Tuesday, 28 February 2012

Exercise like a lion!

Wildebeest wrestling - the ultimate fitness regime? Selous GR, June 2010.
I came across a paper this last week - I can't remember how, because it's certainly not my usual reading material (though my wife has just pointed out a report on the BBC today) - but it suggested an answer to one of the things that puzzle me about lions. Like most cats, lions like to sleep. A lot, in fact - they're perfectly content sleeping for 21hrs a day, so it's no wonder tourists don't normally see them doing very much. As a consequence, I think lions are rather boring: I'd rather be birding. Still, on the occasions when I've got visitors staying who need to see lions I do go and look at them, sleeping away, and I wonder. How is it that a lion, sleeping 21hrs per day, can still be so fit and healthy? On the rare occasions when they do shift themselves, wild lions are certainly lean, mean killing machines. But how do they remain in such good condition, when they sleep nearly all the time, and even when hunting tend to walk as slowly as possible, or sit motionless in ambush?

Lions certainly can get fat - zoo keepers have to be careful to maintain the weight of captive lions within normal bounds by restricting their access to food and trying to get them to be more active through 'environmental enrichment' activities. So it's not as though they are somehow immune to such fitness concerns. So what's special about lions that helps them through this? The answer, of course, is that they do sufficient exercise: although they are pretty inactive most of the time, the effort it takes to actually catch and kill an animal is huge, even if the action only occurs for a few short seconds. During those seconds they're giving it everything they have. And if there's anything similar about their metabolism to ours, these short but extremely intense bursts of exercise are more than enough to keep a lion - or us - in good shape. In fact, the paper I read (it's free to view here)  is a review of numerous studies of different fitness training regimes, focusing on comparing 'High-intensity Interval Training' (HIT) to more traditional fitness regimes. In essence, Gibala and colleagues conclude that HIT - involving just a few minutes of absolute 100% capacity exercise per week - offers comparable fitness and health benefits to much more time intensive traditional fitness regimes that raise metabolic rates (or at least Oxygen consumption) to 60 - 75% of maximum. In other words, chasing down and wrestling a wildebeest to the ground for a minute or two each day, can give you the same fitness benefit as doing an hour or so of moderate exercise each day.
Lions do sleep. A lot!

In fact for the lion and other large carnivores it's well known that it's not an easy life at all - obviously lions in the wild don't worry about keeping fit, they worry about meeting their energy requirements. And there's another paper that explores some of the energetic consequences of carnivory that's really rather interesting too (free to view here). This is a theoretical study exploring body size relationships and energetic costs among carnivores. This is interesting, because not only do energetic requirements scale with body size in a non-linear way (if you're bigger you need more energy than small animals, but proportionately you need less: a doubling of body mass corresponds, more or less, to an increase in energy requirements of only 1.7 times), but there's also a distinct and very, very important difference between big and small carnivores (we'll define big as anything over 20kg). Small carnivores like wild cats tend to eat lots of small things - mice, invertebrates, birds, etc - that tend to be smaller in body mass than the carnivore. But above about 20kg carnivores switch dramatically, preferring to prey on animals that are as big, or bigger than themselves. Hunting small and common things is relatively cheap - you can walk along and then do a sudden leap. Hunting bigger things is a whole lot harder - they run and you've got to catch them, then bring them to the ground and deal with a struggling animal. That is going to cost a lot more energy than hunting several small things, so if you are going for this strategy it obviously makes sense (a) to only attack if you think you've a reasonable chance of success (which is why most hunts are called off well before anything interesting happens), and (b) to target something that's going to give a really good return on your investment. In other words, once you start hunting largeish prey, it's worth going for the biggest you can possibly handle.

Carnivore Body Mass and Energy Expenditure. Step occurs at 14.5kg.
From Carbone et al, 2007
Now, consider that the cost of hunting small things is a simply, linear function of number of little things you catch, and the reward (in terms of energy intake) is a similar straight line. For small carnivores, that's fine, as you get bigger, your energetic demands increase, but a linear increase in hunting time will result in a linear increase in energy intake too, and everything can balance out. However, as you get even larger, at some point you'll run out of time to keep hunting like this and still meet your growing energetic costs. At this point, you've got to either stop being a carnivore, or change tactic - and that's where you start going for a few, high reward hunts and become a big carnivore - using data on real energetic costs for a range of carnivore species, Carbone and colleagues show that there really is such a switch - at about 14.5kg. (The graph here shows Daily Energy Expenditure for carnivores of different body mass, based on real field measurements). What's interesting is to then look at actual measured energy intake for the same species and compare with the requirements and another pattern occurs: at the very low range of both sections of the graph (that smallest carnivore on the graph is about 7g, then things just over 14.5kg) they have plenty of food for their requirements. But at just below the threshold - say 14kg - and near the top of the range (like a lion), they're really struggling to make ends meet. And in fact, it turns out that near both these limit, carnivores do a lot to limit their energy requirements - large bears hibernate, lions sleep all the time, so do those 14kg carnivores. And by sleeping so much, they really can cut down on their energetic needs.
They do hunt sometimes, and when they do they look mean!

And so we come full circle: lions sleep all the time, not because they're lazy, but because hunting is such a high energy activity, that they can't afford to waste energy any other way. They don't get fat, because actually they're close to their energetic limits. It's tough being a big carnivore. (Though I still think I'd prefer it to being a wildebeest always on the look out for big carnivores!) It's interesting though - we're certainly in the large carnivore body size step, but our DEE is about 8500kJ/day in normal weight people, well below the line. We're obviously not carnivores! Though what it would be like for, say, the Hadzabe, I'm not sure - my guess is it will still be well below the line. Which isn't, to say, of course, that we can't learn from the lion's exercise regime and forget endurance-training, in favour of wildebeest wrestling. Who's first?!

Main references:

ResearchBlogging.orgGibala, M., Little, J., MacDonald, M., & Hawley, J. (2012). Physiological adaptations to low-volume, high-intensity interval training in health and disease The Journal of Physiology DOI: 10.1113/jphysiol.2011.224725

Carbone, C., Teacher, A., & Rowcliffe, J. (2007). The Costs of Carnivory PLoS Biology, 5 (2) DOI: 10.1371/journal.pbio.0050022


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