Surplus animals in zoos and wildlife parks are handled differently across Europe. Some zoos use the “Breed and Cull”-method where animals are allowed to reproduce, and surplus animals culled when the mature. Other zoos sterilize the animals or inject them with contraceptive rods to prevent reproduction. Naturally secreted hormones affect both behaviour and morphology, but the effect of the injected hormones has not been studied in lions. The aim of this observational study was to compare the behaviour in captive groups of female lions, that had either been injected with contraceptive rods or not. The study was made in eight different zoos in Denmark, Germany, Belgium and The Netherlands. We found no significant differences in the behaviour of the two groups (with and without rods), indicating that there was no effect of contraceptive rods. However, the small sample size, and unbalanced factors such as pride composition, age and weather could have blurred our results. In order to address the challenge of surplus animals, we suggest, future studies with a more optimal experimental design and larger sample sizes and the inclusion of physiological measurement in addition to quantitative behavioural recordings.