Aug 13 Cheetahs have been on my mind, and I suspect I speak for a lot of people. India imported several cheetahs from Namibia and South Africa starting a year ago, in an effort to regenerate an Indian population of these exquisite animals. Sadly, six have died, as also three cubs born here earlier this year.
Hummingbirds, I would venture to say, are always on my mind. Anyway, the physical strain these two creatures put on their bodies routinely always puzzles me. Hummingbirds beat their wings incredibly fast; cheetahs run incredibly fast, if for short spells. How they do it is one question worth asking. But I also ran across humminbird research suggesting that some species have evolved ways to avoid some of this extreme physical exertion. And those species have evolved generally shorter beaks. Intrigued, like I was? Take a look: Birds, cats and evolution's way, https://www.livemint.com/opinion/columns/the-evolution-of-extraordinary-exertion-how-cheetahs-and-hummingbirds-sustain-their-remarkable-abilities-11689273734603.html cheers, dilip --- Birds, cats and the way of evolution Something has always intrigued me about cheetahs and hummingbirds. No, it isn't as if I sit here all day wracking my brains over both gorgeous animals. But every now and then, I do. Like when I began writing this, when there came news of a seventh death among the cheetahs we imported from Africa last year. Tragedy aside, this is what I wonder about cheetahs and hummingbirds: how do these animals sustain the exertions they put their bodies through? Take the cheetah, first. It's a beautiful animal no doubt, but it's also one of the fastest on the planet. How do we know this? As far back as 1965, a tame cheetah in Kenya was trained to run behind a moving car. Well, the car had a piece of meat tied to its back as an incentive for the animal. Over three such runs, she averaged 102.5 kmph. That remains the fastest speed ever recorded by a land mammal. In the wild, cheetahs don't necessarily have to go quite that fast, for their prey is usually slower than they are. But for me, the interesting question is not about that remarkable speed, but how, and how long, the cheetah can sustain it. After all, even the best conditioned human athletes can maintain a flat-out sprint for no more than about 30 seconds. World sprint records tell that story: among men, the 100m record is 9.58 seconds and the 200m, 19.19 seconds - just about twice the 100m record. So top-flight athletes can sprint at their fastest for at least 200m. But the 400m? 43.03 seconds, which is well over twice as long as for the 200m. That's already a sign that over distances longer than 200m, athletes can't do a full sprint. Instead, they must pace themselves. What about the cheetah? Well, about the same maximum time for a flat-out sprint: 30 seconds. In that time, at its top speed of about 100kmph, a cheetah can cover a few hundred metres. Any more, and its heart - pumping at about 250 beats per minute (bpm) by then - will probably buckle under the strain. Of course, often enough a cheetah will catch up with its prey within 30 seconds. But if it doesn't, the big cat will have to give up the chase and take a rest, let its beating heart slow down to its normal resting rate of about 120bpm. What about hummingbirds, then? They don't run, of course, so what is the connection to cheetahs? Their exertions and metabolism, that's what. There are times - freezing nights when there's nothing to be gained by flying about - when these little creatures become semi-comatose. In that state, their pulse rate can be as low as 75 beats per minute, comparable to a healthy human being at rest. But when they are flying from flower to flower during the day, that rate can rise to over 1200 bpm. Why so? Consider what the hummingbird is doing as it flies - flapping its wings frenetically. That makes enormous energy demands on the bird. Besides, being so small means it loses body heat faster than larger animals, more quickly using up the oxygen in its blood. So just to keep up the oxygen supply, a hummingbird's heart has to pump faster than in other birds. The heart has also evolved to be larger in relation to the bird's body than in other animals: about 2.5% of its body weight, compared to about 0.3% for humans. All due to beating wings. In normal flight, hummingbirds average about 50 wing beats per second (bps). That's already too fast for the human eye to see. But with some of these birds, there are times when that can rise to 80 bps. If you do the arithmetic, that's one beat of the heart for every four beats of the wings. Naturally, like with cheetahs breaking into a full-tilt sprint, this rapid wing action cannot sustain beyond a certain length of time. I don't have an idea of how long that is. But even though we know of hummingbirds from their hovering and flitting from flower to flower, we also know that they spend a large part of the day at rest, perched somewhere. Still, the bird's hover uses up calories at a fearsome rate. Those calories must be replenished. This is why hummingbirds flit about so much, dipping their beaks into flowers to sip nectar. Ornithologists believe they have to consume nearly their full body weight in nectar each day simply to survive. (By comparison, the average human eats about 2kg per day - only about 3% of her body weight.) It's obvious, then, that some hummingbirds will seek to reduce the need to hover quite as much while extracting nectar: more bang for the buck, if you like. How do they do that? By finding ways to perch. Indeed: there are some species of hummingbirds that have evolved to cling to a twig or a neighbouring flower. Clinging like that, they can sip on nectar without beating their wings, thus saving themselves for later, more energetic pursuits. But evolved how? These species have bigger feet than other hummingbird species. On those feet, there's a more developed innermost ("hallux") claw than other species have. That claw acts almost like a thumb does for us, allowing for a more secure grip on a perch. And something in all this speaks even more of evolution. A team of scientists found that these clinging hummingbird species, with their bigger feet, have generally shorter beaks than those who hover more. ("Repeated evolution of unorthodox feeding styles drives a negative correlation between foot size and bill length in hummingbirds", The American Naturalist, https://www.journals.uchicago.edu/doi/epdf/10.1086/726036.) Why would that be? Partly because a long beak can get in the way while perching, in ways it would not while hovering. But also because from their perch, these birds don't need to insert their beaks all the way to the bottom of the flower to get at its nectar. Instead, they can poke through the base and directly slurp up the sweet stuff they find there. Evolution: full of enduring mystery. I'll leave you wonder why cheetahs have evolved those black lines running down from their eyes. -- My book with Joy Ma: "The Deoliwallahs" Twitter: @DeathEndsFun Death Ends Fun: http://dcubed.blogspot.com -- You received this message because you are subscribed to the Google Groups "Dilip's essays" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To view this discussion on the web, visit https://groups.google.com/d/msgid/dilips-essays/CAEiMe8rhTBkn-oe3G7udmP2iBPno3F3Zqo66u6MczRSF22%2BErA%40mail.gmail.com.
