Are you smarter than a Hyena?

3 07 2007

Spotted Hyena (Crocuta crocuta), the “bad guys” of the African plains. My wife is repulsed by them, but I find something oddly intriguing about these predators; any social carnivore in which the female has a functioning pseudo-phallus nearly indistinguishable from the male’s is bound to stand out.

Spotted Hyena Skeleton

Spotted Hyena are extremely effective, social predators, being much more effective hunters than many people think. The classic African battleground, at least in the mind of the public, features a pride of lions valiantly defending a kill from a pack of dastardly hyenas, laughing and yipping in a way that can only be described as disturbing. Just as likely as not, however, the favored lions are trying to steal prey killed by hyenas, both groups of carnivores vying for the top position in their habitats (cheetahs, wild dogs, leopards, and other smaller predators typically give up their kills to the more formidable lions and hyenas). The social structure and coordination utilized by hyenas requires a fair amount of intelligence, however, and I have previously hypothesized that binocular vision (accompanied by the existence of lips and eventually shortening of the face) had some role to play in the evolution of intelligence, allowing carnivores a greater ability to form complex social structures based upon facial/body cues (binocular vision is the key step to the beginnings of having a “face”) than herbivores with eyes on each side of their head. Likewise, a new study in the Journal of Mammalogy by Holekamp, et al. looks at how the intelligence of Spotted Hyena may be convergent with that of some primates with a similar social structure.

Intelligence is a costly thing to have, especially metabolically speaking; it takes a lot of energy to run what’s inside your noggin, so whatever selective forces produced our big brains must have been of considerable advantage (or allowed to keep acting as we were better able to literally feed our brains). The authors of the paper outline two hypotheses about the circumstances that favored the evolution of great intellectual capacity;

The 1st hypothesis suggests that these traits were favored by selective forces in the physical environment, such as the need to learn and recall when fruit might become available on trees that fruit sporadically, how to use tools to extract difficult foods, or how to navigate visually through a 3-dimensional arboreal world (e.g., Milton 1981; Povinelli and Cant 1995). The 2nd hypothesis, known broadly as the ‘‘social complexity’’ hypothesis, suggests instead that the selective force favoring big brains and advanced cognition in primates was the need to anticipate, appropriately respond to, and manipulate the social behavior of conspecifics (reviewed in Byrne and Whiten 1988).

I look at this problem not from the angle of one over the other, but rather “Which came first and which had the largest impact at any given period?” but the authors state the social complexity hypothesis seems to have more evidence to support it, although studies in non-primate animals are lacking. If the social complexity hypothesis is correct, however, we should see evidence of convergence among other groups of animals similar in social structure to primate groups, and this makes the spotted hyena an ideal subject to test this hypothesis. Indeed, the social structure of the hyenas seem to have parallels with the primate subfamily Cercopithecinae (“baboons, macaques, vervets, guenons, talapoins, patas monkeys, and mangabeys”), the carnivores providing a good “independent test” given that the last common ancestor they shared with primates existed during the late Mesozoic.

Like primates, spotted hyenas do not produce large amounts of offspring per pregnancy (usually 1 to 2 cubs are born at a time) and the young a fed milk for an extremely long time, up to 24 months (the copious amounts of bone hyenas ingest keeps the milk flowing for this extended period). In fact, the period during which hyenas are nursed surpasses that of the primate analogs considered in the paper, but the hyenas do become reproductively ready in about the same time-frame as the primates (male hyenas are ready essentially when weaned {although this doesn’t mean they successfully mate at this time}, females begin mating between 3 and 4 years). Indeed, the authors of the study have produced an entire table for comparison between hyenas and the primates, and the lack of substantial differences are surprising. Let’s take baboons as our prime analog for the hyenas; both live in large groups of animals related through mothers that stay in the group, dominance being determined via a hierarchy of display and combat, cooperative resource sharing, individuals maintaining bonds with kin, individuals preferring higher-ranking members for same-sex social partners, etc. Indeed, the primary differences seem to be that hyena clans are female-dominated and that hyenas engage in cooperative hunting, the overall social structure of the distantly-related mammals being surprisingly alike.

While we do benefit from the overall comparison of behaviors shared by certain primates and hyenas, the ability to recognize 3rd-party relationships is an important sign of the intelligence an animal may possess. It’s easy to remember who is higher or lower than yourself in a social hierarchy, but to be able to recognize the status of, for example, two fighting hyenas and side with the higher ranking one to gain favor can be quite useful. The authors describe the test of 3rd-party relationships among hyenas in this way;

Thus, a hyena considering an attack might benefit, for example, when attempting to displace a larger subordinate animal from food, by delaying its attack until the arrival of a potential ally who is higher ranking than the target animal. If a hyena increases its rate of aggression only after a hyena that is higher ranking than itself arrives on the scene, then the animal might be following a simple rule of thumb, such as ‘‘Only attack a larger subordinate when another individual is present who is higher ranking than yourself.’’ Use of this sort of simple mental algorithm would not require that hyenas be able to recognize 3rd-party relationships among their group-mates. However, if a hyena’s attack rate also increases after the arrival of an individual who is dominant to the victim but subordinate to the attacker, then the attacking hyena must recognize the relative ranks of the other 2 individuals. In the latter case, the hyena would be demonstrating that it can indeed recognize tertiary relationships.

In considering dominance it should be noted that mere size does not mean everything, especially for young hyenas. Young hyenas are born into the rank just below their mothers, and so it was quite easy for a larger hyena to be subordinate to others unless it is able to establish itself through a fight. Being able to call utilize a cohort to prevent a larger animal from winning in a battle would be a good way to preserve the hierarchy in spite of size difference. Even if two hyenas are fighting and a more dominant individual arrives on the scene, it will side with the more dominant of the two individuals in the dispute, and so it is a very bad day for a hyena when a subordinate attacks a more dominant individual without any support, nor is it good for the relatives of the losers. In addition to the imposed dominance hierarchy, hyenas also recognize kin relationships, and after fights whichever hyena was the aggressor is more likely to act aggressive to the kin of the victim, even taking out aggression on the kin of their opponent more than on unrelated lower-ranking members (which Sapolsky described many times among baboons in A Primate’s Memoir). Still, there’s much we don’t know about hyena interactions, and more work needs to be done before we can more completely compare it to the behavior of baboons.

The spotted hyena is just one of several related animals belonging to the Family Hyaenidae, and all of its relatives are less gregarious, even to the point of not living in groups at all. The authors call for further research into cognition and intelligence with these animals as well, being that if the close relatives of the spotted hyena lack some of its cognitive abilities we can better discern how social complexity has influenced the evolution of intelligence. Just as well, study of the brains of hyenas is desperately needed; not only do we need to observe social structure, but to see if these animals have “social brains” as well. If the social complexity hypothesis is correct, we would expect the frontal cortex of the spotted hyena to be more developed than that of its less-gregarious relatives, and the same type of study could be carried out with lions in relation to other big cats. Speaking of lions, I also have to wonder how lions and spotted hyenas may have driven each other into forming prides or clans; while it’s easier to gain food as a group, it would also be easier to protect or steal food in a group rather than acting as an individual. Unfortunately the behaviors of these animals is long gone, but I can’t say that I don’t wonder about it.

But wait, there’s more! In looking for other papers to tie into this discussion I happened across Samuel Gosling’s 1998 paper “Personality Dimensions in Spotted Hyenas (Crocuta crocuta),” describing the various behavioral traits of 34 hyenas based upon “Assertiveness, Excitability, Human-Directed Agreeableness, Sociability, and Curiosity.” This sounds a little anthropomorphic at first, but animals are not all merely carbon-copies of each other, behaving in a certain fashion because they are merely programmed to do it. No, anyone who is familiar with any group of animals (especially social mammals) for any length of time can distinguish distinct personalities in the group, and the existence of varied personalities amongst hyenas speaks to their intellectual capacity as well. Indeed, what Gosling found was that dominance hierarchy alone could not explain all the behaviors of the captive hyenas studied, the only strong correlation made being females are more “assertive” than males (which is to be expected in a female-dominated social species). “Agreeableness” to keepers may also have been influenced by dominance, the keepers being viewed as being more dominant by some hyenas, but this relationship between humans and hyenas likely goes out the window when dealing with wild populations.

Spotted hyena are absolutely amazing social carnivores, one of the animals most indicative of Africa. They are easy to malign, but can be difficult to understand, but gaining such understanding may help us answer some important questions about how we came to be as we are. While I’m sure it will be a bit nerve-racking, I do look forward to my first night in Africa someday, hearing the “whoops” of the hyena across savanna at night.



10 responses

3 07 2007
Zach Miller

Amazing! One wonders why other binocular-vision carnivores (like cats) haven’t developed this level of intelligence!

3 07 2007

Zach; I think the development of binocular vision opens up a larger suite of expressions available to carnivores, but it may not pan out if the predators remain solitary. If they do not need to keep track of other animals and their expressions in a social group, then there would be no reason to continue to develop social intelligence and other related cognitive abilities. I guess what I was hypothesizing was that binocular vision would be a first physical step that would allow some animals to develop more complex social interactions based on expressions, the social aspect being the main force that would push evolution. That’s why I’d like to look at the brains and habits of lions in relation to other cats; they are also social carnivores that need to deal with daily social interactions, so would their brains and habits be different just as the spotted hyenas brains and habits are different from its relatives? I think so, but observation/experiments need to be done.

4 07 2007

I have actually just started a research project comparing the mating habits of the different hyaenid species, and then I got my JM in the mail last week and there was C. crocuta on the cover! I agree the article was very interesting, I thought it was actually kind of a bold claim, but the discussion makes some fascinating points, although hyenas don’t converge on cercopithines in several ways, it just goes to show that in nature there is almost always more than one way to come up with a given feature or suite of features. Thanks for the link to Gosling’s paper!

The talk about sociality and intelligence makes me even more amazed at cephalopods. They have none of the typical features of intelligent animals: they aren’t social, they aren’t long-lived, they don’t have extensive parental care, they don’t have fast metabolisms, etc etc…I know their “intelligence” isn’t directly comparable to that of vertebrates, but they’re still pretty damn amazing.

5 07 2007
Zach Miller

Cephalopods really are the primates of the sea (in terms of intelligence). I wonder what environmental pressures pushed the evolution of intelligence in these multi-tentacled creatures?

17 09 2007

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18 06 2008

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22 10 2012
Laura McKenzie Peterson

Can I ask where you got that image of the hyaena skeleton? I’m trying to put together a presentation and I can’t find any decent postcranial images of anybody in hyaenidae (well, not with reliable sources). It looks like you have lots of pics from different museums, so maybe you even have one of Pachycrocuta brevirostris…? Anyway, if you could let me know, that’d be great, of course I’d cite you 🙂

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