Ant milk: Makes a colony good
Orli Snir, a biologist at Rockefeller University in New York, couldn’t keep her ants alive. She plucked pupae from a colony of invading clone ants, where younger larvae and older adult ants were pestering sesame-seed-sized offspring that looked like puffed-up rice grains. She then isolated each doll in a small, dry test tube. And every time they drowned.
More precisely, each pupa was oozing so much watery, golden-colored liquid that it was difficult to breathe. But they lived when dr. Snir expelled the liquid with a capillary tube. Her modest observation led to a strange path of experiments to a bizarre but inevitable conclusion: this mysterious ant gland acts like milk.
Not a single species of ant uses this milk. Maybe all ants do it, according to a published article led by Dr. Snira on Wednesdays in the journal Nature. It adds ants among other unexpected creatures such as pigeons, spiders and bugs which feed on each other with milk-like fluids. And much like milk in mammals, it brings together ants of different generations—and the larger ant society, too.
After she first noticed the strange secretions, Dr. Snir reviewed the scientific literature and her ant puppets seemed to be leaking something largely unknown to science. She shared her findings with her co-author, Daniel Kronauer, who heads the ant evolution research group at Rockefeller. “My first thought was, ‘This is crazy,'” he said.
In the traditional view, ant pupae are passive, trapped in a boring, transitional phase of life. The insect begins as a wriggling, worm-like larva, then closes into an inert pupa that looks like a living sleeping bag, which emerges as an adult. Inside the living colony, where these three stages are in a frenzy of constant, unfathomable contact, the pupae seem to remain dry.
So where does this doll juice usually go? To find out, dr. Snir added blue food coloring to the doll’s shells. She then returned them to the ant colony, where the pupae were stored in a slimy pile that oddly resembled risotto. Within hours, she found, adult ants and larvae turned a blue hue, a sign that both groups were extracting fluid. The adult ants even seem to take the larvae and put them on the pupae so that the youngest members of the colony can ingest the secretions.
Further experiments by the team revealed a network of interdependent 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 do not drink it, there is less chance that they will survive. As for what adults get from consuming it itself, that’s still unclear, but early studies show it contains hormones and neuroactive compounds.
Next, the team looked at pupae from four other ant species on the other side of the ant tree of life. Each of these species appeared to have a similar fluid, suggesting that the newly recognized “milk” is common among ants.
“I’m absolutely excited about this work, which I think is very important,” said Bert Hölldobler, an ant biologist at Arizona State University who was not involved in the research. In 1977, Dr. Hölldobler, published the observation that adult ants were attracted to chemical compounds on the skin of pupae, but he did not pursue the subject further.
The discovery that intergenerational interactions of ants revolve around sharing “ant milk” may also lead to deeper puzzles.
One is how ants became so social in the first place. Since using the pupal fluid involves entering into an alliance of codependency – the pupae must be cleansed of it, and the larvae must they feed on it, or die – this “milk” could be it an adaptation that first bound solitary ants into colonies.
It may also be the glue that binds today’s ant societies together, acting as a shared endocrine system for the colony to replace the hormones that shape its future development. “I think we’re going to see a lot more studies that build on what’s been discovered here,” said Adrian Smith, another biologist outside the research team at the North Carolina Museum of Natural Sciences.
But one last, pressing question remains unanswered. “I will say that I tried to taste it,” said Dr. Snir. She thought the ant milk was lightly sugared, but in retrospect, she’s not sure if that was just because she expected it to be. She even asked her labmates for a second opinion. “This is the truth: I asked for volunteers, and no one agreed to try it,” she said.
“I’m still looking.”
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