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).

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.

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 mammals, 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:

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, and so on. 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.

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 x^1.5.) 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 (although of course the cow’s brain is bigger). 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.

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Age of mammals

Linnaeus chose one trait – mammary glands / lactation – to define the order Mammalia. This was not a purely scientific decision. Like many authorities in eighteenth century Europe, he was concerned that the common practice of wet-nursing was unnatural and dangerous, and he wrote a pamphlet urging the advantages of women nursing their own infants.

But mammals do not owe their Cenozoic success to any one trait. True, there is a central theme in mammalian evolution.

[T]he over-arching attribute manifested by the origin of the mammals is increasing homeostatic ability: the maintenance of a constant internal environment in the face of a fluctuating external environment, by means of high-energy regulatory processes (Kemp p. 18)

But this homeostatic ability is supported by a whole series of interrelated traits that evolved in tandem. Here’s a summary diagram. 

Evolving a whole set of coordinated traits like this is a much slower business than optimizing a single trait. It is a matter of correlated evolution, in which small changes in one character allow for small changes in other characters, along an “adaptive ridge.”

It took several hundred million years, from synapsids, to therapsids, to cynodonts, to mammals, to put the mammalian package together. And even after mammals had appeared and begun to diversify, it would take an extraordinary catastrophe at the end of the Cretaceous before the Age of Mammals would really begin.

For an excellent popular introduction, try I Mammal: The Story of What Makes Us Mammals

Life at sea: whales and sailors

48.3 – 45.8 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.

(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

54.0 – 51.1 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.

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.)

Hotblooded

Were dinosaurs warmblooded? More precisely, were they ectotherms, with low metabolic rates, like living reptiles, or endotherms, with high metabolic rates, like mammals and birds? (Yes, yes, you and I know that birds are dinosaurs, cladistically speaking, but you know what I mean.) And there are other possibilities: were the biggest dinosaurs, the sauropods, gigantotherms, keeping metabolic rates low, but staying warm through sheer size?

Endothermy is a big deal:

Elevated metabolic rates enable animals to remain active year-round at high latitude and altitude. They also enhance physiological performance, improve endurance, increase activity levels and facilitate rapid niche shift during environmental perturbations.

https://www.nature.com/articles/s41586-022-04770-6

Recently, it has become possible to address this question by looking at chemical signals of metabolic rates in fossil bones. The chart below (see link above) summarizes the results.

The upper branch of the tree are the diapsids – reptiles, dinosaurs, birds, and relatives. The lower branch is the synapsids ­– from dimetrodon way back in the day to mammals today. The chart shows that the earliest dinosaurs had high metabolic rates, as did the closely related early pterosaurs. And the sauropods were true endotherms. But some later dinosaurs actually gave up on endothermy: triceratops, stegosaurus, and the hadrosaurs (duck-billed dinosaurs) seem to be secondary ectotherms. Other dinosaurs went for more intense endothermy, like allosaurs and diplodocus and some close bird relatives.

In short, dinosaurs are diverse.

The Great American Interchange

2.87 – 2.72 million years ago

For most of the last 100 million years, South America was an island continent, like Australia, with its own peculiar mix of species, largely isolated from other continents (although monkeys, and guinea pig relations, rafted across.) By contrast, North America was intermittently connected with Eurasia and exchanged species off and on. South America supported a rich array of marsupials, including a marsupial version of a saber-toothed tiger. It also had predatory flightless “terror birds” that seemed bent on reoccupying the two-legged predatory dinosaur niche.

There was also a profusion of notoungulates (probably distantly related to hoofed animals in North America and Eurasia), and liptoterns. (Below is a reconstruction of a late surviving liptotern, Macrauchenia, looking like a Dr. Seuss invention.)

South America was close enough to North America for the two continents to start exchanging species by 14 million years ago, but the really massive exchange began with the establishment of the Isthmus of Panama, and climate changes, about 3 million years ago. 38 genera of land mammals walked north from South America. 47 genera walked south from North America. So the initial exchange was unbalanced; the subsequent evolution was even more so. Only a handful of South American invaders – notably armadillos and (for a while) ground sloths – succeeded in establishing themselves in North America, while North American invaders generated a profusion of new species. Many of the really distinctive South American forms would go extinct over the next millions of years.

Paleontologists dispute the causes of the turnover, but it looks an awful lot like North American species had a competitive edge. This is one instance of a phenomenon we’ve seen already in animal evolution, and will see again in human history, of large land areas generating more competitive forms.

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).

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.

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 mammals, 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:

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, and so on. 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.

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 x^1.5.) 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 (although of course the cow’s brain is bigger). 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.

Age of mammals

Linnaeus chose one trait – mammary glands / lactation – to define the order Mammalia. This was not a purely scientific decision. Like many authorities in eighteenth century Europe, he was concerned that the common practice of wet-nursing was unnatural and dangerous, and he wrote a pamphlet urging the advantages of women nursing their own infants.

But mammals do not owe their Cenozoic success to any one trait. True, there is a central theme in mammalian evolution.

[T]he over-arching attribute manifested by the origin of the mammals is increasing homeostatic ability: the maintenance of a constant internal environment in the face of a fluctuating external environment, by means of high-energy regulatory processes (Kemp p. 18)

But this homeostatic ability is supported by a whole series of interrelated traits that evolved in tandem. Here’s a summary diagram. 

Evolving a whole set of coordinated traits like this is a much slower business than optimizing a single trait. It is a matter of correlated evolution, in which small changes in one character allow for small changes in other characters, along an “adaptive ridge.”

It took several hundred million years, from synapsids, to therapsids, to cynodonts, to mammals, to put the mammalian package together. And even after mammals had appeared and begun to diversify, it would take an extraordinary catastrophe at the end of the Cretaceous before the Age of Mammals would really begin.

For an excellent popular introduction, try I Mammal: The Story of What Makes Us Mammals