Why do chimps hunt?

A Savannah chimp eating a bush baby she caught with a spear

A Savannah chimp eating a bush baby she caught with a spear

Chimps are the only species of great ape (except for us) that hunts and kills mammals. Sometimes even with spears. However, this makes up a relatively small part of their diet (some estimates put it at only 3%), so it is widely believed that the primary reason they hunt isn’t for food. The groups which hunt together are typically all males, so it has long been thought that it might actually be a social activity with all the men of the troupe bonding over the murder of another animal. But soon to be published research argues that we’ve underestimated the dietary importance of meat in the chimp diet.

The paper in question – creatively titled “Why do chimpanzees hunt?” – notes that although meat makes up a small amount of the food eaten overall, in the day to day life of a chimp its actually a significant lump. Obtaining the same number of calories through eating insects or fruit can take hours longer, so meat is really efficient. Plus it contains a bunch of nutrients not found in the plants which make up a large part of their diet, and is available year round (unlike the insects they often use to supplement their diet). In short, a lot of meat can be obtained really quickly whenever it is needed. It’s a really good back-up option.

But if meat is important for the diet, why do the males hunt in groups? Surely this decreases the amount of meat each individual gets. The female chimps who hunt with spears do it by themselves, perhaps the men simply have to catch up to the huge intellectual advance that is “being alone.” However, the authors have another idea: meat scraps.

A chimp hunting by themselves has a relatively low rate of success. Even when looking at the spear wielding ladies they were only seen successfully killing their prey a couple of times. Working as a group increases your chance of success, so in the long run it pays off. You might get less from each individual kill, but there are more successful hunts overall so it balances out. It would be interesting if someone who knew how computer models work would simulate this strategy to see if it is actually viable. Artem I’m looking at you.

This has some fairly interesting implications for human evolution, which is really the only reason anyone bothers looking at chimps in the first place. Screw understanding the world, how does it impact meeeeee! Our ancestors appear to have been hunting from 2.5 million years ago, and doing so on a much larger scale than chimps by 1.8 million years ago. If this meat scraps idea has any merit (Artem, still looking at you), then this would suggest our ancestors would have to be working in groups. Groups probably larger (given what they’re hunting) and more common (given the amount they’re hunting) than what chimp are doing.

Around this time our brains also began to get bigger. A popular explanation for this is that it is connected to our social lives, with bigger groups putting a strain on our memory (having to remember who is who and how they’re connected and do we like them) so driving us to evolve bigger and better brains. Could it be that the main driver from these large groups in the first place was hunting? Hunting forced us to live in big groups which made our brains get bigger which made us be awesome?

If so lets hope the chimps don’t start eating more than 3% meat. They’re strong, innovative creatures and if their brain begins to get bigger, Dawn of the Planet of the Apes (a film I’m really looking forward to) might become more accurate than we’d like.

Reference

Tennie, C., O’Malley, R. C., & Gilby, I. C. (2014). Why do chimpanzees hunt? Considering the benefits and costs of acquiring and consuming vertebrate versus invertebrate prey. Journal of Human Evolution.

18 thoughts on “Why do chimps hunt?

  1. Great post. Eager to read the article when it comes out. Couldn’t hunting in groups also be a social bonding thing, decreasing fighting between males? I read something about food sharing among chimpanzees increases oxytocin levels in the giver and receiver. As for meat helping our brains get larger because of improved nutrition, we couldn’t get large heads until we got wide, open pelvises to allow for birthing big-headed babies. Walking upright and running after game would have helped there. So it was a combination of things. If it was just protein, cats would have really big brains and that would be scary!

    • I would side with this social glue thing. In particular, if all the males are hunting together then no male is romping around getting an undue amount of mating in. That would have been my first guess since it plays nicely with the gender difference Adam highlighted.

    • Social bonding is probably going on, but if meat eating is important for the diet then that alone could be a sufficient explanation. As such more evidence is needed for social bonding.

  2. You rang?

    The simplest game-theoretic model for this sort of stuff is the Stag hunt (sometimes called assurance games, for those that consider PD a part of stag hunt; game 5 in this classifications). There you have a bistable solution depending on the initial level of hunters, with either everybody hunting together or everybody hunting alone; so that could be used to explain the gender difference.

    However, the stag hunt is really meant to study if you want to actually cooperate in hunting or just tag along and not do too much work. So it might not be the best model. A more appropriate model is to have 3 strategies: (1) hunt alone, (2) tag along with the pack to pick off scraps, but do bare minimum work, and (3) be an active participant in group hunting. I think I saw a paper on something like this sometime in the past (I’ll try to dig it up when time and motivation permit), but a general analysis (in an unstructured population) is not too hard to do. You could add group size to this analysis, as well, but then things can get more complicated because it would see natural to condition participation and level of effort on hunting group size; that is probably only doable with simulation as opposed to the analytics I was describing before.

    • Surely one of the big points would be the relative success of each strategies? If hunting as a group with active participation is more likely to net the kill then this strategy might win out, even though it requires more investment by the participants. Similarly, preferential treatment of active participants could complicate the issue. Chimps are known to give more food to the hunters which did the most (the way chimp hunting tends to work is one chases a monkey through the trees, whilst others will climb up strategic trees to block of routes and funnel the monkey to where someone is laying in weight. The chaser, who puts in the most work gets the biggest reward).

      Presumably the latter shouldn’t be too hard to model, but I don’t know of much data on the efficacy of the various hunting strategies amongst chimps. I guess one could always work backwards, inputting a range of values to see the result. Then you could conclude “active participation has to be x more likely to be successful to make it a viable strategy.” Then you could work with some primatologists to investigate if this is the case and BANG, two papers for the price of one.

      • or better yet, one paper that combines theory and original empirical work. Something that is a rarity

        The first part of what I described is analytic, so “one could always work backwards” is the approach since you can perfectly characterize the parameter space. In a simulation, it would be more difficult since you would usually need to follow a more trial-and-error approach to figuring out what sort of parameters lead to “qualitatively correct” behavior. However, either way can produce some sort of qualitative prediction which could then maybe be used to justify funds or effort for an empirical study.

        • I think one key issue will be that chimps rarely hunt alone, so identifying the real-world parameters will be difficult. But it sounds like there would be ways to work around it, so hooray!

          • Unless you believe male chimps hunting alone would be significantly different than female chimps hunting alone, don’t you have the two main data points you need? Compare the yield obtained by any female single hunter with the average yield obtained by male chimps hunting together?

            FWIW, my intuitive gut guess is that, when hunting live prey, hunting in groups is a much more successful strategy than hunting alone. At least until chimps invent high-power rifles and scopes…

            • The female chimps in question are hunting in a different environment using different tools in different ways; so I suspect they don’t serve as a useful counterpoint to group hunting.

            • This paper seems to be addressing chimps as a whole (so focusing on tropical forests, where most chimps live) whilst the lone hunters are from Savannah chimps; arguably the most anomalous group of them all. So whilst I think a comparison could be made, it would be quite sketchy.

  3. I don’t understand why “why they hunt in groups” was even a puzzle in the first place. I think researchers who study this aspect of chimps’ and early humans’ behaviour would benefit from reading some of the great studies of the predator-prey relations of wild wolf packs by e.g. wolf biologist Dave Mech and the International Wolf Center. Wolves and other wild canines don’t always hunt in groups either – it depends on their situation and (primarily) the type/size and density of available/preferred prey. Canines are meat eaters so their survival directly depends on their hunting success, and they are expert group hunters, so that’ s why they are a great model to look at. Some wild canines (like coyotes) whose diet comprise mainly small prey rarely hunt in packs.

  4. What’s the optimum amount of meat in a (chimp) diet? Also, if chimps became full time meat-eating predators how would this change their environment in terms of prey reduction and change to harder to catch species?

    Some hunter-gatherer evidence suggests that gathering typically produces more calories per hour, so hunting would to be done to improve the diet. (How important is male bonding anyway?:) Also, hunting is less reliable. Does chimps hunting effort vary with the availability of plant food sources?

  5. There might be threshold effects too. While you have the energy you hunt rather than gather; successful hunting allows you to hunt more; enhanced nutrition allows you to fight your way up the social hierarchy. (?)

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