By Victoria Gill
Science and nature reporter, BBC News
How hyenas persuade friends to help
Research has revealed how the very sociable spotted hyena engages its "friends" in group activities.
Female hyenas carry out greeting ceremonies - standing side by side and sniffing each other - before embarking on a group task.
This appears to reinforce social bonds before a potentially risky activity, such as a hunting trip or an attack on an enemy predator.
The findings are reported in the journal Animal Behaviour.
Dr Jennifer Smith from the University of California, Los Angeles, led the team of researchers.
They studied adult female spotted hyenas,
Crocuta crocuta, in a large female-dominated social group in Kenya.
Female hyenas have enlarged penis-like genitalia
Professor Kay Holekamp from Michigan State University, who also took part in the study, explained that the animals, also known as laughing hyenas, live in "fission-fusion" societies.
"They live at the top of a food pyramid, so there's a lot of competition for food and they often separate to hunt," she told BBC News.
"So they're on their own a lot, but they come together when they need to form a coalition - to defend their territory, for instance."
This constant wandering and returning is key to why greetings are so important to the animals.
"You usually see greetings when two animals come together having been separated for an extended period," said Prof Holekamp.
This rather intimate sniffing will be a familiar sight for any dog-owner.
But for hyenas, a very unusual feature of their anatomy is involved in these ceremonies.
Females, which are considerably larger and socially more dominant than males, have enlarged penis-like genitalia.
This pseudo-penis becomes enlarged when the animals are "socially excited".
"Previously, scientists thought that an erect phallus was a sort of flag of submission to another hyena," said Prof Holekamp.
But she and her team have now found that, rather that an indication of lower social status, this excitement, and the subsequent ceremonious sniffing, were a way for the animals to gather support.
"We saw them engage in these greeting ceremonies and then form a coalition to mob a lion," said Prof Holekamp.
Trying to scare away another top predator in this way is very risky, and the greetings appear to "get all the animals on the same emotional plain", she explained.