Dieter Lukas

     Causes and consequences of sociality

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The benefits of high rank

Female mammals who hold a high social rank in their group have higher reproductive success

Summary of our article: Shivani et al. (2022) The effect of dominance rank on female reproductive success in social mammals. doi: 10.24072/pcjournal.158


Living in social groups can create conflict over limited resources that
are important to everyone in the group. In most social mammals,
individuals arrange themselves in dominance hierarchies that govern
access to resources while reducing open aggression. A new study new
study by Shivani (Indian Institute of Science Education and Research
Kolkata), Elise Huchard (Institut des Sciences de lí…volution,
Montpellier), and  Dieter Lukas (Max Planck Institute for Evolutionary
Anthropology) brings together evidence from various mammalian societies to show that differences in dominance ranks among females generally translate into inequalities in their reproductive success.

Previous authors had argued for such an association between rank and
reproductive success, but it was unclear how to reconcile existing data
where authors had not found such an association. "We developed a series of predictions to assess when we would expect rank to lead to
differences in reproductive success in females", says Shivani. "We
checked these predictions by external reviewers, before testing them
with comparative data from 86 different mammalian species".

The researchers found that higher dominance rank can be linked to different measures of higher reproductive success. In particular, higher ranking females often have more and larger offspring, and they have shorter intervals between births compared to lower ranking ones. While the particular fitness link might differ depending on local conditions, dominant females appear better off in one way or another in almost all societies.

In addition, high rank was found to be more strongly related to higher
reproductive success in species in which females have large litters with
short inter-birth intervals, while differences among females were not as
pronounced when females only have single offspring at long intervals.
Effects were also stronger in societies such as those of meerkats, where
cooperatively breeding groups are closed to immigrants and females have strong social bonds including helping each other raise offspring.
"Surprisingly though, dominance rank was not more important in harsh
environments", says Huchard, "and the inequalities of having different
social ranks also arise under conditions we would consider benign."

The association between rank and reproductive success are far from
perfect, and in most societies, all females, even the lowest-ranking
ones, do reproduce. Nevertheless, even such small initial differences
might have long-term consequences. Dominance rank is often inherited in matrilines, because daughters of dominant mothers often have better
starting conditions plus might receive the support of their mother to
obtain their own social position in the group. Benefits therefore can
build up, potentially leading to large variation in the long-term growth
of different matrilineal lineages. "Dominance structures are often
reinforcing, and through inheritance high ranking females might not only
themselves benefit from their position but also pass those privileges on
to future generations", says Lukas.

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