1Departments of Anthropology, New Mexico State University, 2Department of Biology, New Mexico State University
Friday All day, Plaza Level
Group size, adult sex ratio, and the duration, timing, and frequency of vocalizations of the black howler (Alouatta pigra) were studied at two sites inBelize, La Milpa and the Baboon Community Sanctuary. Semi-deciduous lowland broadleaf forest occurs at La Milpa, contrasting with a mosaic of riverine and dry forest near agricultural land at BCS. Howler population densities are much higher at BCS than La Milpa, and spider monkeys occur only at La Milpa. We expected to find smaller group size and a 1:1 sex ratio at La Milpa with fewer intergroup howling bouts.
Excluding lone males, average group size is 6.55 (range 3-11, n=9) at BCS, and 3.5 at La Milpa (range 3-5, n=4). At La Milpa the ratio of females to males is 1:1, whereas it is 1.95:1 at BCS, an increase from the 1.3:1 ratio observed at the site from 1985-2001. Large group size at BCS may reflect both the difficulty young adult males have in establishing new groups (four lone males were seen at BCS compared to one at La Milpa), as well as an adaptation for successful intergroup defense.
Vocalizations at La Milpa primarily consisted of sunrise relays (5:50-6:30 AM) during which up to six groups were heard. Sunrise relays began even earlier at BCS, sometimes at 4:30 AM, and were of longer duration. Howling bouts resulting from intergroup encounters occurred at several times throughout the day and night at BCS indicative of the higher population density. Similar intergroup bouts rarely occurred at La Milpa.