Energetic Investment in Travel by Black Howler Monkeys: Testing Push-Pull Dynamics.

Abstract

Objectives: Movement decisions reflect trade-offs between travel costs and expected benefits at the destination. We examined whether energetic investment in travel by black howler monkeys (Alouatta pigra), an energy-limited species, is driven by push factors (departure conditions), pull factors (destination characteristics), or their integration.

Materials and methods: We studied 12 groups in Campeche, Mexico (2005-2008), analyzing 7461 travel episodes from 59 adults (3747 observation hours). Energy expenditure was estimated using cost of transport models. We compared three model sets (push, pull, and combined) using mixed-effects modeling.

Results: Pull factors outperformed push and combined factors in explaining travel investment. Travel to unripe fruit trees required nearly twice the energy of travel to resting sites (492.4 vs. 245.0 J). Destination tree size increased costs by 11.9% per standard deviation increase in diameter, while travel to preferred food species required 5.7% less energy. Males expended 19.8% more energy per travel episode than females. Time of day showed a significant nonlinear effect, with costs decreasing asymmetrically across the day.

Discussion: Our findings suggest that black howler monkeys prioritize destination characteristics over departure conditions when investing energy in travel, indicating goal-directed rather than reactive movement despite their energy-minimizing ecology. These results underscore the importance of quantifying energetic trade-offs in travel decisions to understand how animals balance immediate costs against expected benefits, particularly in species where energy limitations constrain behavioral strategies.