PARIS—Mammals suck. The ability to suckle milk is a defining characteristic of the group, and it is no small feat of evolution. Nursing—as well as drinking through a straw—requires complex anatomy to seal off the airway every time we suck and swallow.
But one branch of mammals doesn’t suckle: the egg-laying monotremes, which include today’s platypus and echidna, or spiny anteater. These animals lack nipples. Their babies instead lap or slurp milk from patches on their mother’s skin. Monotremes are thought to have diverged from other mammals roughly 190 million years ago, so most paleontologists figured that suckling evolved after that split.
Now, a close look at modern animals and key fossils from before the split suggests monotreme ancestors could suckle after all, but the animals later lost the ability as their mouths evolved to eat hard-shelled prey. The finding “puts a new light on monotremes” and suggests suckling was part of the original mammalian package, says paleontologist and functional anatomist Alfred “Fuzz” Crompton of Harvard University, who led the new studies.
The work is “incredibly interesting and really important” for understanding mammalian evolution, says neurophysiologist Rebecca German of Northeast Ohio Medical University in Rootstown. “They are beginning to understand the part of the anatomy that is critical to infant feeding.”
Previous research by Crompton and others has identified a suite of muscles that play a key role in suckling. One, called the tensor veli palatini, stretches from near the base of the ears to the edges of the soft palate, the tissue that forms the back part of the roof of the mouth. When you suck on a straw, this muscle pulls the soft palate taut so your tongue can form a tight seal with the roof of your mouth. When the front of the tongue drops, the mouth becomes an area of low pressure and you draw liquid in.
More recently, to better understand how suckling evolved, Crompton and his colleagues analyzed the heads of North American opossums, platypuses, and monitor lizards, as well as fossil skulls. At the 5th International Paleontological Congress here last week, Crompton’s co-author, research technician Catherine Musinsky, described the anatomy of two mammalian ancestors: Thrinaxodon, which lived roughly 250 million years ago, and Brasilitherium, which lived about 220 million years ago, both before the first common ancestor of living mammals. (That animal is thought to have lived in the early Jurassic, which began about 200 million years ago.)
Modern reptiles lack the tensor veli palatini and it seems Thrinaxodon didn’t have one either. But in Brasilitherium, the researchers found that the shape of the bones and the scars where muscles attached suggest that a primitive version of the muscle was present. That, along with other evidence, led them to the surprising conclusion that this ancient mammal relative could probably form a tight seal between its tongue and palate and might have suckled. The idea “is very well supported,” by the researchers’ combination of modern anatomy and fossil evidence, says paleontologist Zhe-Xi Luo of the University of Chicago in Illinois, who has also closely examined Brasilitherium.
To explore monotreme anatomy in more detail, the researchers also painstakingly sectioned and scanned the head of a modern platypus. Although these animals branched off from other mammals well after Brasilitherium, they have lost the tensor veli palatini. Instead, their mouth and jaw have evolved to grind up the hard shells of crustaceans they scoop off river bottoms with their flat snouts. They move their lower jaw from side to side to grind their prey with rough pads on the tongue and palate, which have replaced teeth. These adaptations mean the platypus can’t form the tight seal required to suckle.
Given that suckling is one of the defining characteristics of mammals, “It’s a bit surprising that one of the first groups to branch off lost it again,” Crompton says.
Luo agrees. Because suckling allows newborns to efficiently access high-quality, high-calorie food, platypus ancestors faced a potentially expensive trade-off when they gave it up for specialized feeding, he says. For example, licking milk from their mothers’ skin exposes the animals to a higher risk of infection. But such drawbacks may have led to other adaptations, Musinsky noted: Researchers have studied platypus milk for its potential antimicrobial properties.
German adds that Crompton and Musinsky’s approach offers a valuable way to understand how suckling behavior evolved. “Breasts don’t fossilize,” she says. “And there aren’t going to be many fossil tongues … so anything that we can extract in terms of comparative information is incredibly important.”