Swimming evolved several times in treetop ants

At least 10 ant species can rescue themselves after falling from the forest canopy

KEEP CALM, SWIM ON  Although terrestrial, the tropical ant Pachycondyla villosa will rescue itself from an accidental splashdown by moving deftly through the water, dodging or climbing over floating debris as it heads toward shore. Such swimming ability has evolved multiple times among treetop ants.

S. Yanoviak

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In the tropics, some ants living high in trees can swim well if an accident — or a scientist — drops them into water.

Some form of swimming has evolved independently several times in ants, says Steve Yanoviak of the University of Louisville in Kentucky. In swim tests, the champs powered across the water at more than three body lengths per second, he and Dana Frederick of the University of Arkansas at Little Rock report in the June 15 Journal of Experimental Biology.

Tropical ants have plenty of water to worry about, especially where rivers flood acres of Amazonian forest so deeply that for months each year, fish swim among the trees. Ants that plummet from their treetop homes may drown, get gulped by aquatic predators or, just as fatally, be swept downstream far enough that they can’t find their colony again. “If you’re a wingless animal living high in a canopy, then you have to deal with what happens when you fall,” Yanoviak says.

GOOD MOVES The tropical ant Odontomachus bauri shows what to do with six legs on water. A rear leg provides stability as one front leg moves in sync with the midleg on the opposite side. Dots mark approximate points where a leg contacted water: blue for rear legs; yellow, middle; green, front. Arrows with same color code mark the meniscus shadow. S. Yanoviak
Some ants deal by gliding to a tree trunk, Yanoviak discovered in earlier research that involved dropping ants out of trees ( SN: 2/12/05, p. 101 ). Still musing about falls, Yanoviak took ants to a wooden footbridge over swampy pools in Peru. “I dropped a couple of ants and totally expected them to just struggle at the water surface and become fish food.” Many did. But ants in a few species “basically stood up and ran across the water surface and climbed out — no problem,” he says.

The ants that appeared to run on water did have their feet sink a little, and some ants’ legs sank more deeply but could still row along. To see if ants might have natural tendencies to rush toward tree trunks while swimming, the researchers got a vinyl Winnie-the-Pooh kiddie pool and set a white or black pole in the water against the perimeter. In a species known to swim, ants put into the pool with a white pole oriented in random directions overall as they swam looking for escape. But 87 percent of the ants dropped in the pool with the black pole swam toward it and escaped safely, as if heading for a dark tree. These ants’ behavior reminds Yanoviak of “a Labrador retriever chasing a stick in a pond, making a beeline straight towards it, totally determined.”

Of the 35 common tropical ant species he examined in various tests, 10 proved to be strong swimmers and another 10 showed some signs of directed movement. Biologists already knew that some ants can swim, Yanoviak says. Now he’s suggesting that the ability could be widespread. The ants he and Frederick identified as aquatic-capable were scattered among nonswimmers in several lineages in the genealogical tree, suggesting that the ability evolved independently each time.

Benefits of swimming extend beyond the flooded Amazon, says Walter Federle of the University of Cambridge. He has done some ant-dropping tests in Malaysia and found that ants living in the watery mangrove forests were better swimmers than ants living elsewhere.

Just how many ant species can swim remains to be seen, says Corrie Moreau of the Field Museum of Natural History in Chicago. “I suspect you will soon have ant biologists all over the world dropping ants in buckets of water to find out.”

SLO-MO ANT  In real time, this ant (Odontomachus bauri) is moving about eight times faster across a pan of water than the video shows. This species moves quickly to rescue itself, but about half the species in a recent test just flailed and struggled. The background grid is composed of 1-by-1 centimeter squares. Credit: S. Yanoviak

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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