ABOVE: A blue whale surfacing in the Pacific Ocean.
Olivia Judson, from the WILD SIDE blog on the New York Times
AUGUST 18, 2009, 9:30 PM
The Long and Short of It
During my recent trip to the Oxford University Museum of Natural History, I stopped for a while in front of a glass case of small mammals. I felt a pang as I looked at the “edible dormouse” — it’s got to be bad news to have “edible” as part of your name.
But the animal that really captivated me was the pygmy shrew. It was tiny! Smaller than my little finger. It weighs only a few grams (less than a quarter of an ounce), and is smaller than some insects. The Goliath beetle, for example, can weigh more than 100g (3.5 ounces), and can be as big as my palm.
I contemplated the pygmy shrew, imagining it burrowing through grasses, capturing small insects, perhaps engaging in an epic battle with an earthworm. I thought about its heart beating at 1,200 beats a minute, and about the fact that some species of shrew are so small that they can run on water.
And then I thought how remarkable: the shrew is a mammal, and the blue whale is a mammal. Yet the blue whale is the largest animal ever to have lived. The biggest ones — which, by the way, are females — can grow to be as long as 30 meters (about 100 feet) and can weigh over 120 metric tonnes (118 tons). One of the most massive blue whales ever put on the scales was 190 metric tonnes.
Long ago, the great biologist J.B.S. Haldane wrote an essay called “On being the right size.” It was actually an essay arguing against communism, but it starts out with a sentence famous in evolutionary circles. “The most obvious differences between different animals are differences of size, but for some reason the zoologists have paid singularly little attention to them.”
This is no longer the case. We know quite a bit about the evolution of size — why animals become larger or smaller. For example, bigger, heavier animals tend to win fights — that’s why we have weight classes in sports like boxing. So in species where males gain from fighting for females, males typically evolve to be big. Among southern elephant seals, for example, females come to beaches to give birth and mate. Any male who can fight off his rivals can control the beach — and will thus be able to mate with all the females there, passing on his genes to all their offspring. Sure enough, in this species males have evolved to be six times heavier than females. It’s the legacy of generations of male aggression.
In other species, it’s females that are under pressure to grow large. Often, larger females are more fecund or have healthier offspring. Among insects, for example, larger females generally lay more eggs. In red deer, larger hinds tend to produce healthier offspring. Larger female spotted hyenas are higher in the dominance hierarchy and tend to produce more cubs.
The balance between fecundity in females and aggression in males is shown nicely in some sex-changing fish. (Many animals — not only fish, but also some shrimps, limpets, and the like — change sex during their lives.) In species where fighting is important — that is, where males can monopolize lots of females — individuals usually start out as female and become male when they reach a sufficiently impressive size (or when the resident impressive male dies or disappears). In species where fecundity is more important, fish usually start out as male, and switch to female at the point where they can make more eggs.
Many other factors contribute to pushing size one way or another. Larger animals need more food, but they are less vulnerable to cold (and more prone to overheating). Smaller animals generally take less time to become adults, and so tend to start reproducing at a younger age. And so on.
But here’s the thing. While we know quite a bit about the forces that cause animals to change size, we know rather little about how an animal’s body “knows” what size it is supposed to be. Let me show you what I mean.
Take a salamander. Let’s say it’s a certain size, and it has a certain number of cells. Suppose you double the size of the cells. Do you get a salamander that is twice as big? No. You get a salamander that’s the same size as it was before. But it has half the number of cells. Somehow, the salamander’s body can measure how big it is and stops growing when it gets to the right size.
(These animals look like regular salamanders, and are perfectly healthy. However, they are a bit stupid, apparently because they have half the number of brain cells. They’re less good than regular salamanders at solving mazes.)
We’ve known this since the 1940s. Yet we are not much closer to solving the problem. We have identified some of the genes that affect size; we can, for example, make some extremely small roundworms — roundworms that are one tenth the size of wild worms. (Unlike salamanders, adult roundworms have a fixed number of cells. So if you make the cells smaller, you get smaller worms.) But as one author wrote recently, “We still understand little about how size is actually sensed.” Or how the pygmy shrew “knows” how small it should be.
A newly discovered patch-nosed salamander.
Olivia Judson, from the WILD SIDE blog on the New York Times
AUGUST 18, 2009, 9:30 PM
The Long and Short of It
During my recent trip to the Oxford University Museum of Natural History, I stopped for a while in front of a glass case of small mammals. I felt a pang as I looked at the “edible dormouse” — it’s got to be bad news to have “edible” as part of your name.
But the animal that really captivated me was the pygmy shrew. It was tiny! Smaller than my little finger. It weighs only a few grams (less than a quarter of an ounce), and is smaller than some insects. The Goliath beetle, for example, can weigh more than 100g (3.5 ounces), and can be as big as my palm.
I contemplated the pygmy shrew, imagining it burrowing through grasses, capturing small insects, perhaps engaging in an epic battle with an earthworm. I thought about its heart beating at 1,200 beats a minute, and about the fact that some species of shrew are so small that they can run on water.
And then I thought how remarkable: the shrew is a mammal, and the blue whale is a mammal. Yet the blue whale is the largest animal ever to have lived. The biggest ones — which, by the way, are females — can grow to be as long as 30 meters (about 100 feet) and can weigh over 120 metric tonnes (118 tons). One of the most massive blue whales ever put on the scales was 190 metric tonnes.
Long ago, the great biologist J.B.S. Haldane wrote an essay called “On being the right size.” It was actually an essay arguing against communism, but it starts out with a sentence famous in evolutionary circles. “The most obvious differences between different animals are differences of size, but for some reason the zoologists have paid singularly little attention to them.”
This is no longer the case. We know quite a bit about the evolution of size — why animals become larger or smaller. For example, bigger, heavier animals tend to win fights — that’s why we have weight classes in sports like boxing. So in species where males gain from fighting for females, males typically evolve to be big. Among southern elephant seals, for example, females come to beaches to give birth and mate. Any male who can fight off his rivals can control the beach — and will thus be able to mate with all the females there, passing on his genes to all their offspring. Sure enough, in this species males have evolved to be six times heavier than females. It’s the legacy of generations of male aggression.
In other species, it’s females that are under pressure to grow large. Often, larger females are more fecund or have healthier offspring. Among insects, for example, larger females generally lay more eggs. In red deer, larger hinds tend to produce healthier offspring. Larger female spotted hyenas are higher in the dominance hierarchy and tend to produce more cubs.
The balance between fecundity in females and aggression in males is shown nicely in some sex-changing fish. (Many animals — not only fish, but also some shrimps, limpets, and the like — change sex during their lives.) In species where fighting is important — that is, where males can monopolize lots of females — individuals usually start out as female and become male when they reach a sufficiently impressive size (or when the resident impressive male dies or disappears). In species where fecundity is more important, fish usually start out as male, and switch to female at the point where they can make more eggs.
Many other factors contribute to pushing size one way or another. Larger animals need more food, but they are less vulnerable to cold (and more prone to overheating). Smaller animals generally take less time to become adults, and so tend to start reproducing at a younger age. And so on.
But here’s the thing. While we know quite a bit about the forces that cause animals to change size, we know rather little about how an animal’s body “knows” what size it is supposed to be. Let me show you what I mean.
Take a salamander. Let’s say it’s a certain size, and it has a certain number of cells. Suppose you double the size of the cells. Do you get a salamander that is twice as big? No. You get a salamander that’s the same size as it was before. But it has half the number of cells. Somehow, the salamander’s body can measure how big it is and stops growing when it gets to the right size.
(These animals look like regular salamanders, and are perfectly healthy. However, they are a bit stupid, apparently because they have half the number of brain cells. They’re less good than regular salamanders at solving mazes.)
We’ve known this since the 1940s. Yet we are not much closer to solving the problem. We have identified some of the genes that affect size; we can, for example, make some extremely small roundworms — roundworms that are one tenth the size of wild worms. (Unlike salamanders, adult roundworms have a fixed number of cells. So if you make the cells smaller, you get smaller worms.) But as one author wrote recently, “We still understand little about how size is actually sensed.” Or how the pygmy shrew “knows” how small it should be.
A newly discovered patch-nosed salamander.
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