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This episode is about the birds and the bees and the butts.

Hosted by: Stefan Chin (he/him)

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Thank you to Guardio for  sponsoring this episode of SciShow!

Guardio is a secure browser extension  that keeps you and your information safe. You can get a seven day free trial of Guardio by clicking the link in the description and  experience a secure digital world today. [ intro ] 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 sight lines 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 males 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!

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