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Variation in female leverage: The influence of kinship and market effects on the extent of female power over males in Verreaux’s Sifaka

Citation

Lewis, Rebecca; Bueno, Gabrielle; Di Fiore, Anthony (2022), Variation in female leverage: The influence of kinship and market effects on the extent of female power over males in Verreaux’s Sifaka, Dryad, Dataset, https://doi.org/10.5061/dryad.ngf1vhhwh

Abstract

Female mammals employ reproductive strategies (e.g., internal gestation) that result in power asymmetries specific to intersexual dyads. Because the number of eggs available for fertilization at any given time for most mammals is quite limited, having a fertilizable egg is potentially an important source of economic power for females. Control over mating opportunities is a source of intersexual leverage for female Verreaux’s sifaka (Propithecus verreauxi). We examined economic factors thought to influence the value of mating opportunities, and, thus, the extent of female leverage: kinship and market effects. Using a longitudinal dataset of agonistic interactions collected during focal animal sampling of all adult individuals in 10 social groups from 2008-2019, we tested the effects of relatedness, female parity, reproductive season, and adult sex ratio (population and group) on (1) the direction of submissive signalling and (2) which sex won a contested resource. While 96% of the acts of submission were directed from males towards females, females only won a third of their conflicts with males. Thus, our study has implications for evolutionary explanations of female-biased power. If female power evolved due to their greater need for food and other resources, then intersexual conflicts would be expected to result in males more consistently relinquishing control of resources. As expected, males were more likely to chatter submissively towards successful mothers, during the mating season, and when the sex ratio was male-biased. Although females generally had less power to win a conflict when their fertilizable egg was less valuable (when they were nulliparous or unsuccessful mothers or when interacting with male kin) and with an increasing female-bias in the sex ratio, this ability to win additionally was influenced by which sex initiated the conflict. Our study demonstrates that female leverage can be influenced by the supply and demand for mating opportunities, but evoking submission does not translate into winning a resource. Indeed, intersexual power is dynamic, contextual, and dependent on the individuals in the dyad.

Methods

We collected all occurrences of intragroup agonistic intersexual interactions during 1-hour focal animal sampling sessions (Altmann 1974) of all adult and subadult sifaka, for a total of approximately 14,000 hours of observation from 2008 through 2019. Insufficient behavioral and demographic data were available for 2009-2010 because Cyclone Fanele interrupted data collection and thus were excluded from our analysis. The identity of the initiator and of the receiver was recorded for each interaction, and all behaviors occurring during the interaction were recorded as occurring either in isolation or as part of a sequence. Agonistic behaviors were defined according to the Brockman (1994) ethogram with the following additions: “food rob +” (X tries to take the food away from Y and is successful), “food rob –” (X tries to take the food away from Y and is unsuccessful), and “snap at” (X bites in the direction of Y but does not make contact). In addition, “proximity” (a concept implicit to the definition of certain behaviors) was defined as occurring when individuals were within 1 m of each other (Lewis 2019).

For all agonistic encounters, we scored an individual as “winning” a conflict if the other individual in the dyad moved at least 1 m away from the “winner” within 10 seconds of the agonistic interaction. Note that we did not limit our analysis of “winning” to the feeding context because sifaka compete for other resources in addition to food (e.g., water, space, sun, shade, grooming partners, huddling partners). Moreover, our definition of a “win” included a broader set a behaviours than merely “supplant” (X moves toward Y, Y immediately changes location [within 5 seconds], X occupies the location previously held by Y: cf. Brockman 1994)). If neither individual withdrew after the agonistic interaction, neither individual was considered the winner and the outcome was scored as “neutral”. The one exception to this rule was for the “food rob” behaviors because “food rob +” is defined as an initiator successfully gaining control of the food resource, while “food rob –” necessarily means that the initiator was not successful. Therefore, the identity of the sifaka that had control of the food resource at the end of the “food rob” behavior was scored as the winner.

Sifaka often exhibit multiple aggressive and/or submissive behaviors within an agonistic interaction. We thus used the following rules for scoring an interaction as “win” versus “neutral” when multiple behaviors occurred in a sequence. If a social interaction began with an approach, the 10 second rule started with the time of the first non-approach agonistic behavior. If an individual used multiple types of aggressive behaviors essentially simultaneously (e.g., lunge and cuff), then we only scored the first aggressive act. However, if an individual used repeated, successive acts of aggression towards another individual (e.g., three cuffs within 10 seconds) and the receiver chattered submissively immediately after each individual aggressive act (e.g., cuff then chatter response, cuff then chatter response, cuff then chatter response), we scored each aggressive act independently, with the assumption that the additional acts of aggression were needed because the first aggressive act was not successful. When animals repeatedly made a submissive chatter vocalization spontaneously (i.e., without receiving aggression within 10 sec beforehand), we scored the agonistic events as independent when there was at least 5 seconds between the end of the first chatter and the beginning of the second chatter. To address the issue that these repeated aggressive or submissive acts are not entirely independent of one another, we assigned a corresponding proximity “bout identity” to each agonistic act and then used bout ID as a random factor in our statistical models. A proximity bout was defined as a period of time in which the members of a dyad were continuously within 1 m of each other within a given 1-hr focal sample, and all behaviors occurring during this period were assigned with the same bout ID. Finally, for each agonistic interaction, we also scored the identity of the initiator of the interaction. Note that by definition, the winner of an interaction was always scored as the initiator for supplants.

We conducted monthly censuses of the population in the Ankoatsifaka grid system of trails (Leimberger & Lewis 2017; Lewis et al. 2020). In addition to locating all groups with radio collars and recording the identity of each individual present in the group, we located unmarked groups and solitary individuals by walking the trail system. While sifaka live in cohesive groups, they sometimes visit other groups, and males occasionally roam independently during the mating season (Richard et al. 1993; Brockman 1999; Leimberger & Lewis 2017). On the rare occasions when a known individual was not observed on the day of the census, we nonetheless retroactively added them to the census data for that month if the individual was observed during behavioral data collection within seven days of the census.

Funding

National Science Foundation, Award: BES# 1719654

Leakey Foundation

Primate Conservation, Inc.

University of Texas at Austin

Multiple private donors