On 14 July 2005, Petra Löttker defended her doctoral thesis on aspects of group dynamics and reproductive monopolization by female moustached tamarins (Saguinus mystax) at the University of Münster, Germany. Her supervisors were Prof. Dr. Norbert Sachser, Department of Behavioural Biology, Institute of Neuro- and Behavioural Biology, University of Münster, and Dr. Eckhard W. Heymann, Department of Behavioural Ecology and Sociobiology, German Primate Centre, Göttingen. The study was supported by DFG (HE 1870/10-1,2). The following is a summary of the thesis.

Callitrichids are the only family of primates in which group members are characterized by a cooperative breeding system with only one, rarely two breeding female(s) per group, often polyandrous matings, dizygotic twin offspring with high neonatal body mass, delayed offspring dispersal, and intensive helping behaviour shown by all group members. To date, the factors shaping the evolution of cooperative breeding in general, and the proximate mechanisms and ultimate causes of female reproductive monopolization in particular, remain largely unknown. A number of models have been put forward to explain single-female breeding. They incorporate various factors influencing the degree of monopolization, including relatedness between dominant and subordinate females, dispersal costs, the risk of inbreeding depression, infanticide by dominant females, and availability of helpers. In general, two (non-exclusive) scenarios are possible: dominant females can suppress reproduction in subordinate females, or subordinate females can restrain themselves from reproduction. Both dominant suppression and subordinate self-restraint can be manifested behaviourally and/or physiologically by preconception and/or postconception mechanisms. Which mechanisms are actually involved seems to depend on a variety of factors such as species, demographic and social context, reproductive/physical status of the dominant female, age of subordinates, and environment (captive versus wild).

Against this background, the aim of this study was to investigate different aspects of the cooperative breeding system, focusing on female reproductive monopolization in a wild tamarin species by investigating the demographic back ground as well as underlying physiological and behavioural mechanisms. We studied eight wild groups of moustached tamarins (Saguinus mystax) at the Estación Biológica Quebrada Blanco (EBQB) in north-eastern Peruvian Amazonia over four years, using a combined approach of detailed, long-term behavioural observations and modern noninvasive techniques of genetic and endocrine analyses from faecal samples.

The demographic and genetic data revealed that wild moustached tamarins exhibit a clear monopolization of reproduction by one female per group (Löttker et al., 2004a). Despite the presence of up to three adult females per group, in none of the groups did more than one female breed successfully at any time. Relatedness within groups was generally high, and most non-breeding individuals were either natal or closely related to the respective same-sex breeder. Apart from members of breeding pairs that were never closely related, only three groups in three group years out of eight groups in 17 group years contained individuals unrelated to any of the group members. Hence, in the majority of groups, non-breeding females were able to gain indirect fitness benefits from helping to rear offspring of a closely related female (mother or sister). Along with assumed low chances for independent breeding in the study population, this situation might induce subordinate females to restrain themselves from reproduction.

Endocrine analyses on two breeding and two non-breeding females from two study groups revealed that the monopolization of reproduction was apparently not caused or maintained through ovarian inactivity in non-breeding females (Löttker et al., 2004b). Non-breeding females demonstrated temporal fluctuations in hormone concentrations and absolute hormone levels that were similar to ones in the breeding females during their phases of ovarian activity. Along with the absence of overt female-female aggression, this questions the action of dominant suppression. Instead, both non-breeding females were daughters of the actual breeding pair and sexual interactions with their group males were never observed. Hence, reproductive inactivity in them is likely to be shaped by inbreeding avoidance. Along with the general constraints of helpers on the successful rearing of the young, this would suggest that females avoid reproducing until prevailing conditions improve. Thus, overall, subordinate self-restraint offers a more useful framework than dominant suppression for understanding single-female breeding in wild moustached tamarins.

Behavioural analyses of grooming relationships in two study groups revealed that breeding females groomed most intensely, and with the most reciprocity, with breeding males and non-breeding offspring compared to potentially breeding males (i.e., males that copulated with the female but did not sire young; Löttker et al., in prep.). Breeding females groomed and were groomed by breeding males more during the phases of ovarian activity, when conceptions were possible, and with potentially breeding males (= individuals that invested highly in helping behaviour) during pregnancies. These results suggest that breeding females might use grooming as an (additional) means of establishing and maintaining the cooperative breeding system by inducing mate association with the breeding male and encouraging helping behaviour in other group members.

By combining demographic, endocrinological and behavioural data, this study has contributed to a better understanding of the proximate mechanisms causing and maintaining female reproductive monopolization and cooperative breeding in wild moustached tamarins. However, further studies — and most importantly, more long-term studies on different callitrichid species, using a combination of methods as applied here — are needed to generalize our understanding of the factors shaping the evolution of the cooperative breeding system and female reproductive monopolization in callitrichid primates.