As their name suggests, some members of big-headed ant colonies (Pheidole megacephala) have disproportionately sized heads. These soldiers use their giant, muscle-bound heads to power their mandibles, allowing them to attack rival ants and cut up prey. But every now and then, they’ll encounter ants who know how to fight back. And that’s when the colony starts growing even bigger soldiers.
“If you think about the worst invasive species, ants frequently show up on those lists, and big-headed ants are among the most problematic,” says Andrew Suarez from the University of Illinois in a news release. They’ve hitched rides with humans to six continents. “They are very aggressive. And unlike a lot of native ants, they produce large numbers of queens, so they have incredibly high potential for reproduction.”
Once they arrive in a nice warm spot, the ants spread out and assemble multiple nests, which all work together on defense, reproduction, territorial expansion, and procuring food. Because all the ants in a colony are sisters, genetics can’t account for their different shapes and sizes: It’s the changes in nutrition during larvae development that creates these variations. Different hormone levels turn on different developmental pathways, resulting in big-headed soldiers and small workers.
To see if environmental cues lead them to produce more and bigger soldiers, Suarez, Bill Wills of Illinois, and colleagues studied Pheidole megacephala at five places: Australia, Hawaii, Florida, South Africa, and Mauritius off the southeast coast of Africa. Australia and Hawaii are the opposite extremes of the group. There are no native ants in Hawaii, “so a lot of the local insect and arthropod fauna are not necessarily adapted to dealing with social insects," Wills explains. “But the native ant fauna in Australia is very dominant and diverse.” The other three sites fall somewhere in the middle: Competitors exist, but none as fierce or diverse as Australian ants.
The team found that big-headed soldiers and non-soldiers are the largest in Australia and the smallest in Hawaii; the ants at the other study sites were intermediate in size. On average, Australian big-headed ant soldiers are three times more massive than their Hawaiian equivalents. The soldier-to-worker ratios were pretty constant in all the populations studied, which means big-headed ants don’t respond to more competitive environments by generating more soldiers.
According to their genetic analyses, ants from all the sites were closely related, ruling out the possibility that this noticeable size variation is the result of evolutionary change. “This shows that they were able to adapt to a new environment relatively quickly,” Suarez says. “It’s kind of exciting and scary that it can happen so quickly.”
The work was published in Biological Journal of the Linnean Society.