Holly, a southern white rhino, and her calf Masamba graze

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Hold The Soy

Above: Mom, Holly, and her calf, Masamba

On April 2, 2016, we welcomed our newest southern white rhinoceros calf, named Masamba—which means leaves or vegetables in Yao—to the San Diego Zoo Safari Park. While every rhino calf is a welcome addition to our herd, the hope was that Masamba’s birth signified a breakthrough in the longstanding problem of sluggish reproduction of southern white rhinos born in zoos.

With 97 calves born at the Safari Park, it is one of the top southern white rhino breeding facilities in the world. However, we and other institutions have seen a significant decline in births over time, as females born in zoos have not reproduced as well as females born in the wild. We decided to investigate the possible role of diet, specifically chemicals called phytoestrogens, in this problem.

Phytoestrogens get their name because they mimic the hormone estrogen and can interfere with normal reproduction. We focused on them because they're produced by plants common to zoo diets, like soy and alfalfa, and because the reproductive problems we have seen in southern white rhinos are similar to those observed in other species eating high phytoestrogen diets. This can affect animals in two ways. In adults, hormone cycles can change and tumors can develop within the reproductive tract, ailments common to female southern white rhinos in zoos. When soy and alfalfa are removed from diets, normal reproductive function typically returns. The second effect occurs when a pregnant female eats a diet high in phytoestrogens, exposing her developing fetus to these chemicals and resulting in compromised fertility that can be permanent in her daughter.

Studying how phytoestrogens affect rhinos requires unique approaches. For example, we can’t directly test the effects of these chemicals on rhinos. Time is also an issue, given that rhinos take five to six years to reach maturity, have a 16-month gestation, and usually wait two to three years between births. Using a lab-based approach, we looked at how phytoestrogens interact with southern white rhino estrogen receptors to understand how they might interfere with normal hormone function. This gave us our first clue that phytoestrogens might be a problem, as southern white rhino receptors interacted with the chemicals more strongly than receptors of other rhino species’ that eat similar diets yet still reproduce well.

We then sampled diets from nine southern white rhino breeding institutions, measuring their phytoestrogen content. For wild-born southern white rhinos, no relationship was seen between a diet high in phytoestrogens and fertility, but we did see a significant negative relationship for females born in protected care, which were less likely to reproduce later in life. With that discovery, our Nutritional Services team developed a new food pellet low in phytoestrogens for our southern white rhinos.

We then asked if changing diets could restore fertility for southern white rhinos born in managed care? The response used to be probably not, although now Masamba has caused us to reconsider. Despite breeding regularly for 10 years, his mom, Holly, failed to conceive. That changed approximately one year after a diet change was made using the new pellet, giving us hope that some females can be helped. In fact, in 2015 we had three rhino pregnancies at the Safari Park—after the mothers’ diet was changed. And as we look to reestablish ourselves as a top rhino breeding facility, with the six rescued females at the Nikita Kahn Rhino Rescue Center, we are optimistic that we can avoid repeating history so all female southern white rhinos born here will reproduce as successfully as their mothers.

Read more about the challenges rhinos face.

rhino mom and baby


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