moms as the default caregivers -- 7/14/21
Today's selection -- from Mom Genes by Abigail Tucker. Outside of mammals, mothers are not always the caregivers:
"Outside mammals' corner of the natural world, moms are not always the default caregivers. Among fish, when parental care is provided at all, males usually step -- make that swim -- up. Many human moms will be (perhaps endlessly) familiar with the dutiful daddy fish from Finding Nemo. In the Pixar spin, Nemo's mom is derelict because she has, most regrettably, been eaten. In real life, however, a mommy fish is more often the eater. Many fish enjoy a rather splendid adaptation called 'unrestricted growth' -- which means the female can get continuously larger throughout her life, gorging and growing and releasing ever more eggs as she expands. (These revelations have made me glare jealously into my daughters' goldfish tank, all of whose occupants are allegedly girls.) Because the female fish's fertilized eggs land on a seaweed frond, or in some other likely spot outside her own body, the father may be thereafter deputized for egg- and larva-minding duty while the mom, finloose and fancy-free, shimmies through the world's oceans looking for the next opportunity to sow her wild eggs.
"Female birds aren't quite so carefree, but about 90 percent of bird species equitably split chick-rearing duties with their mates, as though someone had stuck a chore chart on the refrigerator. Because, once again, the fertilized eggs mature outside the female, eggs and hatchlings can benefit from both parties' steadfast efforts to guard and warm and provision them. Our backyard is home to a pair of hawks, and I often gaze up at their biparental nest in silent salute.
"Mammalian females don't lay our eggs, however. We keep them tucked in our guts, even after fertilization. Internal gestation of embryos helps explain mammals' riotous global success: pregnancy keeps our youngsters warm, fed, and shielded from predators and lets us infiltrate even the harshest environments.
'But the same nifty adaptations that helped us to outlast the dinosaurs have also left females holding the diaper bag.
"In our kind, the males are the gamete-spewers, who can theoretically sire nearly infinite numbers of babies. Females are saddled with a cramped internal tenancy that's often months long. A mammalian mom has no choice but to invest heavily in the fetus(es) already jammed inside her, and to postpone passing on her genes to more. Males, meanwhile, may move on to fertilizing eggs in somebody else's basket.
"Nine -- make that ten, and then some -- months of pregnancy (or twenty-two months, for the unfortunate she-elephant) is actually just the beginning of our maternal predicament. Milk further seals mammal moms' fate. Our 'mammalian' identity flows straight from our mammaries, or breasts. A few other non-furry creatures -- tiger sharks, garter snakes -- have evolved internal gestation. But only mammal moms make milk.
"Sometimes it feels almost liberating to tote around a ready-made food source for a child. I've been caught countless times without a spare onesie, a diaper, and even my stroller. Heck, I've nearly forgotten my kids. But a nursing mother never leaves behind her milk.
"This 200-million-year-old convenience comes with consequences, though. In pre-Enfamil days, a built-in milk bar meant that moms alone could nourish newborns, a vast and intimate undertaking that often halts ovulation, intensifying mammal moms' investment in the hungry youngster at hand.
"To complicate matters for human moms in particular, the type of milk that we make is unusually thin and watery. Other mammals spend far less time nursing their young. For wild rabbits, with their rich milk, it's about five minutes a day. Fur seals may nurse only once a week. Humans, though, can spend half the night trying to get the job done. And while other juvenile mammals are weaned within weeks, humans are built to nurse for years because childhood is so long.
"In this respect, the by-products of our bodies -- eggs and milk -- help make up moms' minds. The way that mammals bear and nourish children means that the onus is on females to care for them, and in turn, that moms' brains are built to emotionally foster this paramount relationship.
"As the maternal behavior scholar Laura Glynn noted in a TEDx Talk, it may be that 'the burden has fallen on the female nervous system to protect our genetic legacy:' In the overwhelming majority of mammalian species, a bundle of joy is exclusively Mom's to lug around. Feeling the joy helps her bear the weight.
"From jaguars to giraffes, most mammal dads have nothing whatsoever to do with their offspring. As much as we human moms justifiably lament bad or absent fathers, maybe we ought to be thanking our lucky stars that we're part of the measly 5 percent of mammal species in which dads chip in any care at all."