Everyone knows that the squeaky wheel gets the grease. After all, in a crowded room, it's often the loudest voice that's going to get heard. And if you're a busy rat mom just trying to get through the day, well, when all your pups are squeaking? Start with the one that squeaks the loudest!
But who DOES squeak the loudest, and what does that do?
(So cute! Source)
Bowers et al. "Foxp2 Mediates Sex Differences in Ultrasonic Vocalization by Rat Pups and Directs Order of Maternal Retrieval" Journal of Neuroscience, 2013.
FoxP2 is is a gene that has been shown before to be correlated with language and vocal communication, particularly in chimps and humans. But it also plays a role in "language" in other species. Birds, for example, show increases in FoxP2 during the time when they learn to sing. So it's possible that FoxP2 could play an important role in "language" and "song" in more than just humans.
So what about in rats?
The authors of this study wanted to look at the role of FoxP2 in rats, particularly in rat PUPS. When a mother rat leaves her pups, even for a very short period of time, the pups will get upset, and start to make little ultrasonic distress calls, that the human ear can't hear. When the mother rat hears these calls, she will usually return to the nest (if she is able), and retrieve her pups, often placing them in a new nest or keeping them nearby.
But there's something a little odd about this. Not all pups squeak equally. In fact, males squeak more often than females. And this has an important effect, the mom retrieves the males FIRST. This means they get put near the center of the nest, where it's warmer.
So what was causing these boys to be so squeaky? It turns out that the male pups at this stage (about four days old), had higher levels of FoxP2 gene expression in their brains than the females did.
But was the FoxP2 causing the squeak? To find this out, the authors used a small interfering RNA (siRNA) that prevents the expression of a single gene. In this case, the prevented the expression of FoxP2.
When the siRNA was inserted into male and female pups, the squeaking equalized. You can see the control males in the clear bars and the control females in the black bars. The females emitted fewer calls. But when the siRNA was on board (the red and blue bars), the number of calls was equal.
And this completely changed how the rat mom treated her pups.
What you can see above is the rank order in which the rat mom retrieved her pups. On the far left are the males. They are low because they have a high rank order, being retrieved first, second, and so on (the numbers get pretty high because rat litters can get pretty big). The females (second from left) have higher rank orders, and are retrieved last. But when the siRNA is onboard, it almost looks like the trend is reversed, but in fact there is no significant difference between how males and females get retrieved.
So the squeaky wheel, or the squeaky rat, gets the grease, and in this case, the squeaky rat has higher FoxP2. Why does this happen? It could be that, in the long run, it's better to retrieve males than females, as more male offspring gives the mom a higher chance of passing on her genes.
But then they tried to extend their findings to humans. In theory, if humans were like rats (in this respect), this would mean that little boys would (a) "squeak" more (perhaps cry more?), and (b) that little boys would have higher levels of FOXP2 in the brain than little girls.
But in this case, it turns out that humans are not like rats. In fact, in a set of postmortem (sadly) samples from 4 year old boys and girls, FOXP2 was LOWER in the boys than in the girls.
I'm actually not surprised by this, and I think there are several reasons why this might be the case. First, girls have been shown to have increased vocabulary development early in life compared to boys, which would predict increases in language related genes compared to boys. So I'm not sure this developmental time point was the best one, an earlier one might have been better. But even then I don't think they would have seen anything. After all, WHY would being a squeaky wheel matter in humans? After all, humans, unlike rats, don't have LITTERS of pups. Just one or two (or rarely three) at a time. A human baby does not (generally) need to compete against other babies for attention and care, and so I'm not sure why something like this would be selected for.
But while they didn't see differences in humans (and after all, rats and humans don't have to be alike in all things), the differences in rats are interesting on their own. In rats, the squeaky wheel (or pup) gets the grease.