A story about squid has been making the rounds in news sources and blogs this weekend. Just two days ago, a paper came out showing that male squid (loligo peleii) react with extreme agression to a certain protein found on the surface of squid eggs. The paper was written by a group of researchers (including the illustrious cephalopod biologists Jean Boal and Roger Hanlon) from all over the world.
The authors of the study noted that, when this species of squid get together in big groups, the males interact with the eggs that the female attach to the sea floor in a curious way – they approach eggs that are already attached to the sea floor, blow water on them, and then touch the eggs with their head and arms. This occurs before and in-between fights that the males have with each other, in which they compete over the opportunity to mate with females.
To confirm that some component of the eggs was causing the squid’s aggression, the experimenters put pairs of male squids into tanks with eggs. Sure enough, touching the eggs made the squids more aggressive.
The authors then made an extract of the squid eggs, and used a technique called high-performance liquid chromatography to separate their extract into different parts. HPLC works by forcing a solution through a tube (called an HPLC column) that the various substances in the solution stick to in some way. Depending on the specific conditions, different substances will get stuck in the column for longer or shorter times. Substances that don’t stick to the column very well come out the other side quickly, and substances that stick to the column very well will come out more slowly. The solution coming out of the column at a certain time will contain only some of the substances that were in the original solution, and so it is a useful way to separate a complex mixture up into parts, called fractions.
After the egg extract was divided up into fractions, the various fractions were tested to see if they made males aggressive. After finding which fraction was responsible for this effect, the authors purified it further and identified a specific protein within it and figured out its amino acid sequence.
The culpit, it turns out, is a small protein that is related to a family of proteins called the Beta-microseminoproteins, which have been found in species from humans to scallops to ostriches. The female squid’s reproductive tract secretes this protein as part of the coating of the eggs she lays.
In humans, this group of proteins is found in the male reproductive tract, where it attaches to the outside of sperm cells and may be related to a man’s level of fertility. Nobody has asked whether it also makes males of mammalian species more aggressive, although I suppose it is only a matter of time now.
How could the behavior we’re looking at (that is, the fact that males of the species L. pealeii just flip out when they sense this protein) be good for the squid? Behaviors have to evolve, and so they need to help the individual to reproduce in some way.
It’s easy to see how being aggressive and competitive would help a male mate. The real question is: why use a pheronome? And why on the eggs? In a squid mating frenzy, there is ample stimulation – why don’t male squids simply become intensely aggressive at the site of a large group of other squid? That would seem to involve less energy (and less evolutionary backflips) than first seeing the mating frenzy, then seeing the eggs on the seafloor, then approaching and touching them, and only then being spurred into super-aggression.
For starters, it’s good to remind ourselves that none of the components of that behavioral sequence, odd as it is, evolved specifically for that purpose. Because the type of protein that is used as a pheremone in this case is found in many animals, it very clearly has been around for a long time before squid evolved specific mating habits, filling other necessary (and currently unknown) functions. Similarly, squid were probably spawning in groups and competing for mates before they developed the ability to respond to this protein with aggression.
Of course, this could be wrong – if it turns out that beta-microseminoproteins is used as a pheremone in lots of different species, then it may have evolved this function very long ago. Only more research will tell.
Functionally speaking, though, why would a pheremone evolve as a cue for aggression, even when there are plenty of other cues in the environment for the male squid to respond to (the sight of eggs and other squids being the easiest to think of)? My thought is that, in a system that can change as quickly and as unpredictably as an ecosystem, it always helps to have reduntancy. Having two seperate senses that both cause the right behavior in the right context is better than having just one, because either one might fail at an inopportune time and leave the animal (figuratively) screwed.
The other idea that I came up with is that it might help the squids recognize when a spawning group is made up of the right kind of squid – squids of their own species or sub-species. There are many types of squid that look alike to me, and while I’m sure that squid are better at telling the difference between their own kind than I am, it seems like a relatively easy mistake to make in poor conditions (dark or murky water, or the added confusion of predators being present.) Any squid who couldn’t clearly tell when the squids available for mating were squid they could actually produce offspring with would be at a huge disadvantage in terms of making babies. Again, this question could only be answered with more research.
Thanks for reading!
Scott F. Cummins, Jean G. Boal, Kendra C. Buresch,, Chitraporn Kuanpradit, Prasert Sobhon,, Johanna B. Holm, Bernard M. Degnan, Gregg T. Nagle,, & and Roger T. Hanlon (2011). Extreme Aggression in Male Squid Induced by a b-MSP-like Pheromone Current Biology : 10.1016/j.cub.2011.01.038
Anahí Franchi N, Avendaño C, Molina RI, Tissera AD, Maldonado CA, Oehninger S, & Coronel CE (2008). beta-Microseminoprotein in human spermatozoa and its potential role in male fertility. Reproduction (Cambridge, England), 136 (2), 157-66 PMID: 18469041