Ant Brains Grow Proportionally to Communal Responsibilities

A new study suggests behaviorally limited army ants have proportionately smaller brains than their worker counterparts.

Like other social insect colonies (think bees), ants are divided into castes—classes of individuals that play distinct roles.

Army ants in particular feature three or four diverse body types, each built to perform different jobs.

Soldiers, with their large, muscle-filled heads and long, powerful mouths, defend their community by biting and stinging enemies. And … that’s about it. They can’t carry the young during emigrations, hunt, or kill prey; they can’t even feed themselves.

So it’s no surprise that the latest data points to a reduced brain size.

As described in a paper published by the journal BMC Zoology, scientists measured the head size and brain region of several species of Eciton army ants, comparing workers and soldiers.

“Even though worker brain volume increased with body size, that increase slowed or stopped in the soldiers,” lead study author Sean O’Donnell, a professor at Drexel University, wrote in a BioMed Central blog post. “Even though soldiers were larger than the workers, their brains were similar in size.”

They also discovered that two key brain regions were smaller than expected, based on soldiers’ overall brain size. One processes chemical information (the insects’ sense of smell), the other is critical for learning and memory.

This research supports the idea of evolution favoring reduced brain size in castes with limited behavioral demands. (i.e. The less work an ant does, the smaller its intellect. Or is it the other way around?)

“These findings are exciting,” O’Donnell wrote, “because they support the idea that natural selection for colony efficiency can favor the evolution of reduced brain investment in some group members, when a caste has limited behavioral demands.

“It appears Eciton soldiers give new meaning to the term ‘meathead,'” he added.

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