Once every year, zookeepers at Royal Burgers’ Zoo capture every Seba’s short-tailed bat from their habitat in the tunnel between Bush and Desert. This annual census is an important assessment point, not only to monitor the physical condition of each individual bat, but also to closely evaluate the sex ratio within the colony. The South American bat species lives in harems, where one male shares his roost (territory) with several females. If the percentage of males becomes too large, pressure ensues on the female population and the pups they are rearing.
Knowledge is key
By counting each individual animal and sexing them (determining their sex), staff at the Arnhem zoo can very accurately monitor population growth and the ratio between males and females across the years. 84 males and 103 females were counted in 2014. Of the 103 females, 20 animals were visibly pregnant, six others carried pups on their bodies, and one female even had twins. By 2015, the population had experienced explosive growth: 132 males and 150 females were counted, an increase of almost 50 animals for both sexes! Of those 150 females, 21 animals were visibly pregnant and 11 females were carrying pups on their bodies. On Thursday, November 10, 2016, 163 males and 199 females were counted, of which 16 were visibly pregnant and four carried pups on their bodies. The successful population growth is largely a result of ongoing refinement of the animals’ feed, such as added flying fox nectar, and by creating additional roost areas and potential territories to the habitat.
What happens next?
The explosive population growth is a compliment for animal care, varied feed, and a natural environment, but it also presents a practical problem. The habitat cannot grow along with its population, and Seba’s short-tailed bats live up to twelve years. Because these bats prefer to form harems, the percentage of males in a colony cannot grow too large in order to avoid undue pressure on females and their pups. For this reason, Burgers’ Zoo started a successful bachelor colony of male Seba’s short-tailed bats in the Bush. In order to balance the current sex ratio, 50 males will soon be transferred to the Bush to join the bachelor colony there. The predominantly nocturnal bats thrive in the covered tropical rain forest, and play an important role in the natural pollination of various plant species.