El Niño conditions in the Pacific Ocean wreak havoc on monkey populations, either in the midst of the periodic hot and dry spells or in their chilly aftermath, according to the results of a new study.

The study, published in the October 28 issue of Biology Letters, explored the correlation of El Niño years—when cyclical oceanic-atmospheric oscillations warm the tropical Pacific to above-normal temperatures, affecting global weather patterns—with fluctuations in monkey populations and the abundance of their food resources. It is the first report on the impact of El Niño on monkey species that live in Central and South America, many of which are threatened or endangered. The study focused on four species: red howler monkeys (Alouatta seniculus), which subside primarily on leaves, along with two species of woolly monkeys (Brachyteles arachnoides and Lagothrix lagotricha), and a variety of spider monkey (Ateles geoffroyi), all of whose staple diet is fruit.

"I thought it was interesting when I got [the] results that the howlers were declining in the same year [as an El Niño], and the other three species were declining the same year" that followed El Niño events, says Ruscena Wiederholt, a graduate student of ecology at The Pennsylvania State University's Huck Institutes of the Life Sciences, and first author on the study.

To chart population levels, Wiederholt and her adviser, biologist Eric Post, used data from annual surveys of monkeys from the last several decades. Four groups of primatologists had taken surveys at sites in Brazil, Venezuela, Colombia and Panama. In a period of extreme decline a population of 20 monkeys could lose as many as four members in a year.

Wiederholt and Post also called on another set of surveys dating back to 1987: the annual abundance of fruits and plants on Barro Colorado Island in Panama. These surveys gave the researchers an idea of how abundant food would have been for monkeys, because the vegetation there is representative of the areas where the population surveys were done, Wiederholt says.

As other ecologists have found, Wiederholt and Post saw that fruit supplies went from plentiful during a hot and dry El Niño year to scarce in the cold and wet La Niña period (the cooling phase of the El Niño oscillation) that typically follows. This pattern coincided with rises and falls in populations of fruit eaters. Conversely, whereas leaves were scarce during El Niños, they flourished in the wet periods, as did the populations of leaf-eating howlers.

These findings suggest that El Niño–related weather changes affect Central and South American monkey populations, and point to differences in food abundance as the cause of the population changes. Overall, the numbers of fruit-eaters dropped more than those of leaf-eaters, which Wiederholt attributes to slower reproduction rates.

Although the researchers only studied four sites, Wiederholt expects that the food supply–to-population pattern would hold for other areas of Central and South America. "I would imagine it would be widespread because El Niño has widespread effects on these areas," she says.

The study also suggests that multiple groups of each fruit-consuming species were declining at the same time, one year after the end of an El Niño period. Such synchronized contraction could pose an extinction threat: Whereas if some groups of a threatened species remained more numerous, "the larger populations could send out migrants to 'rescue' the declining populations and save them from going extinct," Wiederholt notes. Instead, each group is shrinking in synchrony, jeopardizing their ability to reproduce and survive.

This study also predicts that the drops in monkey populations could worsen if global warming leads to more intense El Niño weather patterns.