Elephants may be the largest land mammals on the planet, but new research reveals that the typical human is in fact flabbier than these colossal creatures. Appearing in the Journal of Experimental Biology, the new study sheds light on the health of captive Asian elephants, suggesting that their low reproductive rates may not be due to an excess of body fat.

Sadly, many captive populations of Asian elephants are not self-sustaining, leading to concern among biologists that obesity may be affecting the animals’ fertility. However, while it’s true that captive elephants generally tend to carry more body fat than their wild counterparts, the impact of this extra weight on the animals’ health has never been properly investigated.

A team of researchers, therefore, set out to measure exactly how much body fat captive Asian elephants carry relative to their body weight, in order to discern if they can really be labelled obese. Explaining the difficulty in making such a determination, study author Daniella Chusy said in a statement that “obesity is not clearly defined in humans, let alone elephants.”

In their write-up, the authors expand on this issue by explaining that “obesity is a social creation to describe people with high levels of adiposity, and was originally developed based on mortality data.” However, applying such a diagnosis to elephants is fraught with difficulty as “there are no data on the relationship between adiposity levels and mortality in elephants.”

A total of 44 Asian from zoos across the US and Canada were included in the study, 35 of which were female while the remaining nine were male. To measure the animals’ body fat, the researchers fed them slices of bread that had been soaked in heavy water, which contains an isotope of hydrogen called deuterium.

Zookeepers collected blood samples from the elephants in order to allow the researchers to measure the levels of deuterium circulating in each animal’s serum. By subtracting the amount of water in an elephant’s blood from its total body weight, the team was able to determine each individual’s fat level.

On average, male elephants were found to carry about 8.5 percent body fat while females carried around ten percent. By contrast, the average healthy person has a fat content of between six and 31 percent, meaning most of us have a higher ratio of fat to body weight than a captive Asian elephant.

More importantly, results also revealed that the least fertile female elephants tended to be those with the lowest fat levels, suggesting that low reproductive rates are not caused by obesity. Rather, it seems that being underweight carries the greatest risk of infertility among female captive Asian elephants.

Furthermore, observations revealed that the elephants in the study tended to walk between 0.03 and 2.8 kilometers an hour. Previous research has found that wild Asian elephants generally walk between 0.01 and 1.15 kilometers per hour, indicating that the extra weight carried by captive elephants is not due to inactivity.

Finally, the researchers report that insulin levels tended to be higher among the fattest elephants, raising concerns that some may run the risk of developing diabetes-like symptoms. Overall, the study authors tentatively suggest that elephants with a fat content of more than 14 percent could be considered obese, as this threshold was associated with elevated levels of blood insulin.

The researchers are keen to point, however, that more research is needed in order to verify this preliminary conclusion. Overall, though, they are pleased to report that on the whole, captive Asian elephants do not appear to be obese.