In studies published in 2002 and 2004, Dudley reported the alcohol content in wild fruits of the Astrocaryum palm, which Panama's mantled howler monkeys eat. The unripe fruits contain zero ethanol, ripe hanging fruits contain 0.6%, ripe fallen fruits contain 0.9% and over-ripe fallen fruits contain 4.5% ethanol (by weight) on an average.
In Dudley's view, such dietary consumption of alcohol likely shaped the evolution of fruit-eating primates for several million years. The ethanol wafting from fermenting fruits may have been a cue to locate sugary rewards in a vast forest. Besides, ethanol may itself be a source of calories and perhaps even stimulate the appetite.