A Puma's Larder

Most animals, mammals included, spend a considerable proportion of their time either eating or searching for something to eat. Foraging for food is not necessarily difficult in the case of grazing animals, since grass is usually plentiful where they live, but for most others, it is not quite so simple. There is generally a pay-off of some kind to be made between how nutritious the food might be and the amount of energy that would be expended in finding it - as measured by the time spent searching and the area that needs to be covered to do so. And then there's the amount of energy that will be expended in acquiring the food once you've found it; easy enough if you're a herbivore or even a scavenger, but possibly requiring a fair bit of effort if you're trying to kill something.

But that's not necessarily the end of the story. Many animals cache a proportion of their food, saving it for later. One immediately thinks, perhaps, of squirrels storing their nuts so that they can come back to them in winter when food is in short supply. Many burrowing rodents do something similar, hoarding food in underground chambers that can return to at their leisure. But carnivores can cache food, too, despite the fact that meat tends to go off more quickly than properly stored nuts or grain.

We're typically talking about solitary carnivores here, since pack predators such as wolves will quite quickly share a kill amongst themselves, leaving nothing left over. But most carnivores are solitary, so it's not a rare phenomenon, albeit one that has been less extensively studied than most other aspects of feeding.

Why might a carnivore choose to cache meat rather than eat it straight away? Three basic explanations have been proposed, and any, or all, of them might apply to a given species. Firstly, there's the reason that squirrels do it, storing food away in case you need it later. For example, Arctic foxes have been reported to gather literally thousands of goose eggs from large nesting colonies, far more than they could possibly eat, and then store them away for later. They go back to these caches more frequently when lemmings are scarce, suggesting that this is a backup plan used precisely because lemming populations tend to crash and boom.

This works because the foxes live in a climate so cold that the eggs can be preserved for months and still remain edible. This brings us to the second possibility which is that caching food in the right way might extend its 'shelf life', allowing the predator to eat more of a large prey item before it goes off. Wolverines, for example, carry carcasses to relatively inaccessible, cold places before stashing them. Not only does the cold keep the meat preserved for longer, but the fact that it's hard for scavengers to reach means that it's more likely to still be there when they come back, allowing them to get more meals from a single kill than they otherwise would. 

Finally, there's the issue that if the predator has killed something large enough it simply isn't going to be able to eat it all at one sitting. If it at least tries to hide it, there's a better chance that it will still be there when they come back, and they get two meals for the price of one. When grizzly bears kill large hoofed mammals, they are far more likely to store them for later than they are when they kill smaller prey. A leopard hiding its kill up a tree where hyenas can't get at it serves a similar purpose.

The puma/cougar/mountain lion (Puma concolor) is a large, solitary predator that is likely familiar to most westerners to at least some extent, if only from nature documentaries. Despite being widely known under (at least) three different names, it is all one species. "Puma", while sometimes used solely to refer to those subspecies living in Latin America, is also the term used most commonly to refer to the species as a whole, so that's the one I'm going to stick with here. They are, of course, large cats, but they belong to the "purring cat" subfamily, rather than the "roaring cat" group so that they are actually more closely related to house cats than to, say, tigers or leopards. 

Pumas do, as we might expect, cache food. They do, after all, prefer to eat large prey such as elk, moose, and bighorn sheep and, as ambush predators, they put considerable effort into catching and killing their food. When you go to the length of leaping out at a creature that may be larger than yourself and then engaging in a fight to the death with it, clearly you want to waste as little of the resulting meat as you possibly can. Studies as to how and why pumas cache their food are not new, but a particularly large one was recently published, and provides some insight into the lifestyle and ecology of these animals.

The study involved seven pumas living in the Mendocino National Forest in the Cascade Mountains of northern California. They were fitted with GPS collars that allowed researchers to follow their movements and then went to look in places where they seemed to have killed prey or cached food to see what it was and what they had done with it. By the end of the study, they had identified 352 kill sites, although there may well have been others (especially of smaller prey) that went undetected.

Overall, the pumas cached 61% of their kills, although some individuals were more likely to do so. The tactic here is generally to drag the carcass away from the kill site, hiding it somewhere with dense foliage, often at the base of a tree and covering it with leaf litter and the like. Although one might expect them to do this when food is most abundant, thus saving it for later, this didn't seem to be the case here; if anything they were less likely to hide food in summer when there was most of it about. Perhaps that's because, since meat doesn't last in a warm summer, there was little point, but it may also be that a lot of their food at such times consists of deer fawns, which are too small to be worth storing - the puma might as well just eat them all up straight away.

As a defence against other animals stealing the kill, it's likely that this is at least some of the motivation, or why bother trying to hide it at all. And perhaps it is effective as a means of concealing food from scavenging birds. But, as a more general tactic, it doesn't seem to have been very effective. Over 60% of the caches hidden by pumas were discovered by passing black bears, who would then proceed to eat as much of the unexpected free food as they could. What may work for leopards, then, does not work for pumas. (Plus, leopards hide their caches up trees... that really isn't going to work against bears).

Which leaves us with the idea that the main motivation for pumas caching food is simply that they can't eat all of it at once and want to come back to it later. If this were the case, we'd expect that they'd be more likely to cache large prey than smaller animals... but it turns out that it wasn't quite so simple. In fact, the pumas were far more likely to cache mid-sized prey animals than those that were either larger or smaller. In this part of the world, that's mostly adult black-tailed deer, as opposed to say, elk or moose. 

The fact that they aren't caching smaller food - fawns, rabbits, and so on - despite killing such animals, does agree with the "save some for later" hypothesis. It's less clear why they don't do the same for the very largest prey, however. Perhaps the best explanation is that it's simply too much hassle to drag the thing somewhere where you can hide it, and that it might be harder to conceal if you did. 

It's also interesting that when they cached medium to large prey, they were more likely to do so on the edge of their territory than further inside. That seems counterintuitive since, if it makes no difference to the likelihood of a bear finding it, it does make it easier for another puma to do so. Perhaps, if this doesn't benefit the individual, it helps the species, with the animals unintentionally helping one another out with free food supplies...

Synapsida will be taking a break next week, but will return on 6th August.

[Photo by Eric Kilby, from Wikimedia Commons.]