Most female flies take a low-rent approach to parenthood, depositing scores of seed-sized eggs in the trash or on pet scat to hatch, leaving the larvae to fend for themselves.
Not so the female tsetse fly. She gestates her young internally, one at a time, and gives birth to them live. When each extravagantly pampered offspring pulls free of her uterus after nine days, fly mother and child are pretty much the same size.
“It’s the equivalent of giving birth to an 18-year-old,” said Geoffrey Attardo, an entomologist who studies tsetse flies at the University of California, Davis.
The newborn tsetse fly looks like a hand grenade and moves like a Slinky, and if you squeeze it too hard the source of its plumpness becomes clear — or rather a telltale white. The larva, it seems, is just a big bag of milk.
“Rupture the gut,” Dr. Attardo said, “and the milk comes spilling out.”
And milk it truly is — a nutritional, biochemical and immunological designer fluid that the mother fly’s body has spun from her blood meals and pumped into her uterus, where her developing young greedily gulped it down.
Thus fattened on maternal largess, a tsetse fly larva can safely burrow underground and pupate for 30 days before emerging as a full-blown adult with a nasty bite and a notorious capacity to transmit a deadly disease called sleeping sickness.
In a recent chemical and genetic analysis of tsetse fly milk, Dr. Attardo and his colleagues were startled to discover how similar it was to the product of the beloved gland that stamps us as mammals. “I was expecting something completely off the wall and different,” he said. “But there are frightening, fascinating overlaps with mammalian milk in the kinds of proteins we see.”
The new tsetse fly research is just one example of scientists widening the ranks of adherents to nature’s Milky Way. Researchers lately have found the equivalent of mother’s milk in an array of unexpected, breast-free places: in spiders, cockroaches and burying beetles; in great white sharks, male emperor penguins and flamingos of both sexes.
Other scientists have traced the evolution of mammalian lactation and concluded that it arose not as a convenient way to feed baby, but to help our egg-laying ancestors tackle the challenge of water management on dry land.
Still other scientists are seeking to tally and understand the compositional differences in the milks from a broad sample of the world’s 5,500 or so mammals. They have unearthed a number of compelling concordances between the demands of oddball mammals and the makeup of their milk.
Assaying the milk of the nine-banded armadillo, for example, Michael Power, a lactation researcher at the Smithsonian National Zoological Park in Washington, and his colleagues were impressed by the high levels of calcium and phosphorus they detected, and by the even greater concentrations of protein.
“The protein was through the roof, way above anything else in the milk,” said Dr. Power, co-author with Jay Schulkin of “Milk: The Biology of Lactation.”
The elevated mineral counts made sense. “What does an armadillo build? A bony shell,” Dr. Power said. “So there’s going to be a lot of calcium and phosphorus going into this baby.”
But why all the protein? The researchers soon realized it was a matter of chemistry. If you simply dropped large quantities of calcium and phosphorus into most types of mammalian milk, the minerals would glom together into insoluble phosphate compounds.
“They’d get stuck in the mammary gland and never reach the baby,” Dr. Power said. The solution? Throw in extra doses of casein proteins to bind the minerals into compact, usable nano-clusters.
“If I’m going to have a high-calcium, high-phosphorus milk, I have to have a high-protein milk,” Dr. Power said, “because a lot of that protein is a calcium-phosphorus delivery device.”
Olav Oftedal, now an emeritus researcher at the Smithsonian Institution, and his colleagues sought to estimate milk consumption rates among black bear cubs as they nursed in a den. The researchers were startled to detect in the mother bear’s blood and milk traces of the distinctively labeled water samples they’d earlier given to the cubs as part of the experiment.
There was only one way the water isotopes could have ended up in mother’s milk. “She’s lactating in a den,” Dr. Oftedal said. “She’s not eating or drinking. But she is consuming all the excreta of her young, which she then puts back in her milk.”
Small wonder, then, that the amount of milk the denning mother produced exceeded the weight she lost. “She’s recycling everything,” Dr. Oftedal said.
Biologists warn against the wanton use of the word milk — sorry, almond “milk” really isn’t — and some mammalogists would like to restrict the term to the secretions of a dedicated mammary gland, which only their study subjects happen to possess.
But many scientists concur that if a parent synthesizes or highly modifies a substance on which its offspring’s life then depends, that parent is making a milk. By this measure, predigested food alone may not count, but if the parent first adds essential ingredients to the bolus, the regurgitate can fairly be deemed a milk.
Sandra Steiger of the University of Bayreuth in Germany and her colleagues recently reported on an American species, Nicrophorus orbicollis — a handsome, inchlong burying beetle with orange and black stripes — in which both parents care for their young. A parent beetle will eat a bit of carrion, predigest it and, on being tapped on the mouth by an offspring’s front legs, transfer the morsel into the little supplicant’s mouth.
“It’s like kiss-feeding,” Dr. Steiger said. “It looks really nice.” But as the researchers demonstrated, there is more to the osculatory exchange than pulped meat: the parent’s oral fluids are also critical to the young beetle’s survival.
Dr. Steiger’s team showed that if they presented larval beetles with liquefied mice carrion, the little insects could feed themselves just fine, but nearly all of them died before reaching pupation. Only when the scientists stirred parental oral fluids into the mix did the young beetles thrive.
The researchers have yet to analyze this chocolate-colored beetle milk, but Dr. Steiger suspects that it supplies the larvae with gut microorganisms, antibodies, digestive enzymes and other must-haves for mulching cadavers.
Wherever it appears, lactation is expensive and demands evolutionary justification. Flamingos are among the few birds that make milk for their young, and the effort drains them of all color — but at least it’s an egalitarian affair.
A male and female will jointly build a nest, incubate a single large egg and, when the egg hatches, churn out the rich crop milk on which the flamingo chick will feed for nine very long months. Begging calls from the chick stimulate in the parent’s brain the release of prolactin — the same hormone that subserves human lactation — which in turn prompts cells lining the crop, at the base of the parental throat, to swell and secrete the magic formula.
Brimming with protein and fattier than mammalian milk, flamingo milk “has the consistency of cottage cheese,” said Paul Rose, a flamingo researcher at the University of Exeter in Britain.
It is also bright pink. The parents spike the milk with the same carotenoid pigments that normally tint a flamingo’s feathers and that happen to be antioxidants — ideal for promoting a chick’s health and rapid growth.
Weeks and months pass. The parents must steadily step up milk production to meet ballooning demand. By the time the young flamingo is close to full-grown, robust in body and blushing of tone, its parents look thin and depleted, and their once-fuchsia feathers are now winter white.
“All of their energy, all their pink pigment, has gone into the crop milk,” Dr. Rose said. “Raising a flamingo is a very hard job.”
What explains the need for such full-throated lactation? Why can’t flamingos simply feed their young on beetles and flies, the way many birds do? Dr. Rose attributes the practice to the flamingo’s exceptional foraging style and the mouthparts necessary to accommodate it.
Like baleen whales, flamingos are filter feeders, and their distinctively crooked bills act as elaborate sieves. It takes time for a flamingo chick’s straight bill to thicken and bend, and longer still to master the practice of panning for a meal.
An essential antimicrobial
Only in the class Mammalia do all member species nurse their young, yet evolutionary biologists now believe that the roots of mammalian lactation date back more than 300 million years, a good 100 million years before the first mammals appeared.
The ancestors of modern mammals are thought to have laid the sort of porous, parchment-shelled eggs seen today among lizards, snakes and a couple of weird, monotreme mammals like the platypus. In contrast to the hard-shelled calcified eggs of birds, parchment eggs are at chronic risk of drying out, which means modern snakes and lizards often are constrained by the need to lay their eggs in a relatively damp setting.
Our ancient forebears stumbled on a liberating solution: Make yourself into a watering can, and you can lay your eggs wherever you want.
“The likely first function of milk was to hydrate parchment-shelled eggs laid on dry ground,” said Amy Skibiel of the University of Idaho, an expert in mammalian lactation. By this scenario, pre-mammals dribbled fluid onto their eggs through pores on their chest; nipples came much later.
Parchment eggs also are at risk of microbial infiltration, and genetic studies suggest that the earliest egg hydration solutions were fortified with anti-pathogen components. It’s a moisturizer — no, it’s a cleanser! And why not continue with the watering post-hatch, souping up the fluids, through natural selection, into food for the babies?
Once the mammalian lineage had settled on the secretory approach to parenthood, milks quickly diversified, the recipes dictated by a combination of need, diet and who your relatives were.
Hooded seal pups nurse on rapidly shrinking ice floes for just four days, and during that time they manage to double their weight. Not surprisingly, hooded seal milk is more than 60 percent fat — the fattiest milk among mammals.
It also smells overwhelmingly of fish, as I discovered when I sniff-tested a series of exotic milks at the National Zoo’s world-class milk bank. On the other end of the lipid scale, rhinoceros milk, at 2 percent fat, looks and smells like skim milk.
Elephant milk is less watery, and I was sure I caught notes of ice cream. Lion’s milk has no discernible odor and, like most carnivore milk, is low in sugars; meat eaters are designed to efficiently wrest their glucose from protein and fat.
Human milk, by contrast, is extremely sweet. Dr. Skibiel, who tasted her own milk while nursing her baby, said it reminded her of cantaloupe.
The number and variety of sugars in human milk outstrips that seen in any other great ape, Dr. Power said, and he proposes a surprising reason for that bounty: not to build our big brain, as some have argued, but because we needed sugar’s antimicrobial powers to help us cope with all the novel pathogens we encountered after the agricultural revolution, when we started crowding into villages and living in close quarters with other animals.
“Our ability to use animals in lots of different ways is one of the reasons we’re successful,” he said, “but it was a huge shock to the system.” Luckily, our milk rose to the challenge.
“Our brain made our milk,” Dr. Power said, “not the other way around.”
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