Massive Israeli-led study of data collected over 27 years sheds light on social networks, rank and survival of African spotted hyenas
Spotted hyenas are no laughing matter. Known to experts as Crocuta crocuta), they are commonly known as “laughing hyenas” due to their loud, high-pitched rapid series of staccato “hee-hee-hee” sounds they make – giggle vocalization that resembles hysterical human laughter.
It is today the sole living member of the genus Crocuta that is native to sub-Saharan Africa. Although there are as many as 47,000 of these mammals around the world, their numbers outside of protected areas are declining due to changes in their habitat and poaching.
Thought to have originated in Asia, they used to inhabit most of Europe for at least one million years until the end of the Late Pleistocene period. It has a bear-like body, rounded ears and a less-prominent mane than other hyenas, which include the rare brown hyena and the striped hyena that is native to East and North Africa, the Indian subcontinent, Central Asia, the Caucasus and the Middle East.
The spotted hyena is a very social animals with a social organization unlike that of any other carnivore and closer in behavior to primates like baboon and macaque monkeys with respect to group size, hierarchical structure and frequency of social interaction among both kin and unrelated group-mates. They live in “clans” of up to 100 hyenas of various ages. the size of which depends on the abundance of prey. Life in the clan can be difficult for lower-ranked individuals, as they may be excluded and not get enough food after a hunt. Spotted hyena society is matriarchal; females are larger than males and dominate them.
The social system of the spotted hyena is openly competitive rather than cooperative, with access to kills, mating opportunities, and the time of dispersal for males depending on the ability to dominate other clan members. Females provide only for their own cubs rather than assist each other, and males display no paternal care.
Social networks among animals are critical to various aspects of their lives, including reproductive success and survival and could even teach us more about human relationships. Therefore, scientists at Bar-Ilan University (BIU) in Ramat Gan (near Tel Aviv) have spent much time studying the social networks of spotted hyenas. Dr. Amiyaal Ilany said: “We found overwhelming evidence that social connections of offspring are similar to those of the mother. Even after the mother-offspring bond itself weakens dramatically, the offspring still remain connected to their mother’s friends.”
Ilany, a biologist at BIU’s Goodman Faculty of Life Sciences, integrates behavioral ecology, network science and social science to study broad aspects of social behavior in the wild. As a postdoctoral researcher at the University of Pennsylvania, he developed, together with Dr. Erol Akçay, a theoretical model suggesting that social inheritance – in which offspring inherit their social bonds from their parents, either passively or by copying them – could explain the social networks of multiple species.
In a new study published in the prestigious journal Science under the title “Rank-dependent social inheritance determines social network structure in spotted hyenas,” Ilany and his team show for the first time on such a large scale that their model correctly hypothesized that a process of social inheritance determines how offspring relationships are formed and maintained. Their study also elucidates the major role that social rank plays in structuring the spotted hyena clan and how this affects survival.
To test their model, Ilany and Akçay forged a partnership with Michigan State University’s Dr. Kay Holekamp, who had had spent the previous 27 years observing wild spotted hyenas in Kenya. The researchers pored over Holekamp’s data, which included nearly 74,000 social interactions among the spotted creatures.
“Social affiliations are, indeed, inherited within clusters of hyenas. The plethora of data on spotted hyenas that was collected by Kay Holecamp provided us with a golden opportunity to test the model we developed several years ago,” commented Ilany, the lead author of the study. “We found overwhelming evidence that social connections of offspring are similar to those of the mother. A mother who has social affiliations with another hyena can connect her offspring to that hyena and the two, in turn, will form a social bond.” Even after the mother-offspring bond itself weakens dramatically, the offspring still remain connected to their mother’s friends.
“Rank is super important,” added Akçay. “Those born to a lower-ranked mother are less likely to survive and to reproduce.” Descendants of high-class individuals face fewer constraints than descendants of lower-class individuals in choosing their social partners. The researchers found that offspring born to high-ranked mothers copied their mother’s bonds more accurately than those born to low-ranked mothers.
Social inheritance plays an important role in survival, and the researchers discovered an association between the two in both mothers and female offspring. There was a positive relationship between offspring survival and social associations that were similar to their mothers, but only in offspring of high-ranked mothers. Mothers of offspring who were more similar to them in social association were more likely to survive to the following year, possibly reflecting a change in maternal relationships as they get older.
The results of this study suggest that social inheritance plays an important role in building the social networks of hyenas and further supports the researchers’ hypothesis that in species with stable social groups, the inheritance of social connections from parents is the cornerstone of social structure. In several species successful social integration is associated with higher survival and reproductive success. The results add to this by showing that social inheritance is also associated with both offspring and mother survival.
The researchers noted that social network inheritance likely contributes to a group’s stability and also has implications for how behaviors are learned and spread through groups. The study also underscores how factors other than genetics hold sway in key evolutionary outcomes, including reproductive success and overall survival. “A lot of things that are considered by default to be genetically determined may depend on environmental and social processes,” concluded Ilany.
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