Phytoestrogen binding capacity to in vitro produced southern white rhino estrogen receptors. A comparison of native and captive feeds

The southern white rhinoceros population in captivity has a disappointing reproductive rate and the reason behind this is unknown. On the contrary, the wild population has grown steadily over the last two decades and is still growing. In captivity, white rhinos are given a diet of mainly alfalfa and soy-based products, which contains high concentrations of phytoestrogens. Phytoestrogens are known to bind estrogen receptors and exposure to phytoestrogens is known to impair reproduction in several different species. In Lapalala Wilderness, South Africa, research was conducted to determine the exact grass species consumed by free-ranging white rhinos, and to define the relative distribution of grass species eaten throughout the observed study period. This aim was addressed by observing four female white rhinos during a period of 3 months. Samples of the grass species were collected, identified and extracted using 100% methanol. The relative binding of phytoestrogens present in the dried plant extracts on white rhino estrogen receptors was tested. This was done both with phytoestrogens extracted from alfalfa and with phytoestrogens extracted from the samples of the grass species from Lapalala Wilderness. As a result, we could compare the captive feeds and the native browse. The comparative results show that alfalfa binds approximately 100% of the in vitro produced estrogen receptors. The samples of the identified wild grass species bind at most only 20% of the receptors. This study does suggest that the high concentration of phytoestrogens in captive feeds and the low concentration in native feeds are possibly an explanation for the difference in reproductive success between wild ranging white rhinos and captive white rhinos.