to our understanding of the evolution of reproductive life histories