The results of the team's analyses were spectacular. All four monkey species showed drops in abundance relating to large-scale climate fluctuations. Even though the monkey populations were separated by large distances, the three fruit-eating species had synchronous responses to large-scale warming. During El Niño warming events, trees produced more fruit than usual. Then, during the subsequent La Niña cooling events, the trees produced much less fruit, resulting in a local scarcity or even famine.
Some ecologists have speculated that high production of fruit during El Niño events may have been triggered by the increase in solar radiation, despite lower-than-usual rainfall. However, high productivity during an El Niño event might also use up the stored reserves of the trees, which would have difficulty recovering during the subsequent La Niña events, when weather was wet, cloudy, and cool. This mechanism would explain why the fruit-eating monkeys showed a delayed response to the El Niño events after a lag of one or two years.
Howler monkeys also showed declines with warm and dry El Niño events, but their population fall was out of sync with that of the fruit-eating species. The mechanism is not yet clear, but Wiederholt has some ideas. She notes, "Primate researchers have seen elevated adult female mortality and lowered birthrates among red howlers in drought years. Since leaf flush often occurs at the start of the wet season, a prolonged dry season might delay the availability of this resource for the howlers and perhaps cause them nutritional stress."
Warmer temperatures also may cause leaves -- the howlers' primary food -- to mature faster, which would accelerate the leaves' acquisition of toxins and other chemical defenses against monkeys. The factor that the scientists found was most influenced by changes in climate was the monthly maximum number of tree species that were fruiting. Climate changes also were highly correlated with the monthly maximum number of species that were flowering and with annual seed production. The length of the dry season also was highly correlated with annual flower production. Thus, vegetation responses to climatic conditions substantially altered the food resources available to primates, which in turn influenced the decline or rise in monkey abundance.
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