Most of you have probably read the SV-POW! posts on giraffe and sauropods necks (here, here, here, here), including the latest post and the paper it deals with (Taylor & Wedel 2013 – Yay for open access at PeerJ!). In it, Mike and Matt list a bunch of factors that facilitate a long neck, and show that modern animals are maxed out in one or several of these categories at low values, and thus incapable of developing necks as enormous as those of sauropods.
I was one of the two reviewers of this paper (Yay for review publication in PeerJ!), and previously, as a member of FOR 533, I had heard a lot of stuff on sauropod necks, thought it through, and so on. I guess I am quite qualified to talk about the subject. Still, I missed out on an obvious potential reason why mammals have shorter necks than sauropods!
Before we talk about this, though, let me introduce the cutest long-short neck in Berlin:
Just four days old is this Reticulated Giraffe (Giraffa camelopardalis reticulata) in the Zoo Berlin!
cute, hu? And if you compare to an adult you can see the typical large head of a newborn. But the long legs are there already – can’t shorten them much as the young has to keep up with Mum or it will surely fall prey to predators. And the neck has to be fairly long, too, otherwise drinking water from the ground or milk from Mum gets really difficult.
And now we get to the key issue here: a proportionally shorter neck isn’t really feasible. But what would a longer neck mean for the newborn and the mother? Is it possible that a proportionally much longer neck would give trouble during birth?
If you compare a baby giraffe’s proportions to an adult, you can quickly see that the legs of the baby are maybe a bit longer, but the neck is quite a bit shorter proportionally! See here: I scaled the two giraffes so that the bodies are the same size, roughly, then superimposed them.
Now, proportional differences between babies and adults are normal, but they do not happen just by chance. There are good selective advantages for them. And a baby giraffe might well profit from a longer neck. Remember that giraffes are at a very high risk from predators when they drink. They have to spread their front legs wide to get the snout to the ground, or kneel down. Not a really good set of choices for an animal that needs a long time to get back into a flight-ready upright pose. This is how the baby solved the problem:
Not very comfy, I’d say! A loooooong neck would help – but the baby has a short neck! Apparently, there is a limit on proportional neck length in newborns.
So in sum I think that yes, proportional neck length may be limited by the mammalian birth process! If your neck is too long in relation to the body there is a high risk of getting entangled and stuck. Already, ungulate births are no laughing matter, if the baby is not arranged just right (the same is true for humans, by the way). In an egg you do not face this problem, or at least it isn’t that bad. Also, if the hatchling doesn’t make it out in a few percent of cases that’s much less bad than if the mother dies as well. Egg-laying r-strategists fare much better with high infant mortality, for whatever reason causing it, than live-birthing K-strategists.
Looking forward to hear the SV-POW! crew’s take on this.