Orli Snir, a biologist at Rockefeller University in New York, couldn’t keep his ants alive. She had plucked pupae from a colony of clonal robber ants, where sesame seed-sized offspring that looked like puffed rice cereal were pampered by younger larvae and older adult ants. Then she isolated each pupa in a small, dry test tube. And each time, they drowned.
Specifically, each pupa was leaking so much watery, golden liquid that it was hard to breathe. But they survived when Dr. Snir chased the liquid away with a capillary tube. His humble observation led a strange trail of experiments to a bizarre but inescapable conclusion: this mysterious ant works a lot like milk.
Not just one species of ant uses this milk either. Perhaps all ants do, according to a paper led by Dr. Snir and published Wednesday in the journal Nature. He adds ants among other unexpected creatures like pigeons, spiders, and beetles that feed on milk-like fluids. And just like milk in mammals, it brings together ants of different generations – and the larger ant society too.
After first noticing the strange secretions, Dr. Snir scoured the scientific literature and it seemed his ant pupae were oozing something that was mostly unknown to science. She shared what she found with her co-author, Daniel Kronauer, who leads an ant evolution research group at Rockefeller. “My first thought was, ‘This is crazy,'” he said.
According to the traditional view, ant pupae are passive, trapped in a dull and transitory phase of life. The insect begins as a wiggling, worm-like larva, then seals itself in an inert pupa that resembles a living sleeping bag, emerging as a fully grown adult. Inside a living colony, where these three phases are in a constant and impenetrable hubbub of contact, the pupae seem to remain dry.
So where does this pupa juice normally go? To find out, Dr. Snir added blue food coloring to the shells of the pupae. Then she put them back in an ant colony, where the pupae are stored in a slimy heap eerily reminiscent of risotto. Within hours, she found, the ants and adult larvae took on a blue hue, a sign that both groups were siphoning off liquid. The adult ants even seem to pick up the larvae and put them on the pupae so that the younger members of the colony can suck up the secretions.
The team’s subsequent experiments revealed a web of codependent relationships. If the adult ants do not lick the pupal fluid, the pupae drown or die from fungal infections. And if the newly hatched larvae can’t drink it, they’re less likely to survive. As for what adults get from consuming this product themselves, that’s still unclear, but early studies show it contains hormones and neuroactive compounds.
Next, the team examined the pupae of four other ant species on the other side of the ant tree of life. Each of these species appeared to have a similar liquid, suggesting that the newly recognized “milk” is common among ants.
“I’m absolutely excited about this work, which I consider very important,” said Bert Hölldobler, an ant biologist at Arizona State University who was not involved in the research. In 1977, Dr. Hölldobler added, he published an observation that adult ants gravitated to chemical compounds on the skin of pupae, but he did not pursue the subject further.
The discovery that ants’ intergenerational interactions revolve around the sharing of “ant milk” may also tie into deeper puzzles.
The first is how ants got so social in the first place. Since the use of pupal fluid involves entering into a pact of co-dependency – the pupae must be cleansed of it and the larvae must feed on it, or they die – this “milk” could be the adaptation that first bound solitary ants into colonies.
It may also be the glue that binds current ant societies together, acting as a common endocrine system allowing the colony to exchange hormones that shape its future development. “I think we’ll see a lot more studies building on what’s been discovered here,” said Adrian Smith, another biologist outside the North Carolina Museum of Natural Sciences research team.
But one last pressing question remains unanswered. “I will say I tried to taste it,” Dr. Snir said. She thought the ant milk was slightly sweet, but looking back, she’s not sure if it was just because she expected it. She even asked her lab colleagues for a second opinion. “It’s the truth: I was looking for volunteers, and no one agreed to taste it,” she said.
“I’m still watching.”
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