Dominant macaques in captivity get more overweight than lower-ranking members of their group – without appearing to consume more calories.
The findings suggest that the greatest risk for obesity in these captive primates isn’t how much they eat, nor how much exercise they get, but how high they rank on the social scale, says at the University of Utrecht in the Netherlands.
“The idea was to find behaviours we should target to… make them lose weight,” she says. “But because behaviour wasn’t related to the weight gain, it’s more difficult to find husbandry strategies to really tackle the problem.”
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Obesity can cause health problems like diabetes and heart disease in macaques – which are often used as research models for studies on human obesity – and it can prevent them from carrying out their normal activities like climbing, foraging and grooming. Free-roaming macaques – regardless of their social rank – rarely gain excess weight, but up to 30 per cent of captive macaques become overweight.
Because captive macaques are three times less active than those in the wild and have easy access to sufficient food, it was suspected that some of the monkeys might be getting too much food and too little exercise. To find out, Zijlmans and her colleagues studied three groups of rhesus macaques (Macaca mulatta) with high numbers of overweight individuals, living in large enclosures at the Biomedical Primate Research Centre in Rijswijk, the Netherlands.
The team started by calculating the weight-for-height index – a measure of obesity – for 31 female monkeys, and then observed individual feeding and activity. That included counting the number of chow pieces, apples and bread chunks that each animal consumed over two full days and observing them in 15-minute intervals, for a total of 3 hours each, during normal activity hours. One year and two years later, the researchers calculated the monkeys’ weight-for-height indices again.
By the end of the two-year period, the index had increased in 86 per cent of the monkeys and 71 per cent were overweight or obese. The higher the monkeys ranked on their group social scales – based on behaviour signs, like the space accorded to them by other monkeys – the more weight they gained.
The monkeys that were most obese at the start of the study were the least active, moving for roughly a tenth of the time as the most active group members. Unexpectedly, though, the least active monkeys weren’t necessarily the ones that gained the most weight as the study went on.
“It seems that [lower] activity is more of a consequence of weight gain, than actually a cause or a risk factor,” says Zijlmans.
Daily calorie intake varied dramatically from monkey to monkey, with some consuming three times as many calories as the others. But surprisingly, each monkey’s calorie intake wasn’t related to the amount of weight gained nor their social ranking, she says.
That might be due to some oversights in their observations, she adds, as some monkeys might have packed food in their cheeks without actually swallowing it. But it could also just be due to individual differences in energy requirements.
In humans, social stress can cause changes in metabolism that lead to weight gain, but this doesn’t appear to account for the differences in the monkeys either. Hair analyses revealed that the macaques in their research centre had similar levels of stress hormones regardless of their place in the social hierarchy.
For the moment, the researchers suspect that, somehow, the more dominant individuals are consuming more calories. “Of course the extra weight has to come from somewhere,” says Zijlmans. “So that’s something that should be further studied.”
Applied Animal Behaviour Science
