Why It's Unsurprising That Ants Use Their Babies As Life Preservers During Floods

By Ben Wolford on March 2, 2014 12:53 PM EST

During a flood, ants deliberately use their babies as life rafts. (Photo: Flickr/Daniel Pink, CC BY 2.0)

During a flood, ants deliberately use their babies as life rafts. (Photo: Flickr/Daniel Pink, CC BY 2.0)

Many species of ants have become proficient at turning themselves into life preservers when their nests are washed out. But few experiments had studied the geometry of these rafts in depth. Which ants get stuck on the bottom? When researchers in Switzerland began their experiments to find out, they had assumed the adult worker population would rush to protect their most valuable and vulnerable during floods.

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But according to their research, published last month in the journal PLoS ONE, the biologists were only half correct. While the queens get to ride in the center atop the ant rafts, the vulnerable pupae and larvae don't join her. Instead, they form the base of the raft, exposed to the cold, the potential lack of oxygen, and the predators that lurk in the depths. Why? Because the babies are more buoyant.

These ants (Formica selysi, in this case) join a long list of animals that don't mind sacrificing their own flesh and blood for the sake of collective utility or sexual competition. For most animals, procreation is the whole point. Most evolutionary traits exist because they benefit longer lifespans that increase the chances of reproduction. Aggression between males (and females) of a species are usually rooted in the individual's selfish desire to send more of his or her genetic information down to posterity, and this is also why animals generally take care to rear and protect their offspring.

But not always. Infanticide is surprisingly common in the animal world. It's often brutal and often cannibalistic. In 2009, researchers in Scotland and the United States discovered the carcasses of young bottlenose dolphins washed ashore. Autopsies revealed their insides to have been ravaged as though "someone had taken a baseball bat and just literally beaten these animals to death," one of the scientists said. Further investigation revealed that adult dolphins had been responsible, though they weren't sure why. In 2011, a study of mustached tamarins found that overwhelmed mothers sometimes let their children die, or actively murdered them themselves. Other infanticidal species include prairie dogs, lions, bass, langurs, and even pigs.

It's less surprising then that ants, champions of collective behavior, would risk their children, known as the brood, for the good of the nest. Honey bees, another highly coordinated insect society, have been known for similar infanticide practices for the greater good. Bee hives are sometimes exposed to an infectious bacterial disease called foulbrood. When a larva becomes infected, the entire hive is at risk of destruction. Researchers have discovered that some breeds of honey bees are less prone to the disease because workers actively tease out infected larvae, yank them out of their cells, and toss them out of the hive to die.

One thing to note is that all ants, but especially the baby ones, have incredibly high survival rates after being submerged. After being under water for hours, at worst the ants were no more than dazed; they spent about an hour in a coma-like stupor, then got up and walked away. The babies also recovered. But that was in a controlled experimental environment, the researchers point out. In nature, death rates are likely higher because of predators, temperature fluctuations, and rougher waters. "There are likely to be physiological costs associated with submersion in water, including oxygen deprivation, increased CO2 levels, and possible thermal effects from cold water," the authors wrote.

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