Preserving Wildlife

The Herd Grows by One


I clearly remember when we first suspected something: it was early last year at one of our Reproductive Physiology lab meetings. Like most other meetings, graphs of data flowed in an invisible current around the table as we updated the group on our various research projects. Snippets of conversation could be heard that, in most other settings, would certainly raise eyebrows and likely earn you a sit-down with Human Resources: words like “gonads, ovaries, testes, and sperm” flow freely in our meetings! 

The conversation turned to hormones, and one graph immediately grabbed my attention. It showed the progesterone levels of Kacy, one of our female southern white rhinos. As its name implies, progesterone is a hormone that supports gestation. In other words, elevated progesterone is usually a good sign that an animal is pregnant. Kacy’s progesterone levels were clearly elevated, registering at nearly 1,000 to 2,000 times higher than her normal levels. “She might be pregnant,” I said to no one in particular, almost in disbelief.

There are no rhino take-home pregnancy tests; a rhino pregnancy teaches you patience. A 16-month gestation and the fact that white rhinos notoriously show prolonged but sporadic progesterone elevations mean you never really know if a female is pregnant until a calf is born. Our best evidence was an updated progesterone graph we saw at lab meetings. I prepared myself to see her levels inexplicably return to normal one day, which is, unfortunately, the status quo. But each time we met, they continued to rise.

I became nervously excited. For the first time in nearly a dozen years, it looked like we were going to have a white rhino baby at the Safari Park.

Eat This, Not That

In full disclosure: I was excited, but as Kacy’s progesterone climbed, I started to reevaluate my career choices. I had spent the last four years trying to figure out why white rhinos don’t reproduce in zoos. While searching for a probable cause, we discovered that white rhinos are particularly sensitive to chemicals called phytoestrogens. Produced naturally by soy and alfalfa, both common components of zoo diets, phytoestrogens mimic the hormone estrogen.

In other species, phytoestrogen-rich diets cause reproductive problems like those faced by white rhinos. We had determined that many zoo diets, including ours, contain phytoestrogens, and for years, white rhino reproduction here has been sluggish. Yet here was a graph suggesting that a rhino at the Safari Park, eating our diet, was expecting.

As is often the case in research, an apparent dead end can become an opportunity by forcing you to reexamine a problem from a different perspective. In Kacy’s case, digging into her history actually supported our phytoestrogen hypothesis. A central tenet of toxicology is “the dose makes the poison.” While true, the timing of the dose is equally important. If exposure to phytoestrogens occurs during adulthood, effects are typically subtle and reversible. If a developing fetus is exposed to phytoestrogens, effects can be drastic and permanent. The majority of female white rhinos born in captivity never reproduced.

Kacy, however, was born at an institution that feeds a low phytoestrogen diet. With this, we have started analyzing phytoestrogens levels in diets from zoos across North America. By tracing the reproductive history of every female born at those zoos, we hope to determine if there is a relationship between phytoestrogen exposure and southern white rhino fertility.

At a meeting this February, I held a graph that showed Kacy’s progesterone levels plummeting. We had seen this before, but unlike most cases, Kacy’s progesterone dropped after nearly 16 months of being elevated. On February 25, 2013, in front of guests and keepers, Kacy confirmed her pregnancy for us and gave birth to Kayode, our first baby white rhino in quite some time.

Christopher Tubbs, Ph.D., is a scientist in the Reproductive Physiology division at the San Diego Zoo Institute for Conservation Research.