[...] Cowan once spent several months observing the behavior of a male lobster she had named M, which lived with one other male and five females in a tank at the Marine Biological Laboratory in Woods Hole, where Cowan later worked as a graduate student. Every night M would emerge from his shelter, boot all the other lobsters out of their shelters, and then return home. The females got the message: M was dominant. The females visited both of the males at their shelters, but M got far more lady callers than the other male. The visits were decorous at first: an interested female would insert her claws into the entrance of M's shelter and wiggle her chemoreceptor antennules to smell him. Then she'd urinate at him from the front of her head, releasing pheromones. In appreciation M would spread her urine throughout his apartment, by standing on tiptoe and fanning the water with his swimmerets—little fins along the bottom of the lobster's tail, arranged in five pairs.
Having ascertained mutual interest, the two abandoned all caution. M's primary concern seemed to be how soon the female would undress for him, and he would show his impatience by boxing the surfaces of her claws with the tips of his. Females can mate only after they shed their shells; Cowan thinks that M's boxing was a way of testing how hollow his lover's shell was in preparation for molting.
"One day I walked into the lab, and I thought there were three lobsters in M's shelter," Cowan says. It turned out to be not a ménage à trois but, rather, evidence of a conventional coupling. It was M, a female, and her molted shell. When a female that wants to mate is ready to molt, she lets the male know by placing her claws on top of his head, in a behavior scientists have termed "knighting." This apparently indicates to the male that he must protect her while she sheds her shell; scientists think the female may also release a sex pheromone that discourages the male from simply eating her, as he might under other circumstances. Once the female is undressed, the male gingerly lifts her soft body, flips her on her back, inserts a pair of rigid swimmerets into a pair of receptacles at the base of her abdomen, and passes his sperm into her. It's like the missionary position, but with double the genitalia. [...]