The following article by Diane Cowan appeared in the July 1990 issue of 'The Lobster Newsletter'. It is reproduced here with the author's permission.
Serial Monogamy in Homarus americanus
Although mating in American lobsters has never been seen in the field, it is easily observed in large aquaria. Here, I will summarize the results of skewed sex ratios on courtship and mating behavior. In my experiments, the sex ratio was skewed in three ways: toward females (2M:5F); toward males (4M:2F); and females in the absence of males (0M:5F) in 5,600 liter naturalistic aquaria. Varying the sex ratio has a dramatic effect on the timing of female molts We recently have reported these results in the literature (Cowan and Atema 1990), but thought they would be'of interest to readers of The Lobster Newsletter.
Typically, when a mature female lobster is about to molt, she seeks a mate and moves into his shelter. A few to several days later, she molts inside this shelter. It takes about 1/2 hour for her shell to harden enough to permit her to stand. The male then approaches, and helps her to roll over. The pair fan their pleopods against one another just before the male intromits. Copulation lasts a few seconds, after which the female tail flips out from under the male. The female remains in the male's shelter for one to several more days, then leaves, never to return.
When two mature male and five mature female lobsters were communally held in large aquaria, the females staggered the timing of their molts at precise intervals throughout the summer to mate with the dominant male. This molt staggering resulted in a mating system of serial monogamy. In this situation, the males lived in shelters at opposite ends of the tank. The females lived in intervening shelters or in cinderblocks at the rear of the aquarium. Each night the dominant male ('M"), made excursions through the tank, without hesitation evicting whoever happened to be in the shelters. When 'M" returned to his own shelter, the other lobsters investigated his shelter entrance.
About a week after the beginning of the experiment, a female moved in with 'M" molted and mated with him in his shelter, remained a few more days, and then moved ouL Another female then moved in, molted, mated, stayed a few more days, moved out, then another female moved in, and so on, until all of the females had mated within the shelter of the dominant male. The order in which the females molted was not related to dominance or to the amount of shelter checking. The subordinate male usually remained alone, but he did sneak at least one copulation while the donminant male was out of his shelter.
This unusual mating pattern was seen in four experiments when the sex ratio was skewed toward females. Molting is an important and inevitable physiological event under hormonal control. What's particularly intriguing about the lobster mating system is that the female molts are timed in such a way that each female in turn mates with the donminant male. How can female lobsters stagger the timing of molting?
Skewing the sex ratio toward males resulted in utter chaos. Initially, Sara Ellis and I introduced 4 males and 2 females into each tank, but the aggression was so intense that we removed one of the males after two days. There was no consistent male dominance and not one male ended the experiment with all appendages intact. One male was crawling around on his maxillipeds at the end of the season. Another male attempted intermolt mating by dragging one female into his shelter, mounting, and trying to turn her over, but each time she escaped. The females in these two experiments delayed molting.. Two out of four molted, but apparently couldn't decide which male to pair with and moved in and out of up to three male shelters each day (for over 10 days) before molting.
When five females were housed together in the absence of males, most simply put off molting. In four replicates of 5 females each, only four out of 20 females molted. All of the females who molted sustained serious injuries (loss of claws and/or walking legs). This suggests that the male protects the female from injury and predation while her shell is soft during postmolt cohabitation.
In summary, when a dominant male is in the tank, females molt; when a male is not present, they generally don't molt. Perhaps female lobsters have some sort of control over their molting and therefore, reproduction. In mammalian social systems such controI over reproductive events is known to be mediated by primer pheromones. For example, olfactory cues synchronize menstrual cycles of women in college dormitories. In mice, exposure to male urine accelerates the onset of puberty while exposure to female urine causes delays. Since lobsters can discriminate sex based on odors and have a sensitive olfactory system it seems possible that chemical communication may similarly influence the timing of molting in female lobsters.
While the female is checking at the dominant male shelter, odors are exchanged via three currents generated by the lobster. First, by fanning his pleopods, the male creates a strong current, drawing water through the shelter. Second, the female gill excurrent projects into the male shelter. And thirdly, the female, by fanning the expodites of the third maxillipeds, reverses the direction of the gill excurrent. Urine is released directly into the gill excurrent. Lobsters, like dogs and other mammals, may have informative urine, in lobsters it may contain metabolites of molting hormones, and indicators of sex, age, maturity, and perhaps dominance and social rank.
Currently, I am testing the hypotheses that male odors accelerate the timng of female molting, that female odors delay the timing of female molt, and some combination of male and female odor causes molt staggering such that serial monogamy results. In the situation with two males and five females, if the first female to mate smelled only male odor when she checked the male shelter entrance, perhaps her molt was thereby accelerated. The next female may have smelled both male and female odor every time she checked the shelter entrance, influencing her to wait. Once a female is in the male shelter she probably gets nothing but male odor, and so she molts.
This work enhances our knowledge of mating system ecology, social behavior, and chemical communication in general. It may also have implications for fisheries management and for lobster aquaculture.
Cowan, D. E. and J. Atema 1990. Animal Behaviour 39: 1199-1206.
Boston University Marine Program, Marine Biological Laboratory, Woods Hole, MA 02543, USA
Diane Cowan can now be reached at:
The Lobster Conservancy