Hippos, cheetahs, polar bears, lions, tigers, cats, dogs, dolphins, gorillas, baboons, humans,
chimpanzees, kangaroos, meerkats, um various rodents, insects, amphibians and lizard, fish,
birds, rotifers? .... I mean these are just some creatures that have been observed killing
infants of their own species. Both males and females do it. Some even killing their own
kids. So... Why do we do it. touchy subject, bit of a disclaimer. Killing
someone's kid is probably still questionable ... even if... you were planning on eating
them after. Fruit flies are promiscuous, they will mate
with multiple partners. Generally, the males compete for female attention and the females
choose the ones they like best. When the mate injects his sperm into the females oviduct,
she can store the sperm in special sperm storage organs for later use.
Because she's going to be so promiscuous, the males have special proteins in their seminal
fluid that help them compete. Some seem to deactivate or kill other sperm of males she
had mated with before. Others act on her nervous system and seem to make her less receptive
to mating, she get's less horny, so she's less likely to mate with other males and she'll
make more kids with his sperm. But unfortunately, some of these proteins
are also toxic to the female. This one that makes her less horny is at least one that
happens to reduce her life span. If this is all for sexual competition, let's
see what happens when we take that away. Let's do an experiment.
We'll take 100 males and 100 female and pair them off randomly. Then put them in what the
scientists call "interaction vials" for 5 days, during which time they will.... iiiinteract.
He'll do a little display for the female, once the female is thoroughly impressed, they
will mate. And although it was an arranged thing they fall in love.
But the hours turn to days, their excitement to indifference. Soon their interactions soon
become routine and passionless. They still use the word "love" but it's meaning had completely
gone. The time they spend apart feels more freeing, and they long for the.
Then you put them in another vial so the females can lay eggs in a fresh habitat. When that�s
done you take them and throw them in the garbage. Over the course of 8 days the larva hatch,
they eat, go pupa, and then emerge as adults. We'll knock them out with CO2, randomly
pair off 100 males and a 100 females again, and restart the process. Then you repeat this
another 50 times? Takes about 2 years to do this experiment like this. In humans this
would take over 600 years. What we get are flies that have evolved for
50 generations under monogamous conditions. We also need something to compare them against.
We also need something to compare them against. We'll do basically the same experiment with
another group, but in this group there's going to be 3 males for each female, this
simulates their wild promiscuous conditions where the compete and the females choose.
So now we have 2 different groups of fruit flies that we can play with.
Researchers that actually did this experiment, mated monogamous and polygamous males against
monogamous females to see how the males hurt the females. When with the monogamous male,
the females made more eggs, those eggs developed faster and she lived longer. So it seemed
like the monogamous males weren't hurting the females as much as the promiscuous ones.
But something else happened. Testing to see if there were differences in how the females
were affected by the males, it seemed that the monogamous females were doing worse. They
made less eggs and didn't live as long. So the monogamous males did better than promiscuous
ones, but monogamous females did worse than promiscuous ones.
So what's going on? The proteins in the male's semen hurt the
female, but help him compete. He's making more kids with her because of them. But when
we take that competition away, those proteins are unhelpful because there's no one to
compete with. Also normally he can go off with other females just planting lots of seeds.
And it doesn't really matter to him that he's hurting them all a bit. Here he can't
do that. Here every little bit of damage to her, reproductively it hurts him.
In this new environment, males who produce more of the toxin will end up making less
eggs or less healthy eggs compared to males who produce less of the toxin. So when we
randomly select from a new batch of eggs, there are less flies with those genes. Each
generation makes a little less toxin and by generation 50, our monogamous males are hurting
the females less. What about the females? Why did the monogamous
females get hurt more? The females may have had genes for some sort of adaptation to protect
themselves against the toxin. I don't know what specifically. But if the defences took
a lot of resources to produce, resources that could have gone into making more eggs, in
an environment with males producing less toxin, females that produced less defences, would
have produced more eggs or healthier eggs. So when the researchers randomly picked the
next generation of flies, more of her eggs passed to the next generation. And then by
generation 50, when she mated with normal flies, the females were less equipped to deal
with that toxin and didn't do as well. But what does this have to do with killing
babies? Let's look at the gelada baboon. They're
polygamous and very competitive. A male will try to maintain sexual dominance over a group
of females. If another male comes in and ousts him as the dominant, one of the first things
the new male will do is try to kill the kids from the previous male.
Because the females won't ovulate they can't make a kid for him if they're nursing. He
doesn't really want to wait for that. Killing babies allows him to make kids faster.
It's the same benefit the male fruit flies got from having proteins that deactivate other
male's sperm or cause the females to become less receptive to mating. It's so they can
be the ones that are making kids with her. Unfortunately the females are getting hurt
in the process. On average the time in between births for females with no infanticide is
about 2 and half years. The time between births for females the victim of infanticide, is
about 3 and a half years. He might be making more kids, but she's making less.
Like the female fruit flies, the females have some. She can, well, try to fight back which
will work sometimes. But the males are way larger. And best not to risk injury or death.
Losing a child will suck but she can make another and be reproductively successful.
If she dies or is injured she might not.
So females with safer defenses would probably do better. With mice in the lab, if after
mating, 24 hours after mating, the father is placed back in near the female, nothing
happens. When another female is placed there, nothing happens. But they observed, if a male
who is not the father is placed near the female a quarter of the tested females miscarried
their litter. When a male who wasn't another albino lab mouse like the female, but a wild
type mouse, about three quarters of them miscarried. I don't know why that's different, but probably
has something to do with the smell? Other researchers just pumped in the pheromones
of strange males and saw the same effect. Geladas have been seen do this in the wild.
Researchers looked at the number of births between troops that had a male replacement
and those that didn't. 6 months before the replacement, seems about the same number of
births. But within 6 months after, very few females produced a child compared to the troops
that kept the same male. And then 6 months after that they made double? Seems almost
like they miscarried, became pregnant from the new male at the same time and then made
a baby at the same time. Also by observing the hormones in their poop, they saw that
the females were miscarrying around the time of the arrival of the new male.
In the end the time between births for females that automatically miscarried their child
was about 2 and a half years. Females that automatically miscarry, will
make more children than females that put all those resources into an infant that just ends
up getting killed by a new male. But this infanticide isn't a male thing,
it's a scarcity of breeding resources thing? Females can do the same thing for the same
reasons. With Wattled Jacanas, it's the females that
maintain sexual dominance over a few males. They're the ones that guard the males and
eggs and they will fight females of neighboring areas for territory and access to their males.
It's the males that sit on the eggs and take care of the chicks. Takes him about 3 months
to wean a clutch of eggs. One researcher tried experimentally removing
the guarding female of one group the night before. Within the first hour of the next
day, the females from bordering territories came in and were trying to kill the chicks
or crush the eggs of the old female. If one succeeds, she will try to court him and get
him to raise her eggs. This way she's not waiting 3 months. It's about 8-10 days before
he's sitting on her eggs. But infanticide is observed even without any
sort of dominance take over like this. In Meerkats most of the infanticide is committed
by females, and they become even more murderous when they're pregnant. A group of researchers
watched 28 Meerkat packs in South Africa. Of the subordinate females that gave birth,
on average 60% of their litters will emerge from the burrow 2 weeks later... if the dominant
female isn't pregnant. If she is pregnant, on average, none of their litters emerged.
For dominants it was about 80% and it didn't seem to change whether the subordinates were
pregnant or not. Most of the infanticide happened underground. A litter was presumed dead when
they no longer have guarding babysitters left for them. Meerkats share the duty of feeding
and babysitting their kids. The infanticide typically happened within the first day or
2 of the kids birth. And of the 6 pups that were observed to be
killed, 5 of them were killed or eaten by pregnant females, again mostly within the
first 24 hours. Why do it and why so much more when they're
pregnant? The idea is less of other people's kids running around means more food and resources
for your own kids. But Meerkats share the duty of nursing the pups, so when they're
pregnant, they go around and try to kill everyone else's kids, so that when their kids are
born, they get all the attention. What about parents killing their own kids?
If reproduction, is what it's all about, then how could this behaviour ever evolve?
Burying beetles are an insect that use a little dead rodent or dead bird as a house for their
kids. They take them into a little den or bury them where they will lay their eggs and
when the larva hatch, the little guys have a place live and eat. It is both food and
shelter. Researchers decided to play with them. They
gave some beetle couples a 30 g mouse corpse, and others a 15g one.
The beetles ended up laying the same number of eggs despite the corpse they got. But after
they hatched, the parents with the 15 g corpses ended up eating about half of their larva
within the first 24 hours of their birth. Why this?
If there's not enough food the parents could leave them alone, but then they would all
end up being malnourished. Maybe so much so that they would die before leaving or be outcompeted
later in life. Or they could kill some to make sure the others are as healthy as they
can be. They'll have less larva but have healthier larva that have a better chance
of making littles larvas of their own. Oh and they eat them too. They do that to'.
You know, get some of those nutrients back. Anyways. What's the lesson in all this?
It's not that monogamy is better than polygamy or anything like that. That's a different,
um, whatever. And there's otherwise nothing here that says how we should be acting. We�re
just contextualizing the behaviours what we see in terms of what works reproductively.
No....the real lesson is... um.... I guess is if you're alive today it's because your
parents, your neighbors, your parent's new lover, didn't try to kill you..... or make
some sort baby tikka masala.
This episode is brought to you by.... McKay's crispy baby nuggets. Pro choices in every bite.