Picks from the Past: June 2000

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Gestation lasts about thirty days in the star-nosed mole. Left to right: Early in development, the embryo shows no indication of the appendages that will later form its star. The first signs are slight swellings on the side of the snout. These soon become more pronounced.

Kenneth C. Catania

Left to right: At birth, all appendages are still attached to the nose. After a few days, the first of the upper appendages detach and swing forward. By the time the young mole is a week or two old, all the appendages have broken free and the star is ready to go to work.

Kenneth C. Catania

The developmental sequence of the star differs from the way other animal appendages are known to form. In his book The Blind Watchmaker, biologist Richard Dawkins gives examples of unusual developmental sequences in other animals, such as the sole, a bony flatfish that spends most of its life lying on one side and has both its eyes on the upward-facing side of its head. As a young sole develops, it essentially “pulls” one eye across its face, grotesquely distorting its The mole uses the 25,000 minute touch receptors on its exquisitely sensitive star to find its way around its subterranean world. skull and facial musculature. This awkward process makes sense only when we appreciate that flatfish were not designed carefully and then created, but rather evolved from upright fish that had symmetrical bodies. As Dawkins points out, the ancestors of flatfish must have begun their evolutionary journey by lying on their side on the ocean floor—a position that would have resulted in one eye facing the bottom. In each subsequent generation, the most successful offspring would have been those in which the eye shifted slightly closer to the other side of the face. Today we can read the course of this evolution in the development of a flatfish.

What clues does the unusual development of C. cristata’s star provide about the evolution of this animal? Perhaps the ancestors of the star-nosed mole had strips of Eimer’s organs along the side of the face, and perhaps, in the course of evolutionary time, these strips slowly elevated and eventually “peeled” forward to form separate appendages. If so, the stages of this evolutionary sequence may have been conserved in the present sequence of embryonic development. While this explanation of the scenario seems reasonable, we can’t be certain of it without further evidence. Hoping that an analysis of living species could provide insights into the star-nosed mole’s past, we began to examine other moles from around the world.

Our studies showed that nearly all of the roughly thirty species of mole have some Eimer’s organs on the tip of the snout (usually 1,000 to 2,000, distributed evenly around the nostrils) but no indication of starlike appendages. Three species in the genus Scapanus from western North America, however, seemed to provide what we were searching for. The coast mole (S. orarius), for example, has—in addition to Eimer’s organs around its nostrils—small, raised modules of these sensory receptors surrounding the center of the nose. Moreover, the adult snout of the coast mole bears a strong resemblance to the early embryonic snout of the star-nosed mole, with “proto-appendages” pointing backward and adhering to the sides. This arrangement is just what we would predict for an ancestor of the star-nosed mole.

The coast mole, of course, is not ancestral to the star-nosed mole; in fact, the two are not even especially closely related. But the existence of the coast mole’s proto-appendages supports the proposition that the ancestors of the star-nosed mole had similar structures. Embryonic development in the coast mole stops at the proto-appendage stage but continues in the star-nosed mole, leading to the unfolding of separate appendages that form the marvel of sensitivity we see today.