Variation in aggression rates and urinary cortisol levels indicates intergroup competition in wild bonobos.

Intergroup competition is a widespread phenomenon across taxa and groups typically compete over access to limited resources, such as food and mates. Such competition may be quantified by changes in individuals' behavioral and physiological status in response to intergroup encounters (IGEs). Bonobos, one of our closest living relatives, are often regarded as xenophilic and exhibit high tolerance towards out-group individuals. This tolerance between groups may still be accompanied by intergroup competition over resources. We hereby compared variation in aggression rates and urinary cortisol levels of bonobos during and outside contexts of IGEs in the Kokolopori Bonobo Reserve and investigated whether food and mate availability influenced males' and females' aggression and cortisol levels, when controlling for dominance rank and the number of individuals present. We found that although females had higher aggression rates and urinary cortisol levels during than outside contexts of IGEs, these increases were not related to food availability or changes in between-group dynamics when maximally tumescent females were present, rather than absent. Furthermore, males showed higher aggression rates and urinary cortisol levels during than outside contexts of IGEs. However, males' responses during IGEs were not related to the presence of maximally tumescent females and food availability. Taken together, while competition intensified during seemingly tolerant IGEs in bonobos, such competition was unrelated to short-term changes in food and mate availability. Despite physical and physiological costs of aggression, bonobos associate with out-group individuals frequently and for extended periods. This suggests potential benefits of bonobo intergroup associations.

Importance of subterranean fungi in the diet of bonobos in Kokolopori.

Nonstaple food is a food resource which sole consumption does not allow the maintenance of regular physiological functions, thus constituting a minor portion of an individual's diet. Many primates consume nonstaple food such as meat, insects, and fungi. Hypotheses on the dietary importance of nonstaple food include its role as fallback food and as source of specific nutrients. We tested these two hypotheses by investigating mycophagy (i.e., the consumption of fungi) in a population of wild bonobos in the Kokolopori Bonobo Reserve, DRC. Specifically, we examined the relationship between fungus consumption and various factors relevant to bonobo feeding ecology (i.e., fruit abundance and the consumption of other food types). Additionally, we measured the deviation from linear travel when bonobos searched for fungi to evaluate the nature of fungus consumption (e.g., opportunistic or targeted). Lastly, we examined the nutritional content of the major fungus species consumed (Hysterangium bonobo) to test whether this food item was potentially consumed as source of specific nutrients. We found that bonobos spent a higher proportion of their time feeding on fungi when fruit abundance was higher, indicating that fungi were not consumed as a fallback food. Moreover, bonobos deviated from linear travel when visiting fungus patches more than observed when visiting fruit patches, suggesting that they actively sought out fungi. Lastly, initial analyses suggest that H. bonobo samples contained high concentration of sodium. Collectively, these results suggest that subterranean fungi appear to be attractive food source to Kokolopori bonobos, and that mycophagy may serve to supplement nutrients, like sodium, in bonobo diet.

Evaluating the efficacy of a consumer-centric method for ecological sampling: Using bonobo (Pan paniscus) feeding patterns as an instrument for tropical forest characterization.

Characteristics of food availability and distribution are key components of a species' ecology. Objective ecological surveying used in animal behavior research does not consider aspects of selection by the consumer and therefore may produce imprecise measures of availability. We propose a method to integrate ecological sampling of an animal's environment into existing behavioral data collection systems by using the consumer as the surveyor. Here, we evaluate the consumer-centric method (CCM) of assessing resource availability for its ability to measure food resource abundance, distribution, and dispersion. This method catalogs feeding locations observed during behavioral observation and uses aggregated data to characterize these ecological metrics. We evaluated the CCM relative to traditional vegetation plot surveying using accumulated feeding locations across 3 years visited by a tropical frugivore, the bonobo (Pan paniscus), and compared it with data derived from over 200 vegetation plots across their 50 km2+ home ranges. We demonstrate that food species abundance estimates derived from the CCM are comparable to those derived from traditional vegetation plot sampling in less than 2 years of data collection, and agreement improved when accounting for aspects of consumer selectivity in objective vegetation plot sampling (e.g., tree size minima). Density correlated between CCM and plot-derived estimates and was relatively insensitive to home range inclusion and other species characteristics, however, it was sensitive to sampling frequency. Agreement between the methods in relative distribution of resources performed better across species than expected by chance, although measures of dispersion correlated poorly. Once tested in other systems, the CCM may provide a robust measure of food availability for use in relative food availability indices and can be incorporated into existing observational data collection. The CCM has an advantage over traditional sampling methods as it incorporates sampling biases relevant to the consumer, thereby serving as a promising method for animal behavioral research.