A social network

ambika kamath | Updated on January 24, 2018

Weaving a web:Tent caterpillarsbuild web-like tents,which act ascentrally locatedinformationsources. Ambika Kamath


Can sociality have only one definition? Tent caterpillars show us that they are also ‘truly social’ just like the ants and the bees

Usually, when I receive a reminder that my column is due in a week or so, I’ve been considering its subject for days. This month when the reminder arrived, I hadn’t even picked a creature. Cue an eight-km walk through a nearby forest and frantic sorting through months of nature photos, hoping an idea would strike. When I finally decided to write about tent caterpillars, my enthusiasm for these animals was tepid at best. But in just a week, tent caterpillars have rapidly climbed in my estimation, becoming one of my favourite insects.

Tent caterpillars’ first claim to fame is their group living. A fuzzy, unassuming, brown and white moth, the mother tent caterpillar lays a clutch of eggs together, as moths are wont to do. But unusually, the caterpillars that emerge from these eggs stay close after hatching. They build web-like tents together, synchronously spinning sheets of silk. They make foraging trips together from these centrally located tents, leaving a silk-and-chemical trail behind them. And if a caterpillar finds a particularly bountiful patch of young leaves to feed on, it strengthens the chemical trail on its way back to the tent, allowing its siblings to follow the trail to this greenest of pastures.

The tent thus acts as an information centre for this parentless caterpillar family. And the benefits of siblinghood don’t stop here. The group of caterpillars thrashes and bites simultaneously, deterring predators from attacking them. They huddle together in the patchy sunlight of the North American springtime, enabling the tent caterpillars to reach temperatures that are ideal for growing quickly; temperatures that a single caterpillar can’t attain on its own. There is no question that tent caterpillars benefit from living alongside their brothers and sisters.

Yet insect biologists have been curiously reluctant to call tent caterpillars “social.” This is because the definition of sociality that biologists agreed upon somehow became centred on parents and offspring living together and dividing up familial tasks. This restrictive definition forced tent caterpillars far from the limelight, into lesser categories like “pre-social,” denying that the tent caterpillar’s take on sociality is just as successful as that of the “truly social” ant or bee.

But some insect biologists have persevered in resisting this hierarchy, and the non-traditionally social tent caterpillars have become a deserving poster-child of a more inclusive view of sociality.

Tent caterpillars’ second claim to fame was initially quite mysterious. About 10 years ago, hundreds of mares in horse farms in the US aborted their foetuses at the same time for no readily discernible reason. After some sleuthing, this loss was traced to the horses having swallowed a large number of tent caterpillars along with mouthfuls of leaves. Because tent caterpillar outbreaks aren’t too frequent, these abortion epidemics hadn’t been noticed before, and the phenomenon was described as being new to science.

But knowledge of such interactions was anything but novel. Across the globe in the Sahara Desert, nomadic camel herders known as the Sahrawi have known for centuries that pregnant camels will abort their foetuses after eating a particular species of caterpillar, which appears in large numbers on the tops of acacia trees after heavy rains. This caterpillar looks strikingly similar to, and belongs to the same family as, the tent caterpillar that causes horse abortions. Unlike the scientists, the Sahrawi describe this condition as something that ‘everybody knows’, but their knowledge came to the attention of scientists only in 2013. I think the scientific establishment, to which I belong, comes out from this episode looking rather silly. If we’d stopped to ask the people we share the world with what they know about nature, and considered their knowledge seriously, we’d have known about caterpillar-induced abortions ages ago.

I suspect that I struggled to focus on this month’s piece because my mind was filled with other things. I live in the US at the moment, and the last few weeks here have been tumultuous. Nine African-Americans were gunned down in a church by a man motivated by racial hatred, a cause for deep mourning. A few days later, the US Supreme Court ruled same-sex marriage legal across the country, a cause for celebration. Pondering these events, I felt, confusingly, that real change in how we humans treat each other is both possible and unlikely.

I imagined that thinking about tent caterpillars would distract me from these difficult topics, but I didn’t expect them to inspire me. I never thought that the stories of an insect could upturn stubbornly persistent ideas, ideas as deep and wildly different as what we think society really means and who we think is responsible for generating and safeguarding knowledge about the natural world. If there is room for revolution in the tales of a caterpillar, then people’s stories will spur change too.

(Ambika Kamath studies organismic evolutionary biology at Harvard University)

Published on July 03, 2015

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