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A UVM Study Considers the Sex Life of Ants


Published November 26, 2014 at 10:00 a.m.


Uncapping a plastic box containing a colony of Pogonomyrmex ants, Mike Herrmann says, "There are a lot of parallels between ants and humans." Like human beings, he points out, some ants practice agriculture, growing and cultivating the fungus on which they subsist; other ant species build complex structures that could justly be called architectural. "They're really amazing creatures," he says.

Such human-myrmecological similarities are a major reason why Herrmann, a graduate student of biology at the University of Vermont, opted to study these fascinating insects. He's equally interested in a bleaker parallel: the two species' warlike natures. As Herrmann puts it, "Ants are some of the only other species that bring everyone into a group to attack another same-species group to try to take their resources. We very rarely see this kind of mass mobilization, but humans and ants both do it."

Herrmann's current research, which has attracted the attention of national publications including the Los Angeles Times and the Washington Post, has uncovered yet another correlation: Human beings and ants both have really complicated sex lives.

Traipsing through the Arizona desert at the height of summer in 2010 and 2011, Herrmann tracked down "mating swarms" of two species of Pogonomyrmex — common, desert-dwelling harvester ants — that are sufficiently closely related to be able to interbreed. That hybridization has resulted in an epic war of the sexes.

When a male and female of the same Pogonomyrmex lineage mate, all female offspring will develop into the queens that will eventually found new colonies and thus preserve the lineage. The union of a male and female from different lineages, however, tends to produce only sterile workers, which represent a genetic dead end.

According to a recent paper coauthored by Herrmann and UVM associate professor of biology Sara Helms Cahan, this situation leads to "a conflict of interests, as queens must mate with both lineages to produce both daughter queens and the workforce to care for them, but males gain fitness returns [that is, an evolutionary advantage] only by mating with queens of their own lineage."

Sitting in an office strewn with ant figurines and ant-related cartoons that would elicit chuckles only from entomologists, Helms Cahan says that male harvester ants face a somewhat paradoxical reproductive situation. By mating with an ant of the other lineage, a male Pogonomyrmex "will probably have more daughters than a male who mates with a mate that will produce queens only from his sperm," she explains. "But he will have no grandchildren. So he gets more in the short term, but, over the long term, he gets nothing."

Like many insects, ants depend not on sound or vision for communication but on the secretion and recognition of specialized chemicals. A male Pogonomyrmex therefore cannot know the species identity of his mating partner until the moment of copulation.

So when his chemical sensors alert an ant that he's mating with a female of the "wrong" species, why doesn't he just, you know, pull out? As Herrmann and Helms Cahan have discovered, female Pogonomyrmex ants have evolved a reproductive strategy that serves their own interests: Hold on and don't let go.

Female ants need sperm from males of both lineages, so they've developed a copulatory clutch that prevents males from uncoupling. In an attempt to avoid transferring all of their sperm to a female that will give birth to sterile offspring, males have countered by evolving a technique to slow the flow of their sperm.

For now, however, the female ants' tactic is prevailing: By holding their mates firmly in place for an extended time, they ensure the males will wind up transferring just as much sperm as they would to a female of their own species. Not long thereafter, the males unceremoniously expire, having fulfilled their evolutionary purpose in passing on their DNA — even if, half the time, the effort is futile.

"It's like an arms race," says Helms Cahan. "Everybody pays more in order to have the competitive advantage against each other." Herrmann adds, "It's kind of a high-stakes arms race, too. If the males evolve a trait to mate only with queens that produce new queens ... the queens will have no workers, and the whole system will simply start to collapse."

And you thought creating an appealing online-dating profile was complicated.

The UVM team's observations about ant copulation, which have been published in the biology imprint of the esteemed British scientific journal Proceedings of the Royal Society B, did not reveal themselves solely through fieldwork. The two species of Pogonomyrmex are identical to the human eye, and the colonies' entire mating process occurs in a frenzy that lasts less than two hours. It wasn't until the mating pairs of ants were frozen on the spot, and their microscopic genitalia dissected in the UVM "Ant Lab," that Herrmann detected evidence of this unusual gender conflict.

Helms Cahan says such discoveries were impossible before the genetics revolution that has transformed nearly every scientific field over the past two decades. "Harvester ants have been studied for the last 50 years," she says. "They're the poster children of understanding the ecology of ants. Yet no one knew anything about this super-wacky way that they make their colonies, because nobody could look at the genetics."

Though further investigation is needed to determine the ultimate significance of these entomological findings, the UVM research suggests promising avenues of inquiry. For one, the study hints that the process of interspecies hybridization — which results in sterile offspring — may be a more powerful evolutionary engine than scientists have considered it to be.

Helms Cahan draws a parallel to mules, which are the sterile offspring of a horse and a donkey. "In general, we don't think of hybridizing as being something that generates new traits," she says, "because it's usually just a dead end for everybody who does it. But it turns out that if you are a social animal like an ant that already has offspring that don't reproduce ... you can gain benefits without necessarily paying a cost, as long as those hybrids become workers."

This research has implications beyond entomology, says Helms Cahan. "It's all about your relative performance in a population, as opposed to the absolute performance of your population." In the case of ants, sexual selection pressure actually encourages males to destroy their own lineage, and females to counteract that male instinct because it's in their interest to do so. "So it's that individual interest that ends up preserving the system," Helms Cahan says. "It's a good evolutionary lesson."

Helms Cahan allows that the queens, by effectively stealing their mates' sperm, may ultimately be calling the shots in this unusual mating ritual. "Although it's a weird situation," she notes, "because the end result is no different than if nobody ever did anything." That is, males transfer equal amounts of sperm to females of both species, but in a manner optimal for queens. "[The queens] are winning just by preventing bias," she says.

Perhaps the most important lesson of this research, Helms Cahan says, is that it could shatter our preconceptions about how sexual selection works. "We have a very stereotypical thought of males competing with each other, and the winner 'taking the spoils'; females are passive participants in that process," she says. Pogonomyrmex shows than the mating and reproductive processes are far more dynamic and nuanced than they might seem at first glance, thus chipping away at an old, sexist notion no truer for ants than it is for human beings.

Seems all's fair in love and war, including the erotic battle of the ants.

The original print version of this article was headlined "The Ants Go Marching"