Surprising monkey study: Bad times do not cause group members to change behavior

Researchers have observed an unexpected behavioral pattern in monkeys in Puerto Rico. As the population density in the group rises, the group as a whole produces fewer babies – this is no surprise. But, to the surprise of researchers, it turned out that the group's individual members did not change behavior. How does this add up?

By Birgitte Svennevig

We all know this kind of news story: When there is an economic crisis, society responds by spending less money and saving more. And when times get better, we start spending again.

The same adjustment to shifting resources is found in animal populations: As population density fluctuates, the population will react by getting fewer or more offspring, splitting up or expanding, etc.

We probably think that such group reactions to shifting resources are caused by all members of the society or the animal population; that each individual member changes a little bit in the same direction.

40 years of monkey data

But according to a new Danish/Puerto Rican study this is not the case.

The study was conducted by Adjunct Professor Raisa Hernández-Pacheco from University of Puerto Rico and Associate Professor Ulrich Steiner from University of Southern Denmark and published in journal The American Naturalist.

The researchers have analysed 40 years of data from a monkey colony on the island Cayo Santiago in Puerto Rico. On this basis they know how many and which members of the colony actually changed behavior in reproduction when the population density in the group rose or fell in the period.

Just as many carried on as usual

- We expected to see an increase in the number of monkeys that changed behavior as a reaction to a population density rise or drop. That is, when density is rising, most individuals would change towards not reproducing and when density stays high few individuals would reproduce. In contrast, when density drops,  more females would change towards successfully reproducing. But that did not happen. Instead we saw that no more individuals changed behavior as a reaction to a density rise or a density drop, or when density was stable. There were just as many monkeys who carried on as usual and did not try a new strategy in order to respond to the density change. So it was the same number of monkeys which was responsible for the behavioral changes and adaptations that enabled the group as a whole to adjust to the increased population density, says Ulrich Steiner.

So, there is a constant proportion of group members who display behavior changes, independent of how large or small the crisis might be. Still the population as a whole shows stronger responses to larger crises than to smaller ones.

In average, 67% of the monkeys changed behavior as a reaction to crises, while 37% did not.

During a financial crisis

The study also shows that it is not always the same individuals who account for the group's behavior changes.

- Next time there is a change in the monkeys’ environment, there may be other individuals who react. But the proportion of individuals reacting would stay the same, says Raisa Hernández-Pacheco.

 If we apply these findings to news stories about how people spend less money during an economic crisis, it does not mean that we all spend a little less each. Rather, when the crisis hits, the ones that do not have any money in the first place cannot save much more, those that have some money might spend less, and those that have plenty money are not obliged to change their spending. It is then interesting that if the crisis continues, we would expect the same proportions of people to change their behavior from spending more money to less money as at the onset of the crisis, says Ulrich Steiner.

The colony

Since 1938 a colony of rhesus monkeys (Macaca mulatta) has lived on the island Cayo Santiago in Puerto Rico. Currently, the colony has over 1,400 monkeys.

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Uli Steiner is Associate Professor at Department of Biology and Max-Planck Odense Center MaxO.

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