By using both field observations and a machine-learning algorithm applied to the audio clips recorded in the field, a research team led by the University of Nebraska-Lincoln (UNL) has investigated the “whoops” of spotted hyenas in the Maasai Mara grasslands of southwestern Kenya. The analysis revealed that hyena whoops feature signatures unique to individuals – a form of “caller ID” distinct enough that hyenas can distinguish one from another – and that repetition of such whoops often improves identification.
Since spotted hyena clans are built on hierarchies depending upon social rank, but consist of multiple families that regularly come together and disperse across the savannah, a capacity for identifying specific individuals is particularly important.
“Hyenas don’t treat every individual in the clan the same, so if they’re deciding whether to show up and help someone, they want to know who they’re showing up to help,” said study lead author Kenna Lehmann, a postdoctoral fellow in Biology at UNL.
By recording the sounds of hundreds of hyenas in the African savannah and employing a machine-learning algorithm trained to pair specific hyenas with the whoops they emitted, Dr. Lehmann and her colleagues identified three major traits of the whoops that helped hyenas recognize the individual that emitted them: the duration of the call, the highest frequency of the call, and the average frequency during the portion of the call which was most consistent in pitch. According to their analysis, the greater the disparity in these traits, the more likely the hyenas would be to distinguish among the sources of the whoops.
However, since many acoustic nuances can get lost during transmission – when the calls come from far away, or are masked by environmental noises from wind, precipitation, or other animal calls – repetition of the whoops becomes essential. “There’s an understanding that one of the ways to get your message across is to repeat it, especially if you’re in a noisy environment or if you’re communicating over long distances,” Dr. Lehmann explained. “It’s like getting a little bit more information (each time). The first time you hear it, you might notice: Oh, that was definitely a male or a female voice. Then, the next whoop, you might be able to narrow it down further.”
Although the calls of some animal species also contain signatures that differentiate the groups to which they belong from other same-species groups – somewhat similar to human accents or dialects – the researchers were surprised to find that this was not the case for hyenas. Apparently, the amazing ability to memorize so many individual signatures may have rendered a clan signature either useless or not useful enough to bother developing.
“If you know who the individual is, you know what group they’re in,” Dr. Lehmann said. “Animals are pretty good at associating that information. So if they need individual signatures for other reasons, then there just may have never been a need to also develop a group signature, which is what this finding suggests. They should be able to keep track of all the individual voices and be able to distinguish: If this is Individual X, they’re in my group. I can choose to help them based on them being a group member, but maybe there are more decisions to be made about whether they’re a group mate that I actually want to help,” she concluded.
The study is published in the journal Proceedings of the Royal Society B: Biological Sciences.
By Andrei Ionescu, Earth.com Staff Writer