Wild Bornean orangutan (Pongo pygmaeus wurmbii) feeding rates and the Marginal Value Theorem. Issue 10 (5th August 2020)

• Record Type: Journal Article
• Authors: DiGiorgio, Andrea L.; Upton, Elizabeth M.; Susanto, Tri Wahyu; Knott, Cheryl D.
• Is Part Of: American journal of primatology. Volume 82:Issue 10(2020)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2020-08-05
• Subjects: ape -- ecology -- nutrition -- Optimal Foraging Theory -- primate; Primates -- Periodicals; Primates -- Périodiques; 599.8
• DOI: 10.1002/ajp.23183
• Languages: English
• ISSNs: 0275-2565

Abstract

Abstract: The Marginal Value Theorem (MVT) is an integral supplement to Optimal Foraging Theory (OFT) as it seeks to explain an animal's decision of when to leave a patch when food is still available. MVT predicts that a forager capable of depleting a patch, in a habitat where food is patchily distributed, will leave the patch when the intake rate within it decreases to the average intake rate for the habitat. MVT relies on the critical assumption that the feeding rate in the patch will decrease over time. We tested this assumption using feeding data from a population of wild Bornean orangutans ( Pongo pygmaeus wurmbii ) from Gunung Palung National Park. We hypothesized that the feeding rate within orangutan food patches would decrease over time. Data included feeding bouts from continuous focal follows between 2014 and 2016. We recorded the average feeding rate over each tertile of the bout, as well as the first, midpoint, and last feeding rates collected. We did not find evidence of a decrease between first and last feeding rates (Linear Mixed Effects Model, n = 63), between a mid‐point and last rate (Linear Mixed Effects Model, n = 63), between the tertiles (Linear Mixed Effects Model, n = 63), nor a decrease in feeding rate overall (Linear Mixed Effects Model, n = 146). These findings, thus, do not support the MVT assumption of decreased patch feeding rates over time in this large generalist frugivore. Abstract : Feeding rates of wild Bornean orangutans do not decrease during a feeding bout, as measured by the average feeding rate in each third (tertile) of the bout. Average feeding rates (items per minute) of nine orangutans are presented over the first, second, and third tertiles of a feeding bout. This finding is contrary to the critical assumption of Marginal Value Theorem (MVT), that feeding rates decrease over patch residence, and suggests that MVT is not sufficient to explain all patch departure choices in wild Bornean orangutans. Research Highlights: Wild Bornean orangutan feeding rates do not decrease during a feeding bout. The critical assumption of Marginal Value Theorem, that feeding rates decrease as patch residence, and thus patch depletion, increases, is not upheld in this population of wild Bornean orangutans.