The Very Territorial Caterpillar

As they cling to leaf tips, newborn warty birch caterpillars produce vibrations that can ward off invaders approaching their millimeter-size domain.

When territorial animals are confronted by intruders, they instinctively protect their turf — no matter how small.

For warty birch caterpillars, that means patrolling one of the tiniest territories on Earth: the tips of birch leaves. Scientists observed the caterpillars warding off intruders with loud vibrations that advertise they are in command of a domain that stretches a few millimeters across.

“It’s like rap battles,” said Jayne Yack, a professor of neuroethology at Carleton University in Ottawa and an author of the study, which was published on Tuesday in The Journal of Experimental Biology.

Dr. Yack’s team is the first to observe an insect defending a leaf tip, a discovery that hints at a hidden world of territorial disputes playing out on small scales. These caterpillars are kings of the tiniest castles ever identified.

The behavior of warty birch caterpillars is unconventional. These insects seek turf as soon as they hatch, settling on leaf tips in a “dragon-like” resting stance. While other caterpillar species defend ranges at later developmental stages, they are not as vulnerable to predators and exposure as the warty birches.

“The remarkable thing about these guys is that they’re so small when they hatch, less than a millimeter,” Dr. Yack said. “The mortality rate for a small insect like that is very high, so usually they form groups to survive in that hostile world. But these guys always go to the tip of a leaf. That’s their strategy.”