For the first time in about 12 years, a newborn southern white rhino is running around at the San Diego Zoo Safari Park. His name is Kayode, and he’s a spunky little dude, charging the Cape buffalo with his mother, Kacy, following close behind. He’s a welcome, if not long awaited, addition to the herd. You might be wondering why there aren’t more calves like Kayode cavorting at the Safari Park? Why has more than a decade passed between southern white rhino births?
In zoos around the world this trend appears to be commonplace. Females brought in from the wild to establish zoo-based breeding programs have generally reproduced relatively well compared to females born in captivity. The reason why remains unclear. We suspect that most zoo diets contain plants that produce chemicals called phytoestrogens. During their 16-month gestation, female white rhinos could be exposed to these compounds through Mom’s diet, resulting in permanent fertility issues later in life. In many other species, exposure to phytoestrogens during gestation causes similar reproductive problems.
Notice the words I use here: appears, generally, relatively well, could be. Even coming from a scientist (a notoriously skeptical bunch), these statements do not inspire much confidence. That’s because this breeding problem is particularly complex, and solid data proving that captive-born females have a breeding problem, and that it is caused by diet, is hard to come by. Here are a few questions and answers demonstrating what we know and what we don’t know:
Q: Do captive-born female southern white rhinoceros have trouble reproducing?
A: It looks that way. Depending on the study, estimates of the percentage of captive- born females that have reproduced ranges from 10 percent to about 50 percent. However, a more thorough investigation of individual histories that may preclude reproduction needs to be conducted. In other words, determining if non-reproducers even had access to mates, were housed in properly sized enclosures, or lived in appropriate social groups will give a more accurate indication of the extent of the problem.
Q: Are phytoestrogens a possible cause of this phenomenon?
A: We think so, and we have data to support it! At the molecular level we know southern white rhinos are sensitive to phytoestrogens. We also know that many zoo diets contain phytoestrogens. However, we have not and likely cannot conduct the types of cause- and-effect experiments that could prove it, because that would require having many groups of rhinos eating diets with different levels of phytoestrogens and following reproductive success for multiple generations.
Q: Are there any dietary differences between institutions that have breeding success and those that do not?
A: Anecdotally, yes. At most institutions commercial pellets comprise a large proportion of diets, which we know contain high levels of phytoestrogens. At the four or five institutions that have the greatest captive-born female breeding success, diets appear to consist of mostly grass, and we are investigating to determine this for sure. We have not been able to detect phytoestrogens in grass samples from one of the more successful institutions. Interesting for sure, but not quite a smoking gun.
I hope you can appreciate what we’re up against. As we work toward a solution, we continue to find pieces of information that alone do not meet the burden of proof but together they continue to build a case for phytoestrogens causing reproductive harm in captive-born females. We still have a long way to go, but one day I am certain we will have an answer. In the meantime, come see Kayode and learn for yourself why his name means, “he brings joy.” While you are watching him, consider this additional piece of information that I neglected to mention. Kayode’s mother, Kacy, just so happens to have been born at one of the institutions that feed their rhinos primarily grass. Now THAT’s pretty interesting!
Christopher Tubbs, Ph.D., is a scientist in the Reproductive Physiology Division of the San Diego Zoo Institute for Conservation Research.