Below you’ll find some of the thoughts that have bubbled up in my mind while I’ve been pleasantly bumbling about Kenya. All of these things connect to the project which we’re undertaking in some way, but I hope you’ll indulge by ramblings on natural history just a smidgen, even if they seem somewhat far afield from the task of excavating skeletons shortly at hand.
I visited an elephant orphanage on Saturday, and this is what I think:
They really do.
If you take a gander at the whole spread of animal excellence, from ants to zebras, you’ll indeed find that evolution has especially equipped mammals, above all other critters great and small, to suck. All the other groups of animals, including insects, fish, reptiles, amphibians, and birds, simply do not suck like mammals.
Of course, I do have to be delicate in my distinctions. We can call some of the ingestive functions which other animals undertake sucking, but their sucking has rather different properties than the mammalian form of the activity. Suction is achieved by the mouth of the tarantula, which slurps up the insides of its prey after injecting the victim with a cocktail of liquefying toxins with its fang-like chelicerae. The mosquito, an all too common and all too dangerous winged parasitic beastie, is well known to use its needle of a mouth to extract the blood of its host in exchange for a ruddy bump on the drill site and, potentially, a parting gift of the falciparal malaria plasmodium or something similarly unpleasant. However, the reason that all mammals suck as a class has to do with that classic shibboleth of mammalhood: producing milk.
Unlike the above pseudosuckers such as the mosquito and tarantula, the mammalian oral cavity busies itself during the first stage of neonatal life with little else than sucking. After birth, mammals from whales to mongooses (mongeese?) and bats to people all instinctively migrate their mouthparts to their mother’s (or, in some cases, to simply the nearest working) mammary glands. In the most primitive, evolutionarily speaking, mammals, the monotremes like echidnas and platypuses, the visible bit of the mammary gland on the momma is little more than an overgrown sweat pore which oozes milk for its little wiggling progeny. Of course, both the nipple and the lips become anatomically more committed to this infant care scheme in more derived mammals, including marsupials such as kangaroos, koalas, and opposums (Yes, opposums are marsupials!), and the more familiar Eutherian (Greek: “good or true beast”), true mammal, characters of chimpanzees, cheetahs, bears, and the like. Along with a body covering of hair and three particular inner ear bones, producing milk is what defines a mammal.
Consider the sort of commitment that mammalian mothers make by dint of their hoary phylogenetic inheritance: you not only have to cook your kid inside you and then push it out of its uterine gestational asylum, but you also have to have the thing suck on the spill-proof lids to your mammary glands until weaning, which varies from just days in rodents to over five years in big mammals like elephants and the weird kid down the street.
Mammalian motherhood, for this reason, is a lot of work. You might think all those mommy crocodiles are smiling because they aren’t afflicted with such an involved childcare strategy. Now, I’ll admit that we shouldn’t so quickly accuse these crocomoms of being deadbeats. Rearing their young isn’t necessarily hands-off, as some species of croc go to the trouble of protecting their newly hatched kids in their mouths for months until they’re able to fend for themselves, but they certainly don’t have to deal with that whole sticky lactation issue.
Those crocodiles may smile, but they don’t do much else. It’s not lips that move them to beam their flesh-rending, pearly whites at the world, but rather the immobile configuration of their soft facial tissues and rather conspicuous lack of lips. Try to rip the skin off of a crocodilian’s head. I have. The skin is darn near super-glued to the bone. These guys don’t suck. Their lips are missing and their tongues are about as stiff as boards. Baby crocs don’t drink milk, and their morphology reflects it. Birds, too. Birdy parents may caringly upchuck a meal into the gaping, chirping maws of their chicks, but there is no milk to be had. Those beaks don’t suck. Whether you’re a perpetually smiling croc or a stolid owl, your facial morphology, from the musculature to the bone, is simply non-sucky unless you’re a mammal.
All sorts of things accompany the mammalian need to suck. Consider teething. There’s a reason the first set of teeth are called milk teeth, and there’s a good reason that their eruption is linked to weaning. Consider the mother-child bond formed through the child’s ingestion of the lactating mother’s milk. There’s much to recommend to such a maternal link when you’ve got a kid that needs a parent to show it the ropes of social and survival skills. As a class, it’s fair to say that mammals need to learn more than most any other group of animals to survive and pass on their genes. Many mammals need not only milk but also the instruction of their mothers in (anthropomorphism ahoy!) social etiquette, food capture or location skills, and the like. The maternal bond, a phenomenon detectable not only at the behavioral but also at the hormonal level, is a fantastic way to keep a kid attentive to a teacher that can show it the things it needs to know so that the germ line advances yet another generation. (Happy belated Mother’s Day, Mom! I’m sorry I’m so bad at remembering this stuff!)
Of course, sucking has other things to recommend to it. The tongue and lips must contort in certain ways in order to suck, and this same labiolingual yoga is necessary for speech production in our own species. Furthermore, kissing is a fantastic epiphenomenon of sucking. Without labile lips with which to suckle, pecking in the amorous rather than the avian sense would be darn near impossible. Plant a juicy wet one on the partner of your choice and thank your mammalian mother for lactating! (Thanks, Mom!)
(This post will be continued in a second part: “A Petit Primer on the Genetics of Lactase Persistence”)