By lowering the frequency of its voice, the isolated monkey makes itself heard over greater distances. It increases the chances that its groupmates hear it and call back, allowing it to figure out their location and catch up in little time.
This behavior was reported by German researchers at the German Primate Center (Deutsches Primatenzentrum or DPZ). They released their findings on the science journal PLOS One.
Like almost all primates, spider monkeys are social animals. They form groups called "troops" and communicate with each other through various calls and behaviors regarding the presence of food and water, potential predators or rival troops, and the like.
While the monkeys tend to stay within visual range of each other, they may lose track due to the thick vegetation of the jungles they call home. When that happens, the isolated animal resorts to a type of call described as a "whinny." (Related: Monkeys discovered to use complex grammatical structures in their language.)
Isolated spider monkeys use low-frequency whinnies to call for help over long distances
Earlier studies observed that separated spider monkeys changed the pitch of their whinnies. The DPZ researchers believed that the frequency level of the cry determined the extent to which the animal had gotten separated from its troop.
To test their theory, they paid a visit to the Lacandon Jungle of Mexico. They located a 35-strong troop of adult Geoffroy's spider monkeys and observed the animals' behavior from February to June 2016.
The researchers set up microphones in the territory of the selected troop of monkeys. Whenever an individual drew within 20 m of a phone, the device recorded the calls made by the animal. A total of 566 calls were recorded.
Every day, a troop of spider monkeys divides itself into several subgroups. These smaller groups go looking for food and water before regrouping later in the day.
Under the terms set by the researchers, a member of the subgroup remained with its group if it stayed within 40 m of the other monkeys. However, if a spider monkey strayed beyond that limit, it became isolated and started making whinnies.
The researchers paid careful attention to the variances in the tonal frequency of spider monkeys that got isolated from their subgroup. Furthermore, they analyzed the response of the listeners to the first whinny based on the frequency of the call and the distance between the animals.
The lower the pitch, the faster the call back
The DPZ study revealed that spider monkeys separated from their subgroup uttered whinnies with a lower fundamental pitch than the calls made by animals that remained with their group. Low-frequency sounds carried over greater distances, so reducing the frequency of the whinny might make it likelier for the monkey to contact its subgroup.
Furthermore, the lower the frequency of the whinny, the faster the other spider monkeys responded to it. The listeners also reduced the pitch of their whinnies if the distance between them and the calling monkey was much wider.
Earlier findings indicated that aroused spider monkeys uttered lower-pitched calls more often. The German researchers proposed that animals isolated from their subgroups might also experience similar arousal.
“The acoustic variation of the spider monkeys' contact calls (whinnies) is related to callers' contexts and listeners' responses,” said DPZ researcher Jose D. Ordonez-Gomez, the lead author of the study.
Learn more about fascinating animal behavior at Ecology.news.
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