The reversibility and flexibility of contraceptive methods generally allow for improved genetic and demographic management of captive populations. Earlier studies have produced conflicting results regarding the restoration of reproduction after using melengestrol acetate (MGA) implants in golden-headed (Leontopithecus chrysomelas, GHLT) and golden lion tamarins (L. rosalia, GLT): two closely related species that are physiologically and genetically very similar. The present study investigates the nature of this inter-species difference, presents new data on GHLTs and compares this with published data on GLTs. Analyses showed that around 34% of the GHLTs resumed breeding after their MGA implants were removed or had expired. Non-implanted GHLTs (control group) were significantly more likely to reproduce than females previously treated with an MGA implant, regardless of whether the implant was removed or left to expire. Younger and parous female GHLTs in the control group were more likely to start reproducing. In implanted females, only parity had an impact, with parous females being more likely to resume breeding than non-parous females. In contrast, data published on GLTs indicate that 75% of GLT females resume breeding, and that removing the implant increases the probability of reproduction occurring. Available data suggest that the observed inter-specific differences are related to differences in the weights of the implants used for the two species. For GHLTs, adjusting MGA doses and/or the sizes of the implants currently administered may be required in order to preserve the reproductive potential of individuals. Apart from potentially negative medical and welfare consequences for individual GHLTs, the reduced reversibility of MGA implants also impacts on management practices used to achieve the objectives of conservation breeding programmes. Finally, this study stresses the importance of evaluating the suitability of contraceptive methods at a species-specific level.

Finding a responsible method of population control that does not compromise animal welfare is a pressing problem for zoological institutions and conservation breeding programmes. This is exemplified by the conservation breeding programme of the golden-headed lion tamarin, Leontopithecus chrysomelas. The number of golden-headed lion tamarins in captivity is currently being limited by, among other means, the use of contraception. We have conducted a study on the effects of contraceptive methods used in golden-headed lion tamarins. Data were collected through the distribution of a survey. The use of Melengestrol acetate (MGA) implants in females was by far the most widespread contraceptive method. It was very effective in preventing reproduction, provided that females were not pregnant at the time of implantation. Pregnancies that had commenced before MGA implantation were carried to term and resulted in viable infants, as far as noted. However, the degree of reversibility was very low and, if females did conceive after MGA implantation, infant survival was lower than expected. The widespread use of MGA implants in golden-headed lion tamarins (and probably other species) should be seriously reconsidered. Alternative methods of population control should be investigated. Possible options include the use of other contraceptive methods, limiting the number of offspring through natural factors and the use of euthanasia under very strict conditions. Animal welfare implications associated with the use of euthanasia are discussed.

In order to be suitable, a contraceptive method should have little or no effects on social organization or behaviour. In callitrichids, changes in socio-sexual interactions between group members, due to hormonal changes induced by contraception, may have consequences for the reproductive inhibition of offspring in their natal group. This may lead to an increased rate of inbreeding. In this paper we report on the endocrinological and behavioural effects of contraception in golden-headed lion tamarins, using data obtained through a world survey and an observational study. Hormonal analysis of urinary oestrone conjugate levels in melengestrol acetate (MGA)-implanted females confirmed earlier preliminary findings (Van Elsacker et al 1994): MGA implants inhibited reproduction through the suppression of ovulation and regular ovarian cycles in the implanted female, while the occurrence of ovarian cycles in the oldest female offspring of each group was not affected. Sexual interactions between the dominant adults still occurred but underwent temporal changes. Reproductive inhibition in female offspring was maintained. Social interactions between group members altered in a non-consistent way but did not have an impact on the stability of the study groups during the study. In principle, MGA implants do not have a detrimental impact on the behaviour of group members. The suitability of MGA implants from a behavioural point of view depends on the extent to which those involved wish to preserve the entire range of natural behaviours for this species. The behavioural effects of other contraceptive methods are still largely unknown.