Can Hyenas Count?

During a pivotal scene in West Side Story, the Jets and the Sharks meet to prepare to rumble. As the gangs assembled each side likely tried to assess their opponent’s strength. This ability to assess numerical advantage is highly advantageous in any species where conflict between groups is common.

Spotted hyenas (
Crocuta crocuta) live in large fission-fussion clans where smaller groups often forage separately (Figure 1). The composition and size of these subgroups varies over time as clan members move in and out of these smaller units. Hyenas are also highly aggressive and do not tolerate strangers in their home areas. As a consequence, occasionally lethal attacks occur when small groups from different clans meet.

Jurvetson
Figure 1. A group of spotted hyeans (Crocuta crocuta) detect the kill made by a lion. (From Flickr/jurvetson)

Do individual hyenas assess numbers of opponents before engaging in aggressive conflicts? Game theory predicts that they should, especially when large numerical imbalances occur. A group from Kay Holekamp’s lab at Michigan State University set out to test whether wild hyenas can assess the numbers of potential opponents. They used playback experiments on wild hyenas living in the Masai Mara plains of East Africa.

First, the researchers made digital recordings of the whoop calls of unfamiliar hyenas (i.e. hyenas from Tanzania, Malawi, and Senegal). They organized the calls into three groupings of three calls each: one group consisted of three whoop calls from the same individual (A,A,A); a second included a whoop call from one unfamiliar hyena followed by a call from a second unfamiliar hyena and a third call from the first hyena (A,B,A); and a third recording that contained calls from three different hyenas (A,B,C). The research playback these calls to members of two hyena clans in the Mara and observed their behavior.

Lone hyenas became highly vigilant to playbacks of multiple unfamiliar intruders (Figure 2). Equally important was that hyenas could distinguish the number of unique intruders; they became increasingly vigilant to calls of two and three unfamiliar hyenas (Figure 3).

intruders
Figure 2. Average time 12 lone hyenas spent oriented towards the speaker in 34 playback trials. (From Benson-Amram et al., 2011)

whoopcalls
Figure 3. Mean time spent oriented towards the speaker when hearing calls from one, two, or three intruders. (From Benson-Amram et al., 2011)

Finally, small groups of hyenas were more likely to approach the playback speakers (a potentially risky behavior) when their numbers exceeded the number of “intruders” calling (i.e. three intruders). Thus, it appears that spotted hyenas can recognize individuals by their contact calls and use that information to assess the number of potential intruders before engaging them in battle.

References
Benson-Amram, S., Heinen, V., Dryer, S., & Holekamp, K. (2011). Numerical assessment and individual call discrimination by wild spotted hyaenas, Crocuta crocuta Animal Behaviour DOI: 10.1016/j.anbehav.2011.07.004