Rock Hyraxish Decoded: The Females Coo, the Males Boast

Researching hyrax calls at the Dead Sea may tell us something about the evolution of speech, and more about the evolution of swagger

Rock hyraxes, May 29, 2019.
Yacov Ben Bunan

If you were the only girl hyrax and I was the only boy hyrax, ten to one what we’d be doing isn’t talking. But under normal circumstances, rock hyraxes live in groups of up to 30 ladies and juveniles with a single dominant male, and they evidently have something to say.

The females chitchat and a have wider repertoire than the males, reports the team from Bar Ilan University. The males seem to have less conversation and seem mostly to be bragging: “I am the HYRAX.”

Can communication behavior in desert-dwelling communal animals shed light on the evolution of language and communality in Homo sapiens? Maybe. A group of scientists certainly had a heroic stab at it.

The starting point of this story is Ein Gedi, an oasis near the Dead Sea in Israel, home to wild hyraxes whose behavior Prof. Eli Geffen of Tel Aviv University has been studying since 1999. Vlad Demartsev and Naomi Gordon, supervised by Geffen and Dr. Amiyaal Ilany of Bar Ilan University, took advantage of this profoundly known animal population to test a human linguistics theory dating to the 1930s called the Law of Brevity. Their work on informational coding economy in the rock hyrax was reported in Evolution Letters.

The richer the content we wish to convey, the more time and even muscular effort we have to invest in conveying it, hence the evolutionary logic in optimization. The Law of Brevity, postulated by George K. Zipf, an American linguist and philologist who studied statistical occurrences in different languages, says the shorter the word, the more it is used in speech. For example, we say “yes” more frequently than “affirmative.”

Rock hyraxes, January 23, 2019.
Yacov Ben Bunan

That principle was verified in almost 1,000 languages, say the researchers, and can be observed in action: long words get truncated without loss of meaning, such as television to TV; synchronize to synch; whatever to whatev.

Animals communicate. The simplest of creatures may use chemical signals. At the more complex end of the rainbow, advanced animals can vocalize too, from croaks to meows to whale song. The more complex the animal’s social life, the more necessary communication becomes. So: Could some form of the Law of Brevity be identified in animals too? Optimizing vocal communication effort with short meaningful sounds rather than long ones would make evolutionary sense.

Alternatively or in addition, among animals, might optimization of vocal repertoire be achieved by modulating amplitude – producing more least-effort soft calls than loud ones (leaving aside the need to produce long-distance calls)?

If a hyrax meets a hyrax coming through the wadi

Wait. Why study this in hyraxes, of all animals? “Hyraxes are an interesting model system with complex social and communications systems we’ve been studying for 20 years. Of course they’re not the only option,” Ilany tells Haaretz. They also turn out to have quite the repertoire.

Pups grow up in the predominantly female group and as the males mature, they move out. A typical hyrax group will have a single dominant male and, it appears, a bunch of adolescent loudmouths. The dispersing bachelors lead largely solitary lives and don’t seem to have much to talk about, only encountering females during the short mating season. When they meet other males, they fight.

And how was the hypothesis that the Law of Brevity might apply to rock hyraxes tested? Ilany and Koren trapped and anesthetized 19 of the animals, from multiple groups and some solo males, and fitted them with tiny audio recorders on collars. They then recorded and logged all their vocalizations for about a week.

The researchers then removed the devices (which were about the size of a one-euro coin), listened to the recordings and labeled them and created a “full rock hyrax vocal repertoire,” at least as evinced in that week.

Rock hyrax relaxing, August 29, 2011.
Amiyaal Ilany

We are apes

How could the Law of Brevity be identified? Absent our ability to identify Hyraxish words, if they have any, the scientists suggest two potential equivalencies: the duration of vocalizations and their amplitude, from soft to loud. The law would suggest that the most frequent calls would be short and soft.

The female hyrax emitted more long calls than the theory would predict, and had a wider range of call types, including affiliative calls (such as cooing, sounds related to social bonds). “This was not surprising, as hyrax females maintain stable social relationships within a group,” writes the team.

Their amplitude followed the “least effort” paradigm: soft calls were more frequent than loud. It will take more research to elucidate whether they’re mellifluously gossiping at length about the males vocally beating their furry chests.

Males had soft short calls too, but “frequently sing complex and loud self-advertising songs, transmitting their individual quality to both females and neighboring males,” write Ilany and Koren.

Males may have a wider range too, but they didn’t experience it in that one week of observation, Ilany told Haaretz.

Separate research found that female fruit bats in a captive colony also have a lot to say and they’re usually the ones who get loud – yelling at the males.

The relationship between call duration and usage frequency has been tested in several animals, chiefly primates. Results differed between species: some such as marmosets and golden-backed uakaris showed no evidence for such informational economy. Dolphins and Formosan macaques did, on the other hand.

Researchers suggest that the need to bray some message far and wide could explain the lack of a clear fit of animal repertoires to the brevity principle. Let’s say that the results don’t contradict the possibility that the Law of Brevity is a basic animalian condition and that the human habit of brevity stems from the evolutionary origins of animal communication. We really are just apes.

Mind you, Ilany clarifies, they aren’t claiming the rock hyraxes have language. Just sayin’.