A 14-year demographic study shows that group size affects geladas’ fitness and survival, a finding that could impact other grouped mammals. Photo credit: Elizabeth Tinsley Johnson
Scientists have long struggled to determine the group size that would best enable animals to live long lives and raise many surviving offspring. But now a research team that includes anthropology researchers at Stony Brook University used demographics from several groups of wild geladas (a species of Old World monkey) spanning 14 years to determine that living in medium-sized groups is best for fitness , essentially survival and reproduction. Details of the results are published in the Proceedings of the Royal Society B.
The relationship between group size and individual fitness has long been the focus of socio-ecological theory, but is difficult to prove for long-lived species such as geladas.
In this study, the researchers used a large sample of wild geladas (200 adult females from 46 groups) that live in the Simien Mountains National Park in northern Ethiopia. These animals have been observed for much of their lives, which allows researchers to study reproductive outcomes and survival rates of females and their offspring.
They found that females in small groups had the highest mortality rates (perhaps due to higher predation rates), while females in medium-sized groups produced the most surviving offspring. Women in small and large groups had the highest infant mortality rates. Child murder is the largest source of child mortality in Geladas and only occurs when a new male overthrows the breeding male of a Gelada group. While infant mortality was high in large groups because they are more attractive to bachelors who wanted to “take over” a group, infant mortality in small groups may have been high because those infants have fewer protectors.
“Overall, our study results show that optimal group size may indeed be linked to infanticide risk as well as predation,” says co-author Amy Lu, Ph.D., assistant professor in the Department of Anthropology at the College of Arts and Sciences at the Stony Brook University. “This has implications for a wide variety of group animals, such as other primates and lions, where infanticide can be a risk factor for female fitness and infant survival.”
The research supports the hypothesis that infant homicide risk is a major factor in group size in leaf- and grass-eating mammals, where food competition is relatively low. However, females in groups that are too large and too small cannot be doomed.
“Although small groups find it difficult to increase their group size to a more optimal size, large Gelada groups sometimes split into two or more smaller groups,” says co-lead author Jacob Feder, Ph.D. Candidate in the Interdepartmental Doctoral Program in Anthropological Sciences at Stony Brook University. “By getting closer to the optimal group size, these females could improve their lives. But separation means breaking social bonds, so women must carefully choose who to stay with and who to break up with.”
According to Lu, Feder, and co-authors, the research suggests that female geladas achieve the highest levels of fitness in medium-sized groups and shed light on how and why group size affects the fitness of long-lived species like geladas.
Female mammals kill their competitors’ offspring when resources are scarce
More information:
Elizabeth Tinsley Johnson et al., The Goldilocks Effect: Female Geladas in Medium-Sized Groups Have Greater Fitness, Proceedings of the Royal Society B: Biological Sciences (2021). DOI: 10.1098 / rspb.2021.0820 Provided by Stony Brook University
Quote: Study of Wild Geladas Shows Medium Group Life is Best for Survival and Fitness (2021, June 2), accessed June 2, 2021 from https://phys.org/news/2021-06 -wild-geladas-reveals-mid- size group.html
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