In 2017, the hurricane Maria devastated Puerto Rico, killing thousands of people. He entered from the southwest, first devastating Cayo Santiago, a small island where hundreds of rhesus macaques live (Mulatto macaca). For almost a century they have been studied by biologists around the world. After the disaster, scientists found that these monkeys, one of the most violent primate species, had increased their tolerance towards others and reduced their aggression. Now, in the second part of this work, published in Sciencehave shown how animals that became more tolerant by sharing their main resource, the shade, have reduced their probability of dying by almost 50%.
In Cayo Santiago, also known as Monkey Island, the temperature in the central hours of the day exceeds 40º almost all year round. The photographs (see below) before the hurricane hit show it practically covered with trees. But, after María, category 4 out of 5, the bare trunks of dead trees remained. Years later, the tree cover has still not recovered. So shade, with up to 7º less and abundant in the past, had become a scarce resource. There wasn’t something for everyone. Scientists feared the worst.
“We expected that the monkeys would become more aggressive by competing for shade, given that this is a society known for its aggressiveness,” says the professor of ethology at the Animal Behavior Research Center at the University of Exeter (United Kingdom) and senior author of the research, Lauren Brent. They were wrong: “Instead, they became more tolerant of others and less aggressive.” Using data from the five years before the hurricane and the five afterward from seven groups made up of a total of 790 adult macaques, they observed that social tolerance tripled (measured with acceptance of the presence of another specimen less than two meters away). meters away) after Maria.
With the latest records available, those from 2023, this behavior, although it has decreased somewhat, has continued to double that prior to the disaster. Acts of aggression, common in this species, have also remained well below pre-cyclone levels. The population density on Monkey Island is very high, similar to that of a city like New York (with more than 10,000 inhabitants per km²). But it hasn’t changed during this decade of study, so this wouldn’t explain the changes seen in the macaques’ sociability.
This represents a sudden change in selective pressure, in the benefits or costs of traits or behaviors, within a very aggressive and hierarchical society. With the climate crisis underway, there is an increase in extreme events, including hurricanes, which are profoundly altering ecosystems. Such a widespread transformation cannot be validated in a laboratory. Natural experiments like this are a unique opportunity for researchers. Therefore, they looked for what consequences the increase in sociability and decrease in aggressiveness has had on the adaptive aptitude of the macaques. On this occasion, environmental degradation would have altered the adaptive value of greater social tolerance. With what result?
What they did was study 431 adults and their proximity networks after María. With an average life expectancy of about 25 years (and without predators), they counted the deaths of 155 of them. When they reviewed the files for the period before the disaster, between 2013 and September 2017, they saw that out of 617 adults, 111 died in that period. The absolute numbers do not give the key, many died in the days after the hurricane. The key is which ones died.
What they discovered is told by University of Pennsylvania researcher and first author of the study, Camille Testard: “We had behavioral and survival data (whether or not the monkeys died during the study period) for all the individuals. “We were able to model the relationship between the sociability of individuals and their probability of survival.” Thus they found that after the cyclone, the monkeys that were more tolerant of others, sharing the shade, were more likely to survive. Specifically, 42% more likely. “This was not the case before the disaster, when whether a monkey was tolerant or not did not predict its survival,” adds Testard. “We believe that tolerance to more monkeys facilitates access to shade, which is now a scarce resource, allowing for better thermoregulation and, ultimately, better chances of survival,” she completes.
The need to share the shade when it was 40º outside reinforced social ties. Thus, they saw that also in the early hours of the morning, the tolerance of others, somewhat less than at noon, but always well above the period before Maria. “To access the shadow, they need to tolerate (and be tolerated by) others, and we found that this tolerance extends to other daily interactions,” says Testard.
Regarding the logic behind the change, his colleague Brent recalls that “competing for shade is something different than competing for, for example, food.” And the difference is in the type of social relationships necessary. “Tolerating others does not cost much; “Unlike supporting someone in a fight, you are very unlikely to get hurt or expend a lot of energy tolerating others, so you don’t need to invest a lot in someone to create a relationship in which you tolerate others,” he details. . There may also be a strategic calculation: “When there are few shady places to sit, your best friend may not be sitting in the shade, which means you can’t either. But one of your 10 acquaintances could be, which means you can too.” So, “getting shade is a numbers game in which it is better to tolerate others than to lock yourself in small competitive social groups,” completes the professor of ethology at the British university.
As for the survival of this behavior, we will have to wait until the trees of Cayo Santiago recover. But Brent wants to highlight the adaptability of these monkeys. “For animals that live in groups, social relationships can allow them to cope with environmental perturbations, including human-induced climate change. The lesson we can learn from this is that we need our social relationships to help us overcome life’s major challenges, and being tolerant of others can lead to biological success.”
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