Variation in mantled howler monkey (Alouatta palliata) roaring behavior across ecologically diverse sites and seasons

Howler monkeys are well-known for producing long calls ("roars") as a means of long-distance communication. The ecological resource defense hypothesis posits that animals roar to defend food resources; thus monkeys should modify roaring behavior across seasons and between sites with varying food resource abundance. We investigated mantled howler monkey roaring behavior between the wet and dry seasons at a large, continuous lowland wet forest (La Selva Research Station) in northeastern Costa Rica as well as between La Selva and a nearby small forest fragment (La Suerte Biological Research Station, LSBRS). We quantified a proxy of resource abundance at each site by measuring tree diameter at breast height (DBH) and canopy cover and used all-occurrences sampling to collect data on roaring behavior. We calculated number of roars per minute, length of roar bouts, and roar bouts per hour. Number of roars per minute was significantly higher at La Selva during the wet season than the dry season. With respect to comparisons across sites, both canopy cover and DBH were significantly higher at La Selva. Correspondingly, the length of roar bouts and the number of roars per minute were significantly higher at La Selva. Roar bouts per hour, however, were significantly higher at LSBRS, which may be explained by the very high population density at LSBRS. The results for our comparison demonstrate that some features of howler monkey roaring behavior vary along with ecological conditions and that other non-ecological factors may also drive elements of roaring.