And likewise saying, here is a picture of a pair in this species and you can’t tell which one is who by gender, that tells you a whole world of predictability at the other end.
So a first example of how much explanatory power you can get out of this. And on the website, I will find all sorts of appropriate pictures of tournament and pair-bonding species for you to look at.
So a first realm of looking at animal behavior and seeing how once we got these principles in hand, individual selection, kin selection, reciprocal altruism, suddenly an awful lot of stuff makes sense. Next domain where that’s the case. And this is one that has had a real challenge to the, “Ooh, isn’t nature benign and animals behave for the good of the species” viewpoint. This one domain probably proves an individual selection framework more than anything else out there.
Back to our inevitable National Geographic specials. And somewhere at the end of whatever the special is, about whatever species, at some point they usually get some narrator with some deep, baritone voice coming at that stage and saying something like, man is the only species that kills for pleasure. Man is the only — and look at how wonderful and benign these rosebushes are. They don’t kill. They don’t have war. Only humans do. And what occurred, by about the mid ’70s or so, was enough field workers from a different species reporting, hey, wait a second. We’re not the only species that kills. And we will see plenty of domains where that makes sense in the lectures to come.
But one particular version really, really demanded some rethinking about animal behavior, which was as follows. What is one of the universals, whether you start with us and go all the way down to slime molds? It’s babies are cute. Everybody likes babies. Babies are adorable. You want to take care of babies. Your eyes dilate as soon as you’re around them. And a longstanding notion that what infants, what babies, what baby features are about are, among other things, are means to reduce aggression. And we will see in the ethology lecture something about that. That was the standard sound bite forever.
And suddenly, you got field biologists coming back from studying various species saying, something’s up there, because in my species I see individuals murdering kids all the time. Whoa, what’s up with that? Infanticide within a species. Suddenly, not only are humans not the only species that kills, we’re like not the only one who goes and kills adorable, little, Disney infants. Suddenly, all sorts of species popping up where there was infanticide. And the huge challenge then became to make sense of this.
This first emerged in the 1970s, studies of langur monkeys in India by someone named Sarah Hrdy reporting this. Soon it was reported among lions, some other species as well. And the first obvious response to this was, wait, this can’t be because I watch all the wildlife specials. There’s something wrong. Oh, there’s some sort of psychopathology going on. This is not a normal population. This is not a normal population because they live close to humans. There’s some disturbance. There’s a toxic waste dump somewhere. They’re not a normal species because you’ve got the wrong color socks on as the observer, whatever it is. And this is not normal. This is pathological behavior.
But long enough time goes by, and it becomes clear in all sorts of species, individuals kill infants. So what is this about? You start to look closely, and there’s patterns. There are patterns to it. The first one being that it tends to be adult males who kill infants. The next being, you look closely, and it’s not random who kills who. It’s males killing infants who are most likely to be the offspring of, you guessed it, individual selection, most likely to be the offspring of other males. Competitive strategies for reducing some other guy’s reproductive success.
So this began to be seen in these species that had infanticide. It was male infanticide of kids who were most likely to be the offspring of other males. But more patterns popped up at that time, which is, well, why don’t you see that in any social species where you have competition? And what wound up being clear after a while is there’s only a certain pattern that you see in species that have competitive infanticide, which is the average interbirth interval among females is longer than the average tenure of a high-ranking male.
What? Here’s what that means. That means you’re some low-ranking guy. You’ve been working out in the gym for years on end. You’re finally in the position to boot out the alpha male and take over the whole group. And damn, every single female in the group has a newborn who they’re going to be nursing for the next two and a half years. So they’re not going to be ovulating, and they’re not going to be ovulating for about three years. And you’re likely to be alpha for only a year and a half or so, where the length of time, on average, that you are going to be able to be reproductively active is shorter than the length of time that females in your species nurse kids and thus are not ovulating.
And suddenly, you have this brutally clear logic that makes perfect sense from everything on Monday, which is, go and kill the kids. Go and kill the kids for two reasons. Number one, by killing the offspring of some other male, you are decreasing that individual’s reproductive success. This competition is leaving as many copies of your genes, et cetera. Number two, by killing an offspring, the female, by stopping nursing, will soon be ovulating. And thus, you see this pattern in langur monkeys, vervet monkeys, patas monkeys, lions, mountain gorillas and such, where it’s always this structure. Competitive infanticide — a male takes over a breeding group and goes about systematically trying to kill the infants.