The two-lined hooktip moth lays its eggs in even rows on twigs and leaves of birch and alder trees. When the eggs hatch, almost inscrutably small larvae emerge, some as short as half a millimeter. But the newborns, called warty birch caterpillars, have a serious task at hand. Despite not even being snack-sized, the caterpillars are easy pickings. Unlike other caterpillars, they do not build shelters, live in groups, or live inside leaves like the extremely flat larvae or some other fliers. Instead, newborn warty birch caterpillars march their itty-bitty bodies to a leaf of their own, which they will defend against any intruder, adult or baby, who happens to wander by.
At least, this is what Jayne Yack, a neuroethologist at Carleton University in Ottawa, suspected. In a 2008 paper, Yack and colleagues observed that freshly hatched warty birch caterpillars lived alone on leaves and produced two kinds of vibrations, which they employed in minutes-long duels with other caterpillars. Now, in a paper recently published in the Journal of Experimental Biology, Yack and a team of researchers have confirmed the unusually territorial nature of the caterpillars and described their vibrational defense mechanisms, which are surprisingly feisty for a creature smaller than a grain of rice.
In 2009, Yack and other Carleton researchers including Sarah Matheson, Leonardo Turchen and Emilie Mauduit collected some wild female two-lined hooktip moths at light traps. After the moths laid their eggs, the scientists transferred the larvae to paper birch leaves. A day later, almost 90 percent of the caterpillars had staked out a home at the very tip of their leaf. Here, they fed and laid out a small silk mat, almost like a bedroll, and rested.
The researchers filmed the babies as they patrolled this tiny territory, occasionally hitting the leaf with their head or thorax like a drum or dragging oar-shaped hairs on their anal segment—their butt, in common parlance—to create a vibrational sound. Of course, these sounds were too faint for humans to hear. "We had to use specialized equipment to pick up the vibrations," Yack said in story about the paper posted on the Journal of Experimental Biology site.
It was clear the caterpillars wanted to be left to their own devices on their leaves. But what happened if an intruder ignored these warnings? To test this, the researchers introduced 18 interloper babies to an occupied leaf, watched, and waited. Would the infants come to blows? Were they even able to fight each other? As a defending warty birch caterpillar sensed an intruder, it drummed its head and scraped its butt against the leaf even faster, a clear confirmation that the vibratory signals were a form of defense.
Fortunately, the babies resolved their disputes bloodlessly. In eight of these encounters, merely brushing against the body of another baby was enough to make a warty birch caterpillar take the plunge—leaping off the leaf, where they dangled from a silken thread. In the other 10 face-offs, the interlopers took the hint and retreated. "Most often, the original resident wins, holding on to its tiny territory," Turchen said in an interview with the Journal of Experimental Biology. In other words, no babies were harmed in the making of this science. Turchen called it "vibratory diplomacy."
On its surface, the discovery of territoriality in a new species of animal sounds somewhat mundane. Vertebrates and invertebrates alike defend their territories in any number of ways: spewing toxic chemicals, creating a ruckus of sound, or even honking through ballooning proboscises and dragging their teeth through the necks of their rivals. But territoriality is often a thing of adulthood. It is much more unusual for a baby, let alone a literal newborn, to open its eyes to the world ready to thump, kick, and rhythmically drag its hairy butt in front of any rival who dares come near.
Why come out of the proverbial womb ready to fight? The researchers raise a number of possibilities. They speculate the tip of the leaf could be more tender or nutritious. Perhaps positioning oneself on a leaf tip makes it easier to sense when a hungry spider or lacewing is on the prowl. Or maybe even the leaf tip acts like a diving board, allowing the leaping larva to launch itself to safety.
But one thing is clear. If you somehow hear the telltale buzz of a caterpillar thwacking its eensy body against a succulent leaf, a miniscule tumbleweed rolling across the distant horizon, you better hightail it out of here. This leaf's no place for a baby like you.
