To examine his first hypothesis — the idea that eschscholtzii and klauberi have trouble recognizing each other as potential mates — Tom performed an experiment. He brought wild salamanders into his field lab and set up the equivalent of a salamander love nest: a damp, dark aquarium with places to hide — a set of conditions designed to encourage the animals to mate. He tested all possible combinations of animals (eschscholtzii couples, klauberi couples, eschscholtzii females with klauberi males, and klauberi females with eschscholtzii males) to try to figure out if the separate subspecies were willing to mate with one another — or more accurately, if the females had any qualms about the males.

In salamanders, the female gets the final say about whether or not a potential mate meets her standards, and the male does his best to convince her that he’s worthy. In Ensatina, this process is elaborate and lengthy. An amorous male will approach the object of his affection and nudge her neck and head with his snout. If she’s interested, she will let him slide underneath her until she straddles his tail. Then the two slowly walk together — often for hours! — until the male deposits a spermatophore (or sperm packet) on the ground. A willing female will walk over the sperm packet and take it up into her body to fertilize her eggs.

Unfortunately, Tom’s love nest may not have been quite romantic enough. Only a few of the salamanders were willing to mate — even with another salamander of their own subspecies. Nevertheless, his preliminary results are intriguing. Klauberi females weren’t picky at all; they mated with males of their own subspecies and eschscholtzii males. Eschscholtzii females, on the other hand, seemed to be choosier; they rejected klauberi males. To be sure, Tom needs to work on his matchmaking skills and convince more salamanders to mate in the lab. Nevertheless, his initial results suggest that eschscholtzii, at least, has evolved such that the females no longer recognize klauberi as potential mates.

These results are also supported by genetic data. Nearly all the wild hybrids that Tom has found so far have mitochondrial genes suggesting that they are the offspring of a klauberi female and an eschscholtzii male.