Significant that when spotted hyena hunts plains zebra, it is males that tend to do the hunting

(writing in progress) 

Some of the earliest studies of the interaction between the spotted hyena (Crocuta crocuta) and the plains zebra (Equus quagga) were done in the Ngorongoro Caldera, by Hans Kruuk in the ‘seventies.

In the Caldera, the spotted hyena of preys mainly the western white-bearded wildebeest (Connochaetes mearnsi). However, it also preys on the plains zebra (E. q. boehmi).

Does this successful hunting of zebra by hyena refute my hypothesis about the adaptive value of striping in zebras (https://www.inaturalist.org/posts/67289-the-adaptive-function-of-striping-in-the-plains-zebra#)?
 
I consulted Estes (1991), who has summarised the relevant findings of the research into patterns of predation at this location.
 
Estes (1991) states ‘Hyenas usually go foraging alone or in pairs.’

To understand this, please bear in mind that the spotted hyena has a complex society, and uses gregariousness against e.g. the lion (Panthera leo). However, it is seldom that one individual of the spotted hyena actually cooperates deliberately or overtly with a conspecific while hunting, in the way that the lion and the African hunting dog do.

Instead, the spotted hyena vocalises when it finds an opportunity it cannot deal with by itself. Conspecific individuals soon arrive.

What follows is a matter of strength in numbers, rather than any attempt to cooperate. Each individual is hunting on its own behalf, but certain prey can be taken down only by many individuals all hunting on their own behalves, together.

A lone individual of the spotted hyena is capable of running down a healthy adult male of Connochates mearnsi, but only with good fortune. I infer that no individual of the spotted hyena is likely to kill a healthy adult Equus quagga boehmi; the zebra is just too big and too defensive.
 
Estes confirms that the spotted hyena "will always select the most easily captured prey. Where ungulates are common, they try to locate very young or incapacitated individuals, relying mainly on their eyesight and sense of smell.

A very common technique, practised on game concentrations of all kinds from wildebeests to flamingos, is to make them run. This is done by suddenly loping toward the concentration. The animals, which hardly move out of the way of a walking hyena, flee in the opposite direction for 50-100m, enabling the hunter or hunters to detect any stragglers. Why prey species allow hyenas to get closer than other large carnivores remains unclear."
 
Please note that prey species do not flee from one, two, or three individual hyenas, as opposed to a group. Instead they tend to approach the predator.

This may have bearing on a pattern of fleeing that I have found so puzzling over the years. When one drives a vehicle past prey species, the animals tend not to flee directly away from vehicle. Instead they run in such a direction as to approach the road ahead of one, at an angle, and then race across it. This has always seemed unnecessary to me. However, I now realise that this behaviour may be an extension of stotting: the prey are actually trying to present themselves to the predator to facilitate the choice of a vulnerable target?
 
Anyway, back to Estes (1991).
 
This author specially mentions, re the spotted hyena, that ‘Its ability to detect something wrong with an animal that seems perfectly normal to us is uncanny.’
 
So far, the point I am making is that, in the hunting strategy of the hyena, everything seems to depend on the selection of a vulnerable individual of the prey species. I infer that this would be particularly so when it comes to the plains zebra. This is because that species is larger-bodied and more feisty than the commonest prey species, the wildebeest – which itself is big enough that a lone hyena usually fails to catch it.

"Few victims put up any resistance after a hard chase". However, a healthy individual of the wildebeest is fast enough to escape from a lone hyena over a lengthy course. I.e. the wildebeest survives mainly owing to speed and endurance, rather than fighting back.
 
Now we get to the really interesting part:
 
"Most cooperative hunting by hyenas comes about thus spontaneously, when individuals foraging for themselves grab the opportunity to be in on the killing and eating of a quarry singled out by one of their number. But spotted hyenas also stage deliberate pack hunts: up to 25 Ngorongoro hyenas (average number 11) forgather and deliberately set out to hunt zebras, walking for miles through thousands of wildebeest until they encounter a zebra concentration. Their concerted effort is needed to circumvent the determined defence of herd stallions and break up zebra families."
 
Now, this may seem to be evidence against the hypothesis I have presented, i.e. that the striping of zebras frustrates the hunting strategy of the spotted hyena. However, the picture is ambivalent, and I argue that it actually supports my rationale. For, as Estes (1991) states: "A female hyena usually leads such pack hunts, but typically more males than females participate."
 
Now, let us think about that for a moment.

I infer that, when zebras are killed by the spotted hyena, it is mainly males that do the killing. Males are always inferior to females in the spotted hyena, and must yield to them in any contest for food. So, it makes sense that males will tend to try prey avoided by females, i.e. to push their luck somewhat.

The zebra is generally just too hard for females. However, males are willing to try, by doing something rather extreme for this species, i.e. to cooperate actively as opposed to merely by default.

I take this as evidence that the zebra is a difficult prey species not just because it is relatively big and feisty, but also because it presents obstacles to suitable choice of a vulnerable target.

What I am basically suggesting is that, in the relationship between prey and predator, it makes sense that the subordinate sex (males in the case of the spotted hyena) should try a marginal species of prey, knowing that it risks failure. In turn, it is interesting to infer that, because males of the zebra react by defending the harem, the contest will tend to play out as male against male: males of the hyenas chasing and trying to get around males of the zebra.

Females of the hyena will tend to stick to the easier option of just hunting the wildebeest, something that males cannot easily capitalise on, because any male approaching the kill will tend to be deterred by a female.
 
To restate the highlights of all this:
 
The spotted hyena, although gregarious in various senses, does not usually set out to hunt cooperately; instead it sets out alone. The benefit of having to share its kill with conspecifics (who cadge, uninvited) is that this keeps the lion (a serious cadger in its own right and more than happy to kill any hyena for no reason) at bay.
 
Zebras, like other 'plains game' as I infer it, will let a lone hyena approach quite closely as it looks for a target. In this situation I suspect that the striping makes it considerably harder for the hyena to choose a vulnerable individual of the zebra than of the wildebeest.
 
Females of the spotted hyena will of course hunt zebras at times. Hiwever, I infer that they will take them less frequently, relative to their numbers, than is the case for the wildebeest. So the zebra, which replaces itself rather tardily, can survive among the wildebeest - despite a population density of the hyena that is supported mainly by the wildebeest.
 
When the zebra is deliberately hunted by the hyena, it takes an unusual level of cooperation. This is something that generally only males are prepared to resort to. Although, even here, the leader is a female, this female would not be in business because it would probably not get female companions to hunt the zebra. Although such attacks no doubt work, I infer that they are the exception rather than the rule, and males would prefer to hunt wildebeests as the females do.
 
(writing in progress)

Posted on 13 de julho de 2022, 05:01 AM by milewski