The spotted hyena whoops, giggles and groans. What do the groans mean?
Frédéric Theunissen, firstname.lastname@example.org
Associate Professor of Psychology and Neurosciences
University of California, Berkeley
Steve Glickman, email@example.com
Professor of Psychology and Integrative Biology
University of California, Berkeley
University of California, Berkeley
Popular version of paper 4pABd
"Vocalizations of the Spotted Hyena (Crocuta crocuta): Eliciting Acoustic Variation in Groans"
Presented at 5:20 p.m. on Thursday July 3, 2008 in ROOM 343
The spotted hyena (Crocuta crocuta), also known as the common hyena or laughing hyena, is the most common carnivore in the Serengeti. It is a beautiful, strong and intelligent animal that would occupy most of the plains in sub-Saharan Africa if it had not been displaced by human activity. Contrary to its reputation as a scavenger, the hyena kills most of its prey, often in successful cooperative hunts. Indeed, one of the reasons that the hyena is so successful is that it lives in large groups or “clans” with established rules for cooperation, resource allocation and the protection of its young. As in all intelligent social species, the hierarchy and bonds between animals in the clans require sophisticated communication skills. And indeed, the nocturnal spotted hyena has an incredibly rich vocal repertoire of calls that they use in communication.
The best known hyena call, is the hoot-laugh, or giggle, which has led humans to designate these animals as "laughing hyenas." The giggle is a high pitch, staccato sound that is not communicating a good time. In fact, it is commonly produced by distressed, or submissive, animals in situations where they are both excited and conflicted between approaching and leaving the situation. For example, giggles are made by submissive individuals at a kill waiting their turn while being chased away by higher ranking animals. The hoot-laugh is a variant of the giggle that has lower pitch and that is produced in more aggressive situations often in a chorus. The vocal repertoire of the hyena, however, is much larger including as many as 14 different sounds produced in different conditions. The call that has been studied the most in the field is the “whoop”: a very loud and musical call that often starts with a very low tone which is then modulated up and down in pitch. The whoop is a distance communication call that the hyenas use to announce their presence when they are out of visual contact. Each hyena also has its unique and stereotyped whoop, so that whoops can also be used for individual recognition. It has been observed that new immigrant males who are approaching a new clan produce a high number of whoops as to carefully advertise their arrival into a group that could potentially reject them (Mills, 1990).
Figure 1. Example of vocal calls from the rich repertoire of the spotted hyena. The pictures show the energy of the sound as a function of frequency (y-axis) and time (x-axis). These diagrams are called spectrograms and can be read like a musical partition. The left panel shows the spectrogram for a giggle chorus (top), a groan (middle) and a growl (bottom). The right panel shows examples of two whoops recorded at different times for two individuals: notice the characteristic signature of the individual’s whoop. Click on the links below to hear the sounds.
LISTEN: Hyena Giggle Chorus
LISTEN: Hyena Groan
LISTEN: Hyena Snarl and Growl
LISTEN: Rocco's First Whoop Call
LISTEN: Rocco's Second Whoop Call
LISTEN: BJ's First Whoop Call
LISTEN: BJ's Second Whoop Call
Besides laughs, giggles, yells and whoops, hyenas produce a series of softer sounds that are much more difficult to study in the field. A group of these sounds has been labeled growls and groans and have been observed when adult hyenas approached each other, or by mothers calling their babies out of underground dens. These growls/groans are often produced in conjunction with complex body language. The meaning of these sounds however remained unclear. We took advantage of the unique conditions provided at the Berkeley Field Station for Behavioral Research to perform the first experimental study, where we systematically elicited groans in hyenas by presenting different objects that where of interest to them: unfamiliar hyena cubs, meaty bones, and an empty transport cage used to contain bones or cubs on other experiments. Being curious, both male and female hyenas approached these objects and vocalized softly during these approaches.
Figure 2. The Berkeley Field Station and two spotted hyenas playing in the pool of one of the large pens.
Figure 3. Examples spectrograms of groans elicited in our experiment by one male and one female subject. The groans are elicited when the hyenas were presented with the empty transport cage (top), the meaty bone (middle) and the cub (bottom). The groans to the cub are more tonal and higher pitch. Click on the links below to hear the sounds.
LISTEN: Bear Groans to Empty Cage
LISTEN: Bear Groans at the Bone
LISTEN: Bear Groans to Cub
LISTEN: Eeore Groans to Empty Cage
LISTEN: Eeore Groans at the Bone
LISTEN: Eeore Groans to Cub
Although the frequency of the approaches was similar to all three objects, the cubs elicited more groans. More interestingly, the groans directed to the cubs were acoustically different to those elicited to the bone or empty transport cage: the cub oriented groans had higher pitch and were more musical. A systematic statistical analysis of the sounds showed that we could separate the sounds into two distinct classes. In addition for both classes of sounds, male calls were always higher in pitch than female calls. We concluded that within the classes of groans different communication signals are produced depending on the behavioral context. It is possible that the lower pitch and less musical growls produced to the meaty bone signal a more aggressive approach (“that bone is mine”) while the more tonal and higher pitch groan signal signifies a friendly approach (“it is ok little cub”). But given that our cubs were unrelated to the subjects in the experiment and that deception is also a possibility (“it is ok little cub but I might kill you”), we have to be careful when we attempt to associate a meaning to the sounds. To further study the value of the acoustical signal, we have also elicited groans of mothers to their own cub in the same situation. We found that these groans are also of tonal class and potentially of higher pitch than those to an unfamiliar cub. In the future we plan to also examine the response of the cubs to the playback of these sounds and record both their approach behavior and the vocal sounds that they produce in response. Human language might be unique in its complexity and flexibility but hyena vocalizations are more telling than any of us could imagine.
The Berkeley field station houses a large colony of captive spotted hyenas for research purposes. Dr. Glickman is the director of a long standing research program designed to study the behavior and the reproductive physiology of the spotted hyena in a semi-natural and controlled environment.