Southern dumpling squids (Euprymna tasmanica) are natives of shallow, sandy Australian sea beds, and their small, squat bodies and huge eyes combine to make them one of the most frickin-adorable animals on the planet. They spend their days hiding under the sand, but at night they're tenacious lovers.
Mature adults don't spend much time on foreplay -- males simply grab females and hang on. Once a male has his arms wrapped around his intended, he blasts her mantle with two or three strong squirts of seawater, moves her into position, and slips his mating arm into her mantle. Within the first five minutes of the initial grab he's finished pushing sperm bundles into the pockets of her sperm storage organ, but he'll keep hanging onto her for up to three hours more.
Given that these guys only live about 5 to 8 months, those three hours represent a significant portion of their lives. These squid are promiscuous: both males and females have many mates, so that's three hours not spent looking for other partners. And males don't even give up their position in potentially life-threatening situations, completely ignoring fish that might want to eat them while they're in flagrante delicto with a female.
A new study from Zoe Squires and colleagues at the University of Melbourne, published in this month's Biological Journal of the Linnean Society, suggests one reason that males might be so single-minded about their grip: It seems female dumpling squid use the last sperm they receive first.
As dumpling squid copulate, sperm bundles embed themselves into the wall of the female's sperm storage organ, the bursa copulatrix, and burst open, releasing sperm cells into its folds. Females can store sperm for months, and sperm from her many mates wind up layered inside her bursa, with the sperm from her most recent lover closest to the opening where she squirts sperm onto her eggs. By using genetic fingerprinting techniques, Squires found that this means the squid's last mate can father up to 75% of her offspring, especially early in a laying period when she has a lot of eggs stored up. That may make hanging on to her at all costs a good strategy for a male, since it keeps other males from bumping his sperm toward the back of the line.
But it also means that even though it looks like females have little choice about who they have sex with, they still have some control over which males become fathers to their babies. If they mate with a male they like, they can lay eggs right away and make him the dad more than half the time. And if they wind up mating with some loser squid, they can look for someone better who'll put his sperm at the front of the line. [Squires et al. 2013, Franklin et al. 2014, Squires et al. 2015]