Morphological appearance and effects of castration on blackback gorillas Gorilla gorilla gorilla in the EAZA ex-situ population

Abstract

Currently, the sex ratio of the western gorilla Gorilla gorilla gorilla ex-situ population shows a tendency to balance due to an increase in the number of males. The gorilla’s social structure is the harem (one male and two or three females) and this increase, together with a lack of space and/or facilities to accommodate new families in zoos, makes it impossible for many males to become breeding males leading a group. This excess of males needs to be managed. Different strategies have intended to alleviate this problem, such as forming bachelor groups or housing solitary males, but they have proven to be insufficient so castration has been considered. Castration could affect the development of these males who could be perceived as non-competitors by the silverback, therefore increasing his tolerance towards them and their acceptance in his family group. However, castration may produce side effects affecting the health of individuals, so that long term monitoring of castrated males is necessary.  This study assesses the morphological development of some castrated and intact blackback males. Weight growth curves were elaborated for castrated and intact males and females of similar ages. Several morphometric measurements of both types of males were compared. Morphological measures are not significantly different between castrated and intact males. The average weight of castrated males is lower than that of intact males and similar to that of females, although weight growth curves show clear differences among the three groups. Castrated males showed none of the secondary sexual traits that are present in intact blackbacks. Overall, castration upon reaching the blackback life-history class promoted a lower average weight and a lack of sagittal ridge development but similar body growth to intact males.