The Territory Is Tiny and So Is the Newborn Caterpillar Defending It

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.”

Dr. Yack and her team collected eggs laid by two-lined hooktip moths, the adult form of the species, and set up new hatchlings on single birch leaves. The newborns overwhelmingly booked it to the tips.

After staking claims, the caterpillars made vibrational signals, called drums and buzz scrapes, produced by striking and scraping their bodies against the leaf. The vibrations are like a “no vacancy” sign that rumbles across nearby stems and branches.

Rival warty birch caterpillars were introduced onto occupied leaves over the course of 18 trial encounters. When confronted by intruders, the resident caterpillars dialed up the signal rate by about four times. If the intruder breached the perimeter of the leaf tip, the defenders escalated the signal rate by about 14 times.

Newborn caterpillars are too fragile to endure the sort of violent conflicts observed in other territorial animals, from ants to elephants, which can be deadly. But intruders did make body contact in eight of the trials. And such encounters highlighted why the tiny tyrants want to live on a leaf tip: It allows an easy getaway. Faced with insistent invaders they’d never defeat, the caterpillars can rappel off the tip on a silk thread, a strategy called lifelining.

It took only a light touch to drive a leaf tip’s resident to retreat on its lifeline, leaving the caterpillar literally hanging by a thread. In the other 10 encounters, the intruders heeded the warnings and left the resident in control.

Dr. Yack and her colleagues have since conducted experiments that suggest caterpillars can distinguish signals from different sources. They might even imitate vibrations made by predators, like spiders.

“The vibroscape of insects is really unexplored,” Dr. Yack said.

The new research opens a window into caterpillar communication. But along with evidence suggesting some wasps or aphids defend territory as small as the tip of a birch leaf, the study also shows that territorial conflicts come in all shapes and sizes.

“Territorial behavior is really important to animals, including humans, and there are a lot more strategies out there than we think,” Dr. Yack said.