Birth sex ratio, infant mortality and rearing type in captive western lowland gorillas

One of the main challenges for the successful captive breeding programme for the western lowland gorilla is the management of males surplus to the population. In addition, infant mortality and rearing type can have a large effect on the development of the population. A better understanding of factors influencing sex ratio, infant mortality and rearing could be advantageous for the management of the captive population. We analysed 1286 births from 378 different dams living in zoos participating in the international breeding programme. Our results show that there is a trend for a male-biased sex ratio with increasing maternal age, but the relationship is not significant and we conclude that limiting reproduction in older females would not sufficiently reduce the surplus of males to warrant the possible negative effects on population genetics and social development. None of the following factors had a significant effect on infant mortality within 30 days after birth: maternal age at conception, birth order, rearing type of the mother, sex of the offspring, survivorship of the previous offspring or population. However, there was a significant decrease in infant mortality over the years, probably reflecting improved captive husbandry practices. Infants born to a hand-reared female had to be hand reared significantly more often as opposed to those of parent-reared and wild-caught females. The ratio of hand-reared offspring showed a significant decrease throughout the study period, showing again the positive results of current management strategies. The study confirms that over the years the management of gorillas in captivity has been considerably improved. Selective breeding with young females will not resolve the problem of surplus males and other solutions will have to be investigated. Neither maternal age at conception, nor birth order, rearing type of the mother, sex of the offspring, survivorship of the previous offspring or population did have a significant effect on infant mortality within 30 days after birth. However, there was a significant decrease of infant mortality through the years, probably reflecting improved captive husbandry practices. Infants born to a hand reared female had to be hand reared significantly more often as opposed to those of parent reared and wild caught females. The ratio of hand reared offspring showed a significant decrease throughout the study period, showing again the positive results of current management strategies. The study confirms that through the years the management of gorillas in captivity has been improved considerably. Selective breeding with young females will not resolve the problem of surplus males and other solutions will have to be investigated.