Intergroup relationships in Verreaux's sifakas (Propithecus verreauxi)
Final Report Abstract
In this study, we examined intergroup relationships in Verreaux's sifakas by using a multi-disciplinary approach, including behavioral observations, spatial data gained from GPS loggers and nutritional analyses of food items. How animals achieve collective action for territorial defense and how they manage relationships with neighboring groups has recently become a central topic in the study of animal behavior. Because interactions between groups are fundamentally agonistic, they are expected to have vital consequences for individual fitness. In principle, the power of a group can be influenced by multiple factors, such as group size or habitat quality, but also by the sum of individual contributions because group members have different motivations and may thus exhibit different levels of participation in between-group conflicts. At present, the factors influencing both individual participation in intergroup conflicts and the odds of winning a conflict in gregarious mammals are generally not well understood. In Verreaux' sifakas individual participation in both sexes was influenced by different but also similar incentives. Male participation was influenced by social status, with dominant males participating in virtually every encounter, and males with fewer opportunities to reproduce freeriding more often. Females did free-ride only when dependent infants were present, most likely to protect them from potential injuries or infanticide. In both sexes, the number of actively participating individuals was influenced by the duration of the conflict and the number of actively participating individuals in the opponent groups. Thus, sifakas appear to base their decision to participate on the actual circumstances of the encounter. Verreaux's use their home range relative flexibly and switch between the most intensively used areas approximately every two weeks, most likely to food availability. Accordingly, location of the encounter in relation to the most intensively used area a month before the encounter predicted winning an encounter. Interestingly, the numerical advantage of one of the opponent groups did not predict the outcome of the encounter, although group size in this study period differed by about 130% between some groups. Moreover, sifakas did not exhibit clear intergroup dominance relationships. Thus, in sifakas the combination of higher motivation of residents in defending intensively used areas and collective action problems in intruders may explain this pattern. These findings highlight again the fact that the variable circumstances and individual features can overcome group characteristics and predict the probability of winning a conflict. In addition, potential costs of intergroup encounters are rarely studied. In this study, we could show that sifakas do not face shortterm costs such as increased travel costs directly after losing an encounter, but rather long-term costs by having reduced access to the disputed encounter area. The nutritional analyses revealed that seasonality impacted the quality of diet in both sexes. In addition, females spent more time feeding and had a higher intake of macronutrients than males. In particular, during late lactation females had a higher intake of nutrients than males, suggesting that this increase in nutrients is not simply reflecting the rise of food availability but most likely due to cover the high energetic demands of reproduction. Since food is a limiting resource for sifakas, indicating a potential for food competition between groups, female participation appears to be influenced by resource defense. In conclusion, sifakas seem to decide joining a given encounter opportunistically, most likely based on a combination of individual incentives and the actual circumstance of each encounter, i.e. location of the encounter and number of actively participating opponents, suggesting that the complexity in intergroup relationships appears to be the product of decisions made by each individual group member. Thus, detailed information on individual participation and the various circumstances of encounters are crucial to understand collective action and cooperation during intergroup conflicts in gregarious mammals.
Publications
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Intergroup encounters in Verreaux's sifakas (Propithecus verreauxi): who fights and why? Behavioral Ecology and Sociobiology, May 2016, Volume 70, Issue 5, pp 797–808
Koch F., Signer J, Kappeler P and Fichtel C
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The role of the residence-effect on the outcome of intergroup encounters in Verreaux's sifakas. Scientific Report vol 6, Article: 28457 (2016)
by Koch F., Signer J, Kappeler P and Fichtel C
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Sex and seasonal differences in diet and nutrient intake in Verreaux's sifakas (Propithecus verreauxi), American Journal of Primatology, 79,4, Special Issue: Recent Advances in Primate Nutritional Ecology, April 2017, Pages 1-10
Koch F, Ganzhorn J, Rothman J, Chapman C and Fichtel C
