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We humans have evolved some pretty complicated means of communication,
including facial expressions, sophisticated speech, and even written language.
While we may be the only species
that currently communicates by texting,
some primate species have evolved an even wilder way
to send information to those around them.
And it’s by using their butts.
That is, the females of many species of monkeys,
and even some apes, develop striking swellings around their bottoms
in order to send a message that it’s time to mate.
These dramatic derriere displays
have a major impact on decisions made
within these groups and affect all kinds of their behaviors.
So have a seat and get ready for more facts about primate butts
than you ever thought you needed!
When a female of any primate species reaches the age
where she can reproduce,
she starts to ovulate on a semi-regular basis.
In some species, when a female ovulates,
there’s no indication she’s doing so -
this is called concealed ovulation.
The alternative is called conspicuous ovulation,
and in the primates we’re talking about,
boy, is it conspicuous.
See, these primates experience what’s called a sexual swelling -
basically, the skin around their genitals and anus swell up,
and then shrink again after the fertile window.
And the peak of the swelling typically
lines up with the peak of ovulation.
The extent of the changes in the skin around their bottoms
differs a fair bit between species, but some are very obvious.
These large, showy, and sometimes vibrantly colored swellings
can be so big that, depending on the species,
the female’s weight can increase
by 14 to 25% during her estrous cycle!
And they don’t just swell -
their butts often turn bright red as well.
These exaggerated swellings happen
in many species of Old World monkeys,
which are found in Africa and Asia,
as well as some species of apes.
Perhaps the most iconic primates with junk in their trunks are baboons.
Most baboon species tend to spend a lot of time strolling on all fours
through open, short-grass savannahs,
which means there’s nothing blocking the sightlines to their back ends,
swollen or otherwise.
Baboons live in large groups with complex social orders,
where certain matrilines are socially dominant to other lines.
For these baboons, social rank really matters
it can influence their access to food, and improve
the chances of forming alliances and relationships in groups.
Being higher status in the group ranks
can even mean they get sick less often
and that their wounds heal faster.
Basically, it pays to be well liked.
Another thing that social rank can affect?
Mate access. Higher ranked male baboons
are much more likely to produce offspring than lower ranked ones,
largely due to access to mating opportunities that lower-ranked males don’t get.
All this means that there’s a lot of pressure
to rise in the ranks within a baboon society,
and the best way to do that is by strategically mating,
which you can only do if you have a signal of when a female is fertile.
Hence the bubble butt.
But its not just baboons -
in all of the species with sexual swellings,
those rumps are sending a message to male primates
on the status of the female’s estrous cycle,
and the males are definitely paying attention.
The primary reproductive goal of males
of these species is generally
to father as many offspring as possible.
So once these primate males see a showy-bummed female,
they know to ramp up their efforts to mate.
They’ll even try to prevent other males
from getting to their chosen female around her highest-fertility window
by guarding her, basically at all times.
This can help assure him that if she does get pregnant,
it's his bun in the oven.
Which is very important to him,
because primate babies are very dependent on their mothers for a long time.
And since having dependent offspring
reduces a female’s fertility,
primate males know that a missed opportunity to father
that offspring can cost him a lot in the long run.
On the other hand,
males will spend more time with the mothers of offspring
they think are theirs,
groom them more often than other group members,
and even might share food with them.
In fact, this fertility information is so important
to these guys that in one species,
when line of sight got blocked to the females’ butts,
they all had to find a new place to show off those same signals.
We’re talking about geladas,
a type of monkey found in the Ethiopian highlands.
They’re unique among primates as the only known grass-eaters,
and they spend their days sitting in tall grass,
picking and eating seeds.
But this tendency to sit and shuffle along on their bums
to forage likely made their back ends less effective billboards
than they are in other species -
so evolution got creative.
See, geladas have large, hourglass-shaped patches on their chests
that turn dark red and develop bead-shaped swollen bumps
when they’re ovulating.
It’s a pretty dramatic look.
Their bottoms do still swell a bit,
but it’s a lot less colorful and obvious than in other species.
Instead, they rely on the chest patches
to let male geladas see at a glance which females are ovulating,
all without them having to stop munching.
So this whole fertility signaling mechanism
seems really straightforward, right?
Well, not so fast.
It turns out that there can be a little ambiguity in these swollen butts -
or chests, as it were.
What’s more, females might be using
that ambiguity to their advantage.
See, the females of some species have swellings
that last longer than just the few days
immediately around ovulation.
During this time, the females
may mate with more than one male
in order to conceal who’s truly the genetic father.
If a male mated with her during
the time she appeared most reproductively ready,
he might be pretty convinced it's his, which is much better for the offspring.
This is because, while the mothers are still producing milk for their babies,
their ovulation is suppressed,
meaning they’re unlikely to get pregnant again until the baby is weaned.
And like we said before, males don’t want to wait around for this,
especially if they suspect the baby isn’t theirs.
They sometimes even resort to killing the baby to speed things up,
since the female will become reproductively available much sooner.
So concealed paternity could help females
protect their babies from aggressive males.
And it turns out these males have good reason
to be choosy about which specific females they mates wit
That’s because the females’ swellings
may indicate how successful they’ll be as mothers.
A 2001 study of olive baboons in Gombe National Park, Tanzania,
found that female olive baboons with larger swellings matured faster
and had more offspring than those with smaller swellings.
On top of that, the showier baboon mothers had more offspring survive.
And it does appear to be beneficial
to advertise your health through your bright swollen bottom
when it comes to mate choice.
This same study also found that baboon males pick up on this cue,
and fight harder to mate with females that have larger swellings.
So these males have good reason to be pretty picky about which females
they dedicate their time to.
Since they end up spending a lot of time guarding their mates from other males,
it leaves them less time for other important tasks, like foraging for food.
It’s also dangerous business fighting off other males,
so they want to ensure the female they’re protecting
is the most likely one to rear the very best bouncing baby baboons.
However, other studies looking at
whether a bright butt signals a bright future as a mother
have been a little less definitive.
A study from 2015 of Amboseli baboons found that bigger-rumped females
weren’t attracting any more mates than their smaller-rumped counterparts.
Not only that, but bigger swellings
didn’t even seem to correlate with fitter females -
reproductively fit, that is.
The study didn’t look at which females bench press the most.
So it doesn’t appear that a bigger butt is reliably an indicator
of reproductive success across species.
And that may be because it’s not just about looking at overall health,
but also the importance of timing.
It turns out the swellings can provide more detailed information about the individual,
which is helpful for males in determining
where to best concentrate their efforts.
See, the first few times a female’s estrous cycle
kicks in after she’s given birth,
she’s not likely to conceive.
She still has the hormone cycle,
just lower odds of making a little sibling for her last baby
at least not right away.
And in some species, the female’s swellings
become a bit larger once the post-pregnancy fertility drop is over.
But looks can be deceiving in this case
as the changes in the size of the swelling during different cycles are subtle.
Regardless, it’s important for males to determine the best times to mate,
so they’re not using up valuable time and energy
if it’s unlikely to result in offspring.
And it seems like these males have found out how to do it.
Baboon males concentrate their efforts on females that are more ready to conceive -
so it appears they’re keeping tabs on these females,
although it’s not clear exactly
how they track the females’ cycles so accurately.
But we also know that honing the skill of butt-reading takes practice,
since males that joined the group more recently
were only able to pick out which females
were the most fertile after being in the group for a while.
So males absolutely alter their mating behaviors
based on their read of female’s fertility.
But surprisingly, it can influence behaviors outside the bedroom too.
Take chimpanzees, for example.
Chimps do hunt for meat, but they don’t do it consistently,
and meat doesn’t make up a significant part of their diet.
What’s more, there’s not a lot of information out there about
what actually makes a chimpanzee foraging party initiate a hunt at all.
But it turns out their occasional choice to pursue a meaty meal
might actually be directly related to how many sexual swellings they can see.
A study conducted in Tanzania in the 1990’s
found that the presence of one or more swollen females
in a group made it much more likely that a foraging party would start hunting.
This may simply be because reproductively-ready females
tend to be surrounded by more males. And the more males in a group,
the more likely they are to have success on their hunt.
But this meat-seeking preference
could also signal that food type may play a role in sexual selection.
Chimpanzee males appear more likely to provide meat to estrous females,
who in turn are more likely to mate with these generous males.
And females that received the most meat
also tended to have more reproductive success.
So even though the hunting of meat is not directly a sex-related act,
it might still be strongly tied to the males’ efforts to woo a mate.
When it comes to the factors that influence these swellings in primate females,
we’re still trying to get to the bottom of it all.
But something tells me that it shouldn’t be too hard
to get researchers interested in this topic,
which could lead to even more derriere discoveries.
and now you know -
there’s a delightfully complex evolutionary story going on
with the back ends of our primate cousins.
No butts about it!
Thanks for watching this SciShow video.
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