Why is it that some mammals with small broods have the baby feeding apparatus near the upper limbs like humans and elephants and some near the lower limbs like cows and horses? As it turns out, this is a very good question. What dictates where the teats are on a mammal? The short answer is, well, no one is really sure, but that's not going to stop us from a bit of deductive reasoning and speculation. And one man who is up to the task is Queen Mary University of London's Chris Faulkes.
Chris - We've got teats in the thoracic or chest region like humans, the sort of intermediate area or abdominal area where we find teats in many species. And also there's the caudal region towards the backend, near the hind limbs as in cows, sheep, and and horses and so on.
Will - So there's something of a set amount of places that teats can occur, but the rhyme or reason behind the placement is far from consistent.
Chris - It doesn't seem to be necessarily a simple answer to who has what and where. So for example, in these hooved mammals with either even or odd numbers of toes like goats, sheep, and horses, they have two inguinal teats near the hind limbs. But then so too do guinea pigs, which are rodents and evolutionarily not closely related at all. So no clear pattern regarding where teats are.
Will - So what could some of the reasons be? Well, in the case of primates, the nipples are higher up and closer to the head because it allows for the ease of parents' movement. Humans can carry offspring whilst walking and baboon teats are placed so the offspring can drink whilst the mother is also on the move. This could be handy because you do not want to be incapable of moving when a predator comes around the corner. Mammals that live in trees such as lemurs often have nipples in the pectoral area, which might help the offspring to grip the parent's body while drinking and not fall out of the tree. Some shrews have teats at the rear end because their offspring follow behind them in a line, and cows have others at the rear end simply because their stomach is so big there's no room for it anywhere else. So perhaps ease of movement and access is what dictates the position of mammal nipples, but for now it remains something of a mystery. Apologies that we can be more definitive, Katie, but on the plus side...
Chris - I think it would make a really great larger scale comparison to study across mammals.
Will - You might have just inspired the next scientific paper. Thanks very much to Katie for sending that question in and to Chris Faulkes for helping answer it. If you have any of your own, the email address is chris@nakedscientists.com.
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