A Non-Breeding Breeding Population

Alright, y’all, I’m back for my second official #scicomm post! Today we’re gonna talk about some more of my favorite things: zoos, conservation, and rhinos!!! All papers used will be cited and listed at the end of this post. Note: some of this knowledge is anecdotal and comes from my experience as a zoo educator and animal keeper! [disclaimer: I can only speak to the care and conservation efforts of AZA accredited zoos]

Zoos serve many purposes, including education, recreation, research, and conservation. Not only do they support and collaborate with field partners for in-situ conservation, but they also protect and foster ex-situ captive populations. For many species, like the black-footed ferret, these zoo-housed populations have been critical to survival and conservation. However, there are many biological mysteries about wild and zoo-housed animals that are still being investigated, like the intricacies of reproduction. Such was the case for the near-threatened Southern White Rhinos (Ceratotherium simum simum), as the zoo-housed population was reproducing at an unsustainably low rate after wild rhino harvest ceased in recent decades. As it turns out, part of the reproductive challenges for zoo-housed white rhinos can be linked to endocrine disrupters in their diet!

The endocrine system is the system in the body responsible for the secretion of hormones and other hormonal processes. Endocrine disrupting compounds (EDCs) are chemicals that may interfere with that system, causing detriment to biological processes, like reproduction which we will focus on here. EDCs include hormones, hormone mimics, and hormone blockers that are found in everyday objects like plastics, food, cosmetics, and medications, but can have dramatic effects on physiological functions like loss of fertility. Some EDCs can be stored in tissues and fats for several years (Endocrine Disruptors, 2019). Phytoestrogens, which are produced by plants and act similar to the estrogens produced in our bodies, are common EDCs. These compounds bind to estrogen receptors and activate them, altering the processes and cycles associated with the estrogens regularly occurring in the body (Tubbs et al. 2012). Extensive exposure to estrogenic substances can result in cysts and tumors in reproductive tissues, as well as general alterations of hormone cycles. As it turns out, common rhino feeds and rhino zoo diet items such as alfalfa and soy are high in phytoestrogens that can cause these reproductive issues (Tubbs et al., 2012, 2016).

Studies of diets and fertility of female white rhinos from multiple zoological institutions were compared, and results showed that diets with high estrogenic content correlated with reduced female rhino fertility and reproductive success (Tubbs et al., 2016). These findings suggested that alterations to the white rhino’s zoo diet would likely be a necessary step in increasing the reproduction and sustainability of the zoo-housed rhino population. Suggested diet changes would reduce the overall estrogenic content by limiting alfalfa and pelleted grains with high soy content (Tubbs et al., 2016).

Likely due to the extensive gestation, long life span, and multitude of historical reproductive issues of white rhinos, robust results of this diet change have not been published. Although the effects of these diet changes may not be reflected in the population for some time, this discovery and management shift are of immediate and critical importance. Cheers to the future generations of zoo-housed Southern White Rhinos!

Fun fact: The Wilds in Cumberland, Ohio was a major component of this study! For a while, they were one of only a few facilities having much success with breeding white rhinos, and other zoological institutions wanted to know why, which led to this investigation. Their rhinos spend much of the year grazing on pasture and have less grain and hay supplements in their diets, so their reproductive health was stellar! I highly recommend you all visit them at the wilds. They’re real cute. Here’s me with one of their Greater One-Horned Rhinos.


Endocrine Disruptors (2019) Natl Inst Environ Health Sci. https://www.niehs.nih.gov/health/topics/agents/endocrine/index.cfm (last accessed 15 March 2019).

Tubbs C, Hartig P, Cardon M, Varga N, Milnes M (2012) Activation of Southern White Rhinoceros (Ceratotherium simum simum) Estrogen Receptors by Phytoestrogens: Potential Role in the Reproductive Failure of Captive-Born Females? Endocrinology 153: 1444–1452.

Tubbs CW, Moley LA, Ivy JA, Metrione LC, LaClaire S, Felton RG, Durrant BS, Milnes MR (2016) Estrogenicity of captive southern white rhinoceros diets and their association with fertility. Gen Comp Endocrinol 238: 32–38.

Zoos & Researchers Team Up to Understand How Climate Change Affects Polar Bears

Below are screenshots of my twitter thread on the collaboration of zoo staff and scientists to understand metabolic rates and energetics in polar bears by training zoo-housed bears to inform us on their wild counterparts! It includes a lot of gifs, videos, and images that I think enhance the thread, so I recommend you check it out on my twitter, @Tori__Roeder. If you feel so inclined, you can follow me for animal and ecology content, though predominantly mammals and zoos.