While in chimpanzees males are dominant over females and reproductive success follows a priority of access pattern (Boesch et al. 2006), in bonobos males are not dominant over females and therefore have to rely on alternative mating strategies. Together with Gottfried Hohmann we investigated male reproductive strategies, dominance, and aggression in relation to testosterone levels in a wild bonobos at Lui Kotal, DRC (Surbeck et al 2012a). We found that although high ranking bonobo males are more aggressive than low ranking males, there was no correlation between rank and testosterone levels. Furthermore, in presence of a receptive female, testosterone levels of high ranking males dropped while those of low ranking males increased (Surbeck et al 2012a). In general it seems that bonobo males invest more in close relationships to adult females than into aggressive interactions to secure mating opportunities. The proximity to receptive females however could lead to increased stress in these high ranking males since they experience elevated cortisol levels during their association with these females (Surbeck et al. 2012b).

Whether or not male reproductive strategies, rank and endocrinological parameters are influenced by ontogenetic parameters is topic of another project in our group. Franka Schaebs investigates in her PhD male reproductive strategies in relation to fecal androgen and glucocorticoid levels in adult and adolescent male white-faced capuchins. This project is conducted in cooperation with Susan Perry at the Lomas Barbudal Forest Reserve, Costa Rica.

Based on the difference in dominance relationships between chimpanzees and bonobos, we can predict that these patterns might also be reflected in female reproductive strategies. In Tai chimpanzees, changes in swelling size indicate the probability of ovulation and males can use this information to allocate their mating efforts accordingly (Deschner et al 2003, Deschner et al 2004). Interestingly, although males are dominant over females, females are successful in exerting their choice of mating partners to a large extent (Stumpf and Boesch 2005, 2006, 2010). Fertile periods can not only be advertised by visual but also acoustic signals. We recently investigated the function of copulation calls in relation to ovulation in cooperation with Simon Townsend and Klaus Zuberbühler in Budongo chimpanzees (Townsend et al. 2008, Townsend et al. 2011). Copulation call rates in Budongo females are not dependent on the reproductive state of the female but females suppressed their calls if high-ranking females were nearby. Furthermore, while we did not find any acoustic differences in calls given by females in fertile and non-fertile periods, as assessed by their hormonal profiles, the calls’ acoustic structure did reliably encode individual identity. We are now complementing our studies on chimpanzees with studies on other African great ape species. A study on mating patterns in relation to ovulation in mountain gorillas in collaboration with Martha Robbins (PhD Sosthène Habumuremy), and a similar study in bonobos in collaboration with Gottfried Hohmann (PhD Heidi Douglas) are underway. Additionally, we investigate in cooperation with Susan Perry the impact of the threat of infanticide by immigrating males on stress levels of white-faced capuchin mothers and allomothers at Lomas Barbudal, Costa Rica (PhD Colleen Gould).

Recently, the adaptive value of strong social bonds has been a topic of a number of studies. How such bonds are established and the physiological mechanisms underlying the establishment of bonding is currently unknown. Oxytocin has been implicated in the formation and maintenance of social bonds between relatives and monogamous mates, but its role in other types of social relationships remains largely unexplored. By exploring the behavioral endocrinology of social relationships among unrelated chimpanzees and bonobos in natural social contexts, we aim at gaining novel insights into the adaptations related to maintaining affiliative and cooperative social bonds outside of kinship. In our first study in cooperation with Toni Ziegler, Kevin Langergraber and Klaus Zuberbühler we investigated how relationship qualities influence the oxytocin secretion after grooming (Crockford and Wittig et al. 2013) and how food sharing influences oxytocin secretion (Wittig and Crockford et al. submitted) in Budongo chimpanzees. In cooperation with Gottfried Hohmann, a  study on relationship quality and cooperation in relation to oxytocin and cortisol levels in female bonobos is currently under way in a post doc project of Liza Moscovice. In cooperation with Roman Wittig and Catherine Crockford, we furthermore aim at deepening our understanding of the influence of post conflict behavior on oxytocin excretion and cortisol levels in Tai chimpanzees (PhD Anna Preis).

Despite four decades of evidence that ovarian steroid levels vary as much as three-fold among healthy pre-menopausal women within a population, and as much as two-fold across populations, the causes of this variation remain obscure. To test several hypotheses regarding the sources of this variation we undertook, in collaboration with Virginia J. Vitzthum (Indiana University, Bloomington), two “natural experiments”.

Our first study is of nomadic Central Asians, who are predominantly dependent on their herd animals for subsistence.  From early childhood they consume at least 40% animal fat, a level comparable to or greater than typical U.S., European, and Asian diets, but have higher physical workloads and lower energy intake than are common in industrialized populations. Despite these energetic demands, we found that these nomads had higher progesterone levels than did Germans (identically assayed), suggesting that life-long high fat intake may influence adult hormone levels.

To assess environmental effects independent of possible genetic differences, our second study compared women born and raised in East and West Germany, prior to reunification. These genetically comparable populations experienced divergent living conditions which affected growth and other aspects of their biology and health. Although West Germans have taller heights, indicative of greater food intake and/or less energetically demanding activities, East Germans had higher progesterone levels, arguably because of the relatively higher proportion of animal fats in their diets.

As intriguing as these findings are, fat intake does not explain all of the variance in hormone levels.  Our ongoing analyses include investigations of genetic, epigenetic, and photoperiodic factors that may also be contributing to hormonal variation in women across the world. Look for results from our next, soon to begin, study in the Arctic circle!

A major task in the daily life of wild animals is to consume enough food to obtain the energy that is needed to maintain basal body function and meet travel costs and thermal requirements. In cooperation with Jürgen Kratzsch, Michael Richards, Ben Fuller and Sylvia Ortmann, we have established methods to measure energy balance in great ape urine via C-peptide levels (Deschner et al. 2008) and stable isotope signatures of δ13C, δ15N (Deschner et al. 2012). We now use the method of measuring urinary C-peptide to monitor variation in energy balance in relation to female rank and seasonality in mountain gorillas in cooperation with Martha Robbins (Grueter et al. in preparation) and Tai chimpanzees in cooperation with Christophe Boesch (PhD Livia Wittiger). Additional projects on energy balance in relation to seasonality are underway in female chimpanzees at Fongoli, Senegal in cooperation with Hjalmar Kühl and Jill Pruetz (PhD Erin Wessling), male bonobos (Surbeck et al. in prep.) and female bonobos (PhD Niina Nurmi) in cooperation with Gottfried Hohmann and Oliver Schuelke.

A number of differences between humans, chimpanzees and bonobos might be related to differences in development. It is therefore of great importance to monitor changes of physiological parameters during ontogeny. One specific event that until now has been only described for humans and chimpanzees is the occurrence of adrenarche characterized by the onset of adrenal secretions of increasing amounts of dehydroepiandrosterone-sulfate (DHEA-S). In a recent study in cooperation with Gottfried Hohmann, we could first validate an EIA for the use of bonobo urine and then show that apart from humans and chimpanzees, bonobos also show the typical pattern of increased DHEA-S secretion around the same age as it is observed in these species and therefore also have adrenarche (Behringer et al. 2012b).

We have developed an LC-MS method that allows us to simultaneously measure 23 different steroid metabolites in only 200µl of urine (Hauser et al., 2008a). Using this method we detected methodological problems in the extraction and measurement of testosterone in great ape urine in all existing studies so far and we were able to propose better alternatives (Hauser et al 2008b). Furthermore, by measuring a number of androgens and their metabolites in serum and urine of chimpanzees from the Ngamba Island sanctuary in Uganda in cooperation with Lawrence Mugisha, we were able to identify which of these were good indicators of testicular activity in urine and which of these were mainly produced by the adrenal glands. Interestingly, although testosterone constitutes only a minor fraction of the overall mass of androgens in urine, it still turned out to be the best indicator of testicular activity (Hauser et al. 2011). LC-MS analysis proved to be a potent tool for the validation of enzyme immuno assays for endocrinological measurements in matrices such as urine and feces. For example, we found that a testosterone assay formerly considered to be inappropriate for measuring gonadal activity in chimpanzee urine actually delivers very reliable results, and that existing inconsistencies were not related to cross-reactivities of the antibody but to inappropriate extraction methods (Preis et al. 2011). In an additional project in cooperation with Susan Perry, we developed a LC-MS method that allows for the measurements of androgens, glucocorticoids, and estrogens in fecal extracts of white-faced capuchins (Weltring et al. 2012).

In order to better monitor stress responses that are mainly triggered via the sympathetic nervous system, we validated in cooperation with Erich Möstl the use of an assay to measure alpha-amylase in the saliva of bonobos (Behringer et al 2012a).

The parameters influencing diseases in wild African great apes are currently unknown. One important factor could be stress, which is generally known to increase the secretion of a number of hormones, including cortisol. While long-term stress leads to an increased susceptibility to a variety of diseases, including infectious diseases, and cortisol is known to have a negative impact on the immune system, infections themselves can also increase cortisol excretion. Therefore, changes in cortisol levels prior and during an infection with pathogens, as well as variation in the degree of sickness behaviour shown, could be indicative of the severity of an infection.

In cooperation with Fabian Leendertz form the Robert Koch-Institute, we investigate the “susceptibility-infection-health-susceptibility” loop of non-lethal infection in wild primate communities. Furthermore, we investigate how times of being “ill” may affect an individual’s social relationships and competitive abilities (PhD Helene De Nys).