Margaret Buehler is a PhD candidate in the Department of Anthropology at Tulane University. She was awarded a Leakey grant for her research titled “Subordinate male roles in multimale primate groups with high reproductive skew.”
My research strives to answer a seemingly simple, yet important, evolutionary question about group-living primates: why do specific primates choose to live together? About 70% of group-living (as opposed to solitary) primates reside in groups with multiple adult males and multiple adult females. An initial hunch is that males and females live together for reproduction, but for many multimale-multifemale primate species, the alpha male gets most, or all, reproductive opportunities (high reproductive skew). So why then are subordinate males living in groups? This question is even more perplexing in species characterized by male dispersal because typically males are not related to any other group members, which makes inclusive fitness benefits unlikely. Even though the males themselves may still need to live in social groups to access key resources and avoid predators, it is unclear why females and the alpha male allow additional males to join their groups. As the number of individuals in a group increases, so does energy-expenditure, within-group resource competition, and disease risk. Allowing males to join social groups clearly comes with added costs, so my current research focus is to understand the benefits of living in multimale groups.
For most primates that live in multimale groups, female reproductive success increases as the number or proportion of adult males in the group increases (ex: white-faced capuchins, chacma baboons, geladas, black howler monkeys, etc.). This is puzzling because all of these species are characterized by high reproductive skew, so if subordinates are unlikely to reproduce, how are they improving female fitness? Many studies, of white-faced capuchins and other primates, have shown that subordinate males improve fitness, but few, if any, have directly tested how subordinate males impact female reproductive success. Our theory is that subordinate males are protecting (a) key resources necessary for reproductive function, (b) ovulating females, and/or (c) their infants from conspecifics and predators.
In January, my team and I set out to see if, and to what degree, subordinate males are participating in resource, mate, and infant defense to shed some light on the benefits individuals can gain from living in multimale groups even with high reproductive skew. We are collecting behavioral and GPS data on 13 males in 3 study groups to determine how male defensive behavior differs according to group demographics (presence of infants and/or fertile females), proximity to resources, and/or location within the home range (i.e. central areas vs periphery). I will use these data to determine if there is any evidence of subordinate male participation in resource, mate, and/or infant defense in this species.
So far, we have experienced success in the data collection process. For the first three weeks, I trained my three field assistants on data collection methods and how to identify individual monkeys. Luckily, capuchins are easily identified by differences in facial/fur coloration, marks, scars, broken digits, etc. (see photos above), so our research team does not have to capture or collar any of our study individuals.
The downside of this method is that we must learn how to identify all individuals in all of our study groups while they move through the trees, which is no easy task! The other struggle of no-contact tracking is that we must first locate our study groups by searching the forest and stay with them from dawn until dusk to ensure that we will be able to find the group on subsequent days. Despite these struggles, my assistants have risen to the occasion. They quickly developed the skills and stamina necessary to keep up with these groups and collect accurate data. Over the past 5 weeks, we have successfully collected over 50 hours of behavioral data on subordinate males (via continuous focal samples in 10-minute increments) and recorded subordinate male behavior during 8 intergroup encounters.
In addition to behavioral data, we are collecting GPS data to analyze how subordinate male behavior varies according to location within their home range. We are taking GPS points at 30-minute increments to establish group home ranges, and we are recording the starting and ending location of all behavioral samples and intergroup encounters. The latter we will use to test how behavior varies by location. For example, we can test if subordinate males more defensive or vigilant when they are near their home range periphery and likely to encounter other groups, or if they are more likely to participate in an IGE if they are near a terrestrial water source. In order to test how ovulating females impact male behavior, we are collecting fecal samples to non-invasively assess female fertility. This process is the most difficult as we must sample each female in each group every 2-3 days, excluding those who are still nursing or visibly pregnant. This requires tremendous patience, as the researcher must follow a single individual closely until she defecates, which can take several hours for each female.
At this point, I cannot offer much insight into the answer to our research question. We cannot know the extent of each group’s home range until we finish data collection, and thus, we cannot determine how behavior varies within it. In addition, I cannot determine female fertility until I transport the fecal samples to our lab in the United States. I will need to aggregate the data from many behavioral samples before we can draw conclusions about general behavior patterns. Over the next two months, we hope to collect an additional 100+ hours of focal data, nearly 120 fecal samples, and a few dozen descriptions of intergroup encounters. I look forward to aggregating these data and discovering what is happening here in Santa Rosa. My hope is to use these data to predict long-term patterns and use the rich dataset available on this primate population to test inferences on how exactly subordinate males impact female reproductive success.