Lobster harvesters know that the number of males vs.females they catch is related to when and where they fish.Although this could have something to do with fishing practices, there is evidence to suggest that sex ratio is related to social behavior.

I stocked four 3,000-gallon aquariums with different ratios of males to females and studied the consequences.Varying the sex ratio had a dramatic effect on courtship and the timing of female molting and mating.

When two male and five female lobsters were held in large aquariums, the females staggered their molts at precise intervals throughout the summer to pair bond and mate in succession with the dominant male.

This staggered molting resulted in a mating system of serial monogamy.

In this situation,the males lived at opposite ends of the tank.The females lived in intervening shelters and in cinder blocks at the rear of the aquarium.

Each night,the dominant male made excursions through the tank, evicting all other lobsters from their shelters.When he returned to his shelter, the other lobsters investigated his shelter entrance.

In each tank,about a week after the beginning of the experiment, a female moved in with the dominant male. A few days later she molted inside his shelter. About a half hour later she approached him and they mated.She remained in his shelter for a couple more days, then left, never to return.

Soon after she left (hours to days) another female moved in, molted, mated, stayed a few more days, moved out, then another female did the same until all of the females had molted and mated with the dominant male.

When a subordinate male molted, he dug a pit behind the dominant male shelter and was unharmed.When his shell hardened, he sometimes sneaked into the dominant male shelter and mated with the female inside.

This unusual mating pattern was seen in four separate aquariums with four groups of lobsters when the sex ratio was skewed toward females.The female molts were timed such that each female mated in turn with the dominant male.

In the next issue we’ll take a look at what happened when there were more males than females in the tank. Any guesses?