Full-figured primates

02 Jul 2019 | Back to News, Publications and Annual Reports

Other primates are a lot like us, including where their weight is concerned. One individual tends to be plump while the other remains slender. How does this work with primates? And what can we at BPRC do to reduce the incidence of overweight in the colony? These are the questions we are attempting to answer during a study of overweight among the macaques living at BPRC.

This study is necessary because overweight – the state in which the body has stored too much excess fat – carries a number of health risks. These include a higher chance of developing diabetes and cardiovascular disease. We are currently conducting research into the causes and effects of under and overweight among macaques. Our larger goal is to ultimately reduce overweight and thereby enhance the animals’ well-being. But how do we measure whether a primate is underweight or overweight in the first place?

Not BMI, but WHI

When it comes to measuring underweight and overweight, most every human is familiar with the Body Mass Index (weight/height2: BMI). Yet for macaques, this method of measurement is less suitable because the BMI in these primate species does not correspond to height in the same way. A better tool is the so-called Weight-for-Height index (weight/heightp: WHI), which has a reliable correlation with indicators of overweight such as abdominal circumference and skin-fold thickness. For the rhesus macaques, for instance, we arrived at a formula of WHI3.0 (weight/height3.0) and for the long-tailed macaques, WHI2.7 (weight/height2.7).

Like in the wild

The next important question is: when, exactly can an animal be considered underweight or overweight? Scientific literature states that a rhesus macaque is overweight if it has a body-fat percentage greater than around 18–23%. This would seem to apply to the macaques living in groups at BPRC as well. And on the other hand, an animal is considered underweight if it has less than 8–9% body fat. By this metric, a large portion of the macaques at BPRC are underweight, even though these ‘skinny’ macaques bear young as usual and look just like the macaques living in the wild. There is one caveat, however: this limit for underweight is based on studies involving animals housed individually in small cages, with little ability to exercise.

At BPRC, the primates live in same-species groups in roomy enclosures with a great deal of ‘enrichment’, such as opportunities for play and exercise. These housing practices – which reduce the chance of overweight – much more closely resemble how the animals live in the wild, where primates have an average body-fat percentage of only 2%. If we apply this percentage to our colony, only a handful of animals could be considered underweight.

Majority of BPRC macaques at a healthy weight

Based on our study of underweight and overweight, we are able to conclude that the majority of the macaques in our colony are at a healthy weight. Some 17–23% of our adult animals are overweight, while underweight individuals are rare (0–3%). Read the full text of the (open access) article here.