In both humans and great apes it has been demonstrated that a female's reproductive cycle varies in response to energy balance. In particular, research has shown that levels of ovarian steroids, such as estrogen and progesterone, fall when diet quality is poor or in response to sustained physical exertion, potentially resulting in infertility. But while this pattern has been documented in humans and their closest relatives, the so-called ecological energetics hypothesis (EEH) has not been tested in any other non-human primate. Spider monkeys provide an excellent model to test this hypothesis because in contrast to many New World monkeys, they share a number of relevant traits with the great apes including non-seasonal breeding, extended reproductive parameters, and a high degree of food specialization. Previous research on spider monkeys has demonstrated a consistent pattern whereby females fail to conceive during times when ripe fruit abundance is low. To date, however, no study has adequately demonstrated a clear link between diet, activity levels, and reproductive condition. This research tests the EEH in free ranging black-handed spider monkeys on Barro Colorado Island, Panama. Diet quality is assessed by combining female spider monkey foraging behavior data with available information on local fruit abundance and nutritional content. These values are then compared to monthly changes in the reproductive cycles of known females.
Results from this study contribute to a better understanding of spider monkey reproductive biology and behavior and improve our understanding of how seasonal changes in resources impact primate evolution. Furthermore, by focusing on specific fruiting species important to spider monkeys the research has the potential to inform future conservation management decisions. Finally, this doctoral dissertation research project also contributes to the professional development of a female, Hispanic graduate student and will involve collaboration with Latin American researchers.
This project tested the ecological energetics hypothesis, which suggests that fertility in female primates is regulated by the interplay between diet quality and energy expenditure. Although this hypothesis has been tested for a small number of apes (including humans), it remains unclear and untested whether the ecological energetics hypothesis applies to any non-ape primate species. We tested this hypothesis in free ranging spider monkeys on Barro Colorado Island, Panama by evaluating the relationship among fruit availability and consumption, energy expenditure, and ovarian function. During a yearlong study period, we collected 1,130 hours of behavioral and ranging data on 8 adult female spider monkey subjects. During full-day observations, we were able to collect 206 fecal samples from our subjects for the extraction of ovarian and cortisol hormones. Respectively, these hormones indicate whether females are experiencing (a) periods of infertility or (b) energetic stress in response to drops in food abundance and/or increased travel. We also successfully collected 307 urine samples from our subjects and other members of the spider monkey group to test for negative energy balance via the production of ketones. Detection of urinary ketone bodies is indicative of carbohydrate shortage and/or impaired ability to metabolize carbohydrates. Therefore, ketones should only be present in urine samples when individuals are burning fat. Our preliminary analysis indicates that female spider monkey ranging patterns correlate with food availability and abundance. For example, during certain months of the dry season spider monkeys ranged further in search of food than during the wet season when food was most abundant. This may imply that female spider monkeys must travel further when resources are scarce in order to meet minimum nutritional requirements. We also found that spider monkeys produce higher amounts of ketone bodies during the dry season when food is less available, further indicating that nutritional stress is a concern for this population. Future analysis will explore the relationship between nutritional stress and fertility. From our eight subjects monitored during the yearlong study period, two females became pregnant and successfully conceived. We hope to determine if there are differences in behavioral and feeding strategies, such as access to higher quality foods, that allow some female spider monkeys to successfully conceive, while others do not. This study proposes to contribute to a better understanding of spider monkey reproductive biology and behavior with which we hope to improve our knowledge of primate ecology and evolution and contribute to the long-term health of this species.
