Building bonds between males leads to more offspring for chimpanzees


Date:
August 17, 2021
Source:
University of Michigan
Summary:
If you're a male chimp looking for love -- or offspring -- it
pays to make friends with other males. A study examined why male
chimpanzees form close relationships with each other, and found
that male chimpanzees that build strong bonds with the alpha male
of the group, or with a large network of other males, are more
successful at siring offspring.



FULL STORY ==========================================================================
If you're a male chimp looking for love -- or offspring -- it pays to
make friends with other males.


==========================================================================
A study led by the University of Michigan, in collaboration with Arizona
State and Duke universities, examined why male chimpanzees form close relationships with each other, and found that male chimpanzees that
build strong bonds with the alpha male of the group, or with a large
network of other males, are more successful at siring offspring. The
results are published in the journal iScience.

"One big question that biologists have had for a long time is why you
see so many friendly behaviors such as cooperation and alliance in
animals," said lead study author and U-M postdoctoral researcher Joseph Feldblum. "One would expect to see these social bonds -- or strong,
friendly social relationships -- only if they provide some sort of
fitness benefit to the individuals. Males wouldn't spend all this
time grooming other males and forgoing trying to find females or food
unless you get some kind of benefit from it." One benefit would be the opportunity to sire more offspring, but no previous studies have looked
at the link between social relationships and reproductive success in chimpanzees. Much of the research in this area has been done in female primates, who are primarily concerned with accessing resources in order
to reproduce quickly. For males, the biggest task is getting reproductive access to females, says Feldblum, also an assistant professor in the U- M Department of Anthropology and member of the Michigan Society of Fellows.

"Chimps cooperate frequently, and often in these very dramatic ways:
You see things like grooming, all kinds of complex alliance formation
and group territorial defense," Feldblum said. "The question is: What
do males get out of it and how?" It turns out, they get babies.



==========================================================================
One function of these social bonds, the researchers found, is to help
males gain access to mating opportunities they wouldn't otherwise be
able to get without help from their friends. To examine the link between sociality and paternity success, the researchers examined behavioral
and genetic data from a population of chimpanzees living in western
Tanzania. The group is part of the ongoing study of chimpanzees in Gombe
Stream National Park, begun by Jane Goodall in 1960.

The researchers began by constructing a base model that captures the
effects of male age, dominance rank and genetic relatedness to the mother
on male siring success. They first used the model to look at 56 siring
events with known paternity between 1980 and 2014. Then, they tested
whether adding measures of male social bonds to the models improved
their ability to predict which male would sire a given offspring.

They found that males with more strong association ties -- males with
the highest number of social bonds with other males -- had a higher
likelihood of siring offspring. In fact, two or more strong association
ties meant a male chimpanzee was more than 50% more likely to sire a
given offspring, after accounting for the chimp's age, relation to the
mother and dominance rank score.

Next, the researchers wanted to understand how a chimp's relationship
to the alpha male underpinned male reproductive success. To do this,
they examined the role of strong bonds with the alpha male, looking at
45 siring events by non- alpha males. They generated the same base model,
this time comparing the model with models that included several measures
of bond strength with the alpha male, among other measures.

The model that fit best included what is called the composite sociality
index, which includes grooming and association with the alpha male. It
showed that subordinate males with strong bonds with the alpha male,
as well as those with many strong association ties, were more likely to
sire a given offspring.



========================================================================== "Sucking up to the boss is nothing new," said co-author Anne Pusey
of Duke University. "We show that it's always paid off." But the
researchers also found that two factors -- a strong bond with the alpha
male and many strong association ties -- both independently contributed
to reproductive success.

In animal behavior, coalition formation is when two or more
individuals jointly direct aggression toward a third or another group
of individuals. According to previous research, individuals that are
more central in the network of coalitions tend to rise in rank and sire
more offspring.

In the current work, the researchers showed that males who form stronger
ties are also more likely to form coalitions, and the researchers
hypothesize that this larger alliance network helps males gain mating opportunities. They also found that forming these many strong bonds
leads to chimps' improvement in rank within the group; those that made
it to the alpha position were also more likely to sire offspring.

A clearer idea of the benefits of social relationships in chimpanzees
provides clues about the evolution of friendship in humans.

"Together with bonobos, chimpanzees are our closest living relatives, and
help us to identify which features of human social life are unique. This
study suggests that strong bonds among males have deep evolutionary
roots and provided the foundation for the more complex relationships
that we see in humans," said senior author Ian Gilby, a researcher at
ASU. "This research also highlights the value of long-term studies like
these, which are essential for understanding the biology of a species
that lives for many decades and is slow to reproduce." Feldblum says
more research is needed to tease out how coalition formations and these
social bonds lead to siring success.

"Is it that if your ally is nearby, you're more likely to mate with an
estrus female, or does having your allies around you protect you from harassment from other males?" Feldblum said. "Or because your ally will
support you if a conflict erupts, your stress levels are lower and you can devote more energy to mating efforts? This last step we still don't know." Gilby is an associate professor at the School of Human Evolution and
Social Change at ASU, and a research affiliate in the Institute of Human Origins, which curates the data used in this study. Pusey, professor
emerita at Duke, has spent the last 30 years of her career assembling, organizing and digitizing this unique dataset.

Study co-authors also include Joel Bray of ASU and Christopher Krupenye
of Duke, Johns Hopkins University and Durham University in the United
Kingdom.

The study, "Social bonds provide multiple pathways to reproductive
success in wild male chimpanzees," will be available when the embargo
lifts at DOI10.1016/ j.isci.2021.102864.

========================================================================== Story Source: Materials provided by University_of_Michigan. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. Joseph T. Feldblum, Christopher Krupenye, Joel Bray, Anne E. Pusey,
Ian
C. Gilby. Social bonds provide multiple pathways to reproductive
success in wild male chimpanzees. iScience, 2021; 102864 DOI:
10.1016/ j.isci.2021.102864 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/08/210817111436.htm

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