About Author: Christopher Tubbs

Posts by Christopher Tubbs


White Rhino Births: What We Know and Don’t Know

Eight-Day-Old Southern White Rhino Kayode

Kayode, at 8 days old, frolics at the Safari Park.

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.


Fascinating Fibroblasts: New Uses for Frozen Zoo Cells

We use fibroblasts to examine how environmental chemicals interfere with reproduction in endangered species.

We use fibroblasts to examine how environmental chemicals interfere with reproduction in endangered species.

As a graduate student, I first learned the story of Henrietta Lacks from a professor of mine. She is a woman that forever changed biological research without being a scientist, physician, or high-level researcher. As told in The Immortal Life of Henrietta Lacks, she was a patient suffering from an aggressive form of cervical cancer. While receiving medical treatment, her tumor was biopsied and researchers tried growing her cells in a petri dish. They discovered that her cells possessed the unique ability to grow almost as aggressively outside of her body as they did inside. As Henrietta’s cells continued to grow in the lab, they were named using the first two letters of her first and last name. And with that the world’s first immortal cell line, would be called HeLa. Today, HeLa cells are still growing in research labs around the world, nearly 60 years after Henrietta’s passing.

HeLa cells became workhorses for biomedical advancement by being used to determine the amount of DNA in human cells, discover new effective cancer treatments, and develop the polio vaccine. Each of these research breakthroughs was possible because of the extraordinary ability of HeLa cells to be kept alive outside of the body. Thanks to a greater understanding of proper cell culture conditions, there are now thousands of cell lines in existence from all kinds of different species.

An unparalleled collection of animal cell lines lives within the Frozen Zoo® as part of the San Diego Zoo Institute for Conservation Research. Currently there are cells, called fibroblasts, from approximately 900 species represented by over 9,000 individuals. It is the largest “zoo” of its kind, harboring samples from some of the world’s most endangered animals. There are even fibroblasts in there from extinct species.

The ability to work with cells growing in culture opens the door to many research possibilities. For example, our Reproductive Physiology lab is interested in testing the effects of environmental chemicals on endangered species reproduction. Let’s say you wanted to conduct this work in mice. All you would need to do is simply get a colony of mice, expose them to whichever chemicals you were interested in testing, and measure how reproduction was affected. But we don’t work with mice. We work with animals like rhinos. We will never have, nor would we ever want, an experimental rhino colony. However, we do take advantage of cell lines to determine how environmental chemicals might affect rhino reproduction.

To do this we take copies of rhino DNA and stick them into primate cells in petri dishes so that they make the rhino proteins that regulate reproduction. Then we expose those rhino proteins to different chemicals and measure how the two interact by measuring the production of a “reporter,” which (are you ready for this?) happens to be an insect enzyme. It sounds absolutely bizarre, but believe it or not, this is a well-accepted, common approach. The problem is, we can’t help but also ask, “How well does what is happening in this petri dish reflect what happens in a 5,000-pound rhino?” An answer to that question may lie in the Frozen Zoo.

In collaboration with the Institute’s Genetics Division, we are exploring new ways to use Frozen Zoo fibroblast lines to get a more accurate idea of how environmental chemicals may affect endangered species reproduction. This idea all started with our interesting discovery that many fibroblasts in the Frozen Zoo make proteins that regulate reproduction, even though they are usually taken from nonreproductive tissues. We have also found that, like HeLa cells, individual fibroblast lines possess their own unique qualities, and we can take advantage of this to address specific research questions. So instead of the science fiction-sounding rhino/primate/insect system, we can treat rhino cells directly with chemicals to which rhinos may be exposed. Everything about those cells, from the membrane to the DNA to protein, is made in the same way it would be in a rhino. In other words, we think what happens in those rhino cells paints a much more accurate picture of what happens in a whole animal. Sure, they are still cells growing in a dish, but cells alone can be extremely powerful tools. Just look at what Henrietta and her cells have accomplished.

Christopher Tubbs is a scientist in the Reproductive Physiology Division of the San Diego Zoo Institute for Conservation Research.