Tag Archives: mammals

Dead baby monkeys

There’s a dark side to being a primate. A few years back a review article summarized data on rates of lethal aggression in non-human animals. The figure below shows some of the results. Several clusters of especially violent species stand out in the figure, including primates (redder is more violent). Bats are pretty nice, though (too bad about all the viruses).

dead monkeys

Much of the lethal aggression in primates involves infanticide. Sarah Hrdy demonstrated back in the 1970s that infanticide occurs regularly in Hanuman langurs, monkeys in India. A male who takes over a group of females will systematically kill offspring sired by the previous male. If you think evolution is about the survival of the species, this is hard to explain. But it makes sense given the logic of the selfish gene. Females who lose an infant return more quickly to breeding again, and the father of the next infant is likely to be the killer of the previous one.

Primates may be particularly vulnerable to this grim logic, because they spend a long time as infants. Among primates, commonly,

           L/G>1

That is to say that the time, L, a female spends lactating for an infant (during which she is unlikely to conceive), is usually greater than the time, G, she spends gestating an infant. This puts particular pressure on males to hurry things along by eliminating nursing infants fathered by other males.

astyanax

Death of Astyanax

As a result, infanticide is relatively common among primates, and females under particularly strong pressure to find ways to avoid it. Hanuman langurs live in one-male units, where a female has little choice about who she mates with. In other species, by contrast (most baboons, chimpanzees), multiple males reside with multiple females. In these species females are often sexually promiscuous, sometimes actively soliciting multiple males for sex. This is probably mostly a matter of confusing paternity sufficiently to suppress the threat of infanticide. There’s a general lesson here: females are not always monogamously inclined, but female promiscuity generally has different evolutionary roots than male promiscuity.

Ground-up monkey brains

Short version: It looks like most mammals, at least most large animals, have the brains they need, while primates, especially large primates, have the brains they can afford.

Longer version: One reason for being interested in monkeys is that they’re brainy mammals. Here’s the conventional graph illustrating that:

brain size

Larger mammals tend to have larger brains, but the relationship is non-linear. Multiplying body mass by x doesn’t multiply brain mass by x. Instead it multiplies brain mass by about x.75. In other words, Brain Mass is proportional to (Body Mass).75. Equivalently (taking the logarithm of both sides) Log[Brain Mass] is equal to .75 times Log[Body Mass], plus a constant. So Log[Brain Mass] plotted against Log[Body Mass] gives a straight line with a slope of .75. That means that if one mammal has 16 times the body mass of another, it’s expected to have 8 times the brain mass. 10,000 times the body mass means 1000 times the brain mass. The thing to note is that primates defy expectations. They have larger brains than would be expected based on their body sizes.

But we’ve recently learned that primates – especially big ones – are even more special than this graph suggests. Susan Herculano-Houzel has pioneered a technique that involves chopping up brains (or parts of brains), dissolving their cells to make a kind of brain soup, and counting cell nuclei. This allows her to estimate how many neurons there are in different brains.

monkey brain soup

Major findings: Among most mammals, the number of neurons increases more slowly than brain size. Increase brain size by x, and you increase number of neurons by about x.67. (H-H shows this flipped around. Increase number of neurons by x and you increase brain mass by x1.5.) But primates are exceptional; the relationship is nearly linear. An x-fold increase in primate brain size corresponds to about an x-fold increase in number of neurons. Humans follow the primate rule here. We have about the same density of neurons as other primates. When you combine the exceptionally large brain sizes of humans (exceptional even relative to our brainy primate relations) with a standard high primate neuron density, you get an animal with an enormous number of neurons. By contrast, a rodent with a human sized brain, if it followed rodent rules for how neuron numbers increase with brain size, would have only 1/7 as many neurons.

Neurons are expensive. Most large animals economize by cutting back on neuron density. A cubic centimeter of cow brain has fewer neurons, and consumes energy at a lower rate, than a cubic centimeter of mouse brain. By contrast, large primates are extravagant, devoting exceptionally large energy budgets to running their brains. And human brains are exceptionally costly. An important question for the study of human evolution is how we paid the bill for such costly brains. That’s a story for later. But another part of the story starts back in the early Cenozoic, when monkeys committed to a different set of rules for building brains.

And here is a chart giving absolute numbers of  cortical neurons (cneurons) for a bunch of species. Scott Alexander has some thoughts about the moral implications. Short version: skip the pork for dinner (and skip the elephant, chimp, and manflesh. But you knew that). Beef might be okay. Better is lobster.

And Werner Herzog is probably okay with you eating chicken.

neuron number

A bear there was, a bear, a bear

48.2-45.7 million years ago

The end-Cretaceous mass extinction knocked off not only the dinosaurs (except for birds), but also air-breathing marine predators like mososaurs and plesiosaurs. Birds and mammals started moving into the empty niche: penguins from early on, and eventually whales.
penguins

(Cartoon by Sam Gross. Not scientifically accurate.)

People around the world seem to be naturally inclined to distinguish major animal life forms according to whether they walk, fly, swim, slither, or creep, so evolutionary shifts in modes of travel – the origin of flight, the return to the sea – really catch people’s imagination – and provoke Creationists. The whale story is particularly dramatic. When Darwin was tried to account for the evolution of whales from a land-dwelling ancestor, he cited accounts of bears swimming and feeding in water, and wrote “I can see no difficulty in a race of bears being rendered, by natural selection, more and more aquatic in their structure and habits, with larger and larger mouths, till a creature was produced as monstrous as a whale.” This statement attracted so much ridicule that Darwin took it out of later editions of The Origin of Species. But he turns out to have been very much on target. We now have a great sequence of whale ancestors. The sequence runs from today’s Pakicetus — a wolf size meat-and-fish eater that splashed along the shores of the ancient Tethys sea separating Africa from Eurasia — to the “walking whale,” Ambulocetus, and on to true whales. We have even begun to detail some of the genetic changes that went with the return to the sea. Darwin was sort of on the right track thinking of bears, but anatomy and genetics put the ancestors of whales firmly among artiodactyls – hooved animals including hippos, pigs, and cows.

Whales are famously large. Marine mammals in general tend toward bigness: one theory is that large body size (low ratio of surface area to volume), and an insulating layer of blubber, are adaptations to reduce heat loss. Whales, particularly baleen whales, take it further with dietary adaptations that let them get huge.

Remarkably there may be a parallel in human evolution. Polynesians have the largest body sizes of any living people, and this too may be an adaptation to conserve heat in a maritime environment.

The Polynesian people who settled a wide area of the tropical Pacific have a large and muscular body phenotype that appears to contradict the classical biological rules of Bergmann and Allen. However, a scrutiny of the conditions actually experienced by these canoe voyagers and small-island dwellers suggests that in reality the oceanic environment is labile and frequently very cold, and from it tribal technology offered little protection. The Polynesian phenotype is considered to be appropriate to, and have undergone selection for, this oceanic environment.

Strange relations and island continents

57.1-54.1 million years ago

We’ve seen a great many catastrophes in the history of life, and been reminded of the role of sheer chance in evolution. But the Cenozoic also sees a dramatic adaptive radiation and the steady progress of arms races among survivors of the great dinosaur die-off. Four large scale groupings of placental mammals have already appeared: Afrotheres (aardvarks, hyraxes, elephants, and sea cows), Xenarthrans (anteaters, armadillos, and sloths), Laurasiatheres (shrews, hedgehogs, pangolins, bats, whales, hoofed animals, and carnivores), and Supraprimates (aka Euarchontoglires, including rodents, tree shrews, and primates). This grouping of mammals is anything but obvious – it’s only with DNA sequencing that it has emerged. What’s noticeable is the association with different continents: Afrotheres with Africa, Xenarthrans with South America, and the others with the monster content of Laurasia (Eurasia and North America). Looking beyond placental mammals we see other continental associations: marsupials flourish in South America and Australia, and giant flightless “terror birds” carry on rather like predatory dinosaurs in South America.

mammal tree

There is a pattern here. Evolutionary arms races are most intense in the supercontinent of Laurasia (eventually joined by India and Africa). The island continents of South America and Australia stand apart, and they fare poorly when they start exchanging fauna with the rest of the world. We’ll see a similar pattern – large areas stimulate more competition, and more intense evolution, isolated areas are at a disadvantage – when we look at modern history, with ocean voyages effectively reuniting Pangaea. (This is a major theme of Alfred Crosby’s Ecological Imperialism and Jared Diamond’s Guns, Germs, and Steel.)

Dead baby monkeys

There’s a dark side to being a primate. A few years back a review article summarized data on rates of lethal aggression in non-human animals. The figure below shows some of the results. Several clusters of especially violent species stand out in the figure, including primates (redder is more violent). Bats are pretty nice, though.

dead monkeys

Much of the lethal aggression in primates involves infanticide. Sarah Hrdy demonstrated back in the 1970s that infanticide occurs regularly in Hanuman langurs, monkeys in India. A male who takes over a group of females will systematically kill offspring sired by the previous male. If you think evolution is about the survival of the species, this is hard to explain. But it makes sense given the logic of the selfish gene. Females who lose an infant return more quickly to breeding again, and the father of the next infant is likely to be the killer of the previous one.

Primates may be particularly vulnerable to this grim logic, because they spend a long time as infants. Among primates, commonly,

L/G>1

That is to say that the time, L, a female spends lactating for an infant (during which she is unlikely to conceive), is usually greater than the time, G, she spends gestating an infant. This puts particular pressure on males to hurry things along by eliminating nursing infants fathered by other males.

astyanax

As a result, infanticide is relatively common among primates, and females under particularly strong pressure to find ways to avoid it. Hanuman langurs live in one-male units, where a female has little choice about who she mates with. In other species, by contrast (most baboons, chimpanzees), multiple males reside with multiple females. In these species females are often sexually promiscuous, sometimes actively soliciting multiple males for sex. This is probably mostly a matter of confusing paternity sufficiently to suppress the threat of infanticide. There’s a general lesson here: females are not always monogamously inclined, but female promiscuity generally has different evolutionary roots than male promiscuity.

Ground-up monkey brains

Short version: It looks like most mammals, at least most large animals, have the brains they need, while primates, especially large primates, have the brains they can afford.

One reason for being interested in monkeys is that they’re brainy mammals. Here’s the conventional graph illustrating that:

brain size

Larger mammals tend to have larger brains, but the relationship is non-linear. Multiplying body mass by x doesn’t multiply brain mass by x. Instead it multiplies brain mass by about x.75. In other words, Brain Mass is proportional to (Body Mass).75. Equivalently (taking the logarithm of both sides) Log[Brain Mass] is equal to .75 times Log[Body Mass], plus a constant. So Log[Brain Mass] plotted against Log[Body Mass] gives a straight line with a slope of .75. That means that if one mammal has 16 times the body mass of another, it’s expected to have 8 times the brain mass. 10,000 times the body mass means 1000 times the brain mass. The thing to note is that primates defy expectations. They have larger brains than would be expected based on their body sizes.

But we’ve recently learned that primates – especially big ones – are even more special than this graph suggests. Susan Herculano-Houzel has pioneered a technique that involves chopping up brains (or parts of brains), dissolving their cells to make a kind of brain soup, and counting cell nuclei. This allows her to estimate how many neurons there are in different brains.

monkey brain soup

Major findings: Among most mammals, the number of neurons increases more slowly than brain size. Increase brain size by x, and you increase number of neurons by about x.67. (H-H shows this flipped around. Increase number of neurons by x and you increase brain mass by x1.5.) But primates are exceptional; the relationship is nearly linear. An x-fold increase in primate brain size corresponds to about an x-fold increase in number of neurons. Humans follow the primate rule here. We have about the same density of neurons as other primates. When you combine the exceptionally large brain sizes of humans with a standard high primate neuron density, you get an animal with an enormous number of neurons. By contrast, a rodent with a human sized brain, if it followed rodent rules for how neuron numbers increase with brain size, would have only 1/7 as many neurons.

Neurons are expensive. Most large animals economize by cutting back on neuron density. A cubic centimeter of cow brain has fewer neurons, and consumes energy at a lower rate, than a cubic centimeter of mouse brain. By contrast, large primates are extravagant, devoting exceptionally large energy budgets to running their brains. And human brains are exceptionally costly. An important question for the study of human evolution is how we paid the bill for such costly brains. That’s a story for later. But another part of the story starts back in the early Cenozoic, when monkeys committed to a different set of rules for building brains.

And here is a chart giving absolute numbers of  cortical neurons (cneurons) for a bunch of species. Scott Alexander has some thoughts about the moral implications. Short version: lobster for dinner, skip the pork. (And skip the elephant, chimp, and manflesh. But you knew that.)

neuron number

A bear there was, a bear, a bear

The end-Cretaceous mass extinction knocked off not only the dinosaurs (except for birds), but also air-breathing marine predators like mososaurs and plesiosaurs. Birds and mammals started moving into the empty niche: penguins from early on, and eventually whales.
penguins

(Cartoon by Sam Gross. Not scientifically accurate.)

People around the world seem to be naturally inclined to distinguish major animal life forms according to whether they walk, fly, swim, slither, or creep, so evolutionary shifts in modes of travel – the origin of flight, the return to the sea – really catch people’s imagination – and provoke Creationists. The whale story is particularly dramatic. When Darwin was tried to account for the evolution of whales from a land-dwelling ancestor, he cited accounts of bears swimming and feeding in water, and wrote “I can see no difficulty in a race of bears being rendered, by natural selection, more and more aquatic in their structure and habits, with larger and larger mouths, till a creature was produced as monstrous as a whale.” This statement attracted so much ridicule that Darwin took it out of later editions of The Origin of Species. But he turns out to have been very much on target. We now have a great sequence of whale ancestors. The sequence runs from today’s Pakicetus — a wolf size meat-and-fish eater that splashed along the shores of the ancient Tethys sea separating Africa from Eurasia — to the “walking whale,” Ambulocetus, and on to true whales. Darwin was sort of on the right track thinking of bears, but anatomy and genetics put the ancestors of whales firmly among artiodactyls – hooved animals including hippos, pigs, and cows.