One of the things I love about biology is that there’s always something new to learn, sometimes literally under the next moist rock. Recently, a completely new (to me) phylum of organisms has come to my attention: Tardigrada, the “water bears”.

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These tiny creatures can be found everywhere there’s a little bit of moisture and a little bit to eat — but when the moisture evaporates or the food is exhausted, they just curl up and wait. In addition to desiccation and starvation, they can also survive freezing, boiling, high pressures and sterilizing doses of radiation.

It’s known that the dormant state can last a long time, though it’s controversial whether this “long time” is on the order of a decade or a century. Their animated lifespan, however, is much shorter, around two months for one well-studied species.

What puzzles me, then, is this: If stress resistance is a major determinant of lifespan, and tardigrades are stress resisters par excellence, why aren’t they extraordinarily long-lived?

At the NAKFI meeting last week, our section deeply explored the key influence of body plan on longevity: For a given body plan (e.g., “all rodents,” which are all laid out in basically the same way), stress resistance (and other features like IGF signaling) does appear to strongly influence lifespan, but the same comparison often cannot be made between organisms with different body plans: a relatively stress-resistant rodent is not necessarily longer-lived than a relatively stress-sensitive primate, even when confounding variables like body size are taken into account.

So perhaps the answer to my question is that my question is poorly motivated, and that body plan is king in the determination of lifespan. In this picture, stress resistance and other features can modulate, but not dramatically alter, the underlying or essential longevity program of the clade in question.