A Star Is Born

The fleshy pink "fingers" on the snout of the star-nosed mole point to this animal's unique evolutionary history.

star-nosed mole nose

The raised patches of Eimer’s organs on the nose of an adult coast mole, left, resemble the swellings of the snout of a star-nosed mole embryo, right. This suggests that the star-nosed mole evolved from an ancestor whose snout looked like that of the coast mole.

Kenneth C. Catania
The touch centers of the star-nosed mole’s brain are organized similarly, with much more space in the cerebral cortex assigned to dealing with input from the central appendages of the star than from each of the less important peripheral appendages. Handling information this way conserves neural tissue, because it concentrates most of the brain’s computing power on only a small part of the sensory world at any given moment. Some scanning time might be saved if large areas of the brain received high-resolution data from the entire star (or from your entire visual field), but to do this, the brain would have to be gigantic. In addition to learning how this mole’s remarkable star works, we have been trying to determine how it evolved. For clues to the history of bony structures, one can turn to the fossil record, but there are no bones in the star and no fossilized mole noses. So we have turned to another, less direct place to look for evolutionary clues: embryonic development.

Most animal appendages (including antennae, wings, legs, fins, and arms) start out as simple extensions of the body wall—essentially as direct outgrowths of the embryonic tissue. Moreover, similar genes are expressed during the early development of appendages in animals as different as humans, fish, birds, and insects. This suggests that a basic “program” for appendage outgrowth evolved hundreds of millions of years ago and has been redeployed many times in the course of evolution.

But what about the star-nosed mole’s novel snout appendages? While we do not yet know the genes involved, we have been able to document the mechanics of the star’s development. As it turns out, the star’s appendages develop unlike those in any other animal, suggesting that it had unique precursors and an entirely independent evolutionary history.

Working in collaboration with Kaas and Glenn Northcutt, of the University of California, San Diego, I examined star-nosed mole embryos at various stages of development. We quickly found that all but the very earliest embryos have a protostar (as well as huge embryonic forelimbs destined to become the digging arms of the adult mole), but that instead of forming as outgrowths of the embryonic nose, the star’s twenty-two appendages first appear as slight, elongated swellings on the embryonic face. In later stages, when the swellings are more pronounced, it almost looks as if the star has been folded back against the side of the face. This impression is not quite accurate but does foreshadow events to come.

During most of the mole’s embryonic development, nothing separates the swellings from the side of its face. But just before birth, a new layer of epidermis grows underneath the swellings. At this point, the appendages become separate cylinders, though they are still attached to the face by this new skin.

Shortly after birth, the back end of each cylinder detaches from the face and swings forward, remaining attached only at its front end (a bit like peeling a banana). What was once the hindmost part of each cylinder thus becomes the forward-facing tip.

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