Between 4 and 16 January 1996, during a period of cool weather, we studied the emergence and foraging behavior of Molossus ater at a site near Akumal, in the Yucatan Peninsula in Mexico. The bats, a colony of at least 32 individuals, roosted in a north-facing cinder block wall, and emerged about sunset. Emerging bats were usuary clustered in time, while those returning usually were not. Radio-cracking revealed that the bars foraged for short periods (mean 26.8 min) and captures of returning individuals indicated that 27 of 28 had fed, taking, on average, 4.4 g of insects, mainly hydrophilid beetles. On some nights, few or none of the radio-ragged bats emerged from the roost. Calculations concerning the costs of flight and roosting show that they were more than covered by the energy intake the bars achieved. Molossus ater have high aspect ratio (8.3-9.1) wings and high wing-loadings of 17.55-24.15 N/m(2). When searching for prey, these bats produce long (12.3 ms), narrowband(3.8 kHz), echolocation calls that sweep from 27.6-23.8 kHz. Energy was not limiting for these bats at the time of our study.
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