Creature Feature. The significant other

This is the 22nd instalment of ‘Creature Feature’, and starting this month, I’m going to broaden the conversation. First, you’ll be hearing from biologists who aren’t me. Most months, I will interview a fellow biologist about the creatures they find fascinating. Second, if you have a creature (animal, plant, fungus, anything) that you want to hear more about, let me know — I’ll do my best to bring you the most compelling science about your organism of choice. Kicking off this slightly revamped column, let’s talk about jerboas.

At first sight, you’d be forgiven for thinking that the jerboa isn’t quite a rodent. In a century-old description, the biologist Robert Hatt described jerboas as “looking like potatoes on toothpicks”. According to Talia Moore, a colleague at the Department of Organismic and Evolutionary Biology at Harvard University who studies jerboas and other hopping, mouse-like animals, these creatures seem more like birds. With a nearly horizontal body perched atop two long legs, tiny hands folded out of sight, their silhouette is certainly a little avian. Their behaviour is too. Unlike many rodents, “they don’t really bite, they take dust baths, they don’t make noises unless they’re babies,” Moore describes, “they’ve got this quirky hopping behaviour that makes them seem like a little robin.”

And it’s this strange hopping that has captured the interest of scientists studying bipedal animals. Bipedality, which simply means moving about on two legs instead of four, pops up time and again across animals with backbones. Because we humans walk on two legs, the trait takes on special, self-centred significance. Naturalists have found bipedality in creatures big and small, from kangaroos to kangaroo rats.

What’s surprised the researchers who study bipedal, hopping animals the most is that small and large members of this group function in entirely different ways. “If you’re a kangaroo,” Moore explains, “you require very little energy to move at high speeds for a long amount of time.” This is because kangaroos have impressive tendons. (Not sure what a tendon is? Feel for the cable-like structure behind your ankle.) Every jump a kangaroo takes, its tendons absorb and release energy like a rubber band, leaving less work for its muscles to do. Kangaroos can thus lope along for hours before tiring. And for a long while, people assumed that small bipedal rodents like the kangaroo rat behave similarly, hopping long distances relatively effortlessly.

But life as a kangaroo rat is not simply a doll’s house version of life as a kangaroo. One stark difference is how likely each of these animals is to get eaten. Escaping predators lies at the centre of a kangaroo rat’s movement, and its well-built, muscular legs allow it to jump suddenly, away from, say, an owl’s talons. Relying on muscles instead of tendons to quickly stop and start lets small bipedal rodents like kangaroo rats and jerboas avoid serious injury —repairing a torn muscle is far easier than fixing an injured tendon.

Jumping costs small bipedal rodents a lot of energy, but it always seems to help them escape being caught. About a century ago, western naturalists in the Middle East avoided these arduous chases by simply extracting jerboas from their burrows. “One may insert his hand in a burrow and with his arm “plow” along until the inhabitant is reached.” reported John Hornung in 1932, “Arab boys capture numbers in this way, and while endeavoring to dash past the hands of their pursuers, the animals often dart up the capacious sleeves of the native costume, from which they are easily extracted unharmed.”

And while such curious collection efforts yielded many jerboa specimens for museums and seeded laboratory-bred jerboa colonies, they left us in the dark about the actual habits of these animals in the wild. Moore recalls seeing her first wild jerboa in the deserts of Western China, after many months observing captive specimens in the lab: “I saw what I thought was a jerboa, and it just hopped away faster than anything...it just disappeared!” she exclaimed. “This was completely different from the locomotion I was [seeing] in the lab.”

This realisation that animals behave very differently in the laboratory and the wild is not as trivial as it may seem. Being able to study an animal in the lab is certainly convenient, and lets scientists control scores of variables — animals’ hunger levels, the temperature, and the surface they’re moving on, to name a few — that can complicate things in nature. But for a biologist, control comes at the price of being realistic. Moore recognised this trade-off the moment she saw her first wild jerboa hopping helter-skelter, successfully evading capture. “Instead of being frustrated by the variability I saw in the [animals’ motion], I actually realised that that was what was interesting, and studied that.”

Moore is currently studying if jerboas’ unpredictable changes in hopping direction, coupled with their muscle-powered starts, stops, and jumps, help them avoid being eaten. But even her five years of research have made only a small dent in what we don’t know about jerboas, and she attributes our lack of knowledge in part to the fact that these desert-dwellers are nocturnal. “Nobody wants to sit out in the middle of the night and watch them.” Those that do, however, are rewarded with the chance to watch haphazardly hopping “potatoes on toothpicks” that are unlike any rodent you may find in your kitchen.

Ambika Kamathstudies organismic and evolutionary biology at Harvard University; ambikamath@gmail.com

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