Getting girls in the animal world is a serious business. Males need weapons such as antlers and canines to fight off competitors and secure mating rights. Because contests among males can be hazardous, it is essential that aggression be limited to occasions where the rewards of success outweigh these dangers. How then do males living in social groups know when it is appropriate to engage in this risky behaviour?
This is the question studied by an international group of scientists led by Roger Hanlon of the Marine Biological Laboratory at Woods Hole. Hanlon and his team noticed that when spawning males of the longfin squid Loligo paeleii came into contact with fertilized eggs they went berserk, squid style. Within moments of contact, males begin attacking one another with raised arms, beating their fins, initiating chases and grappling. The immediacy of the response implied the existence of a chemical signal, applied by females to the eggs, that triggered the males' outbursts.
Using an impressive range of purification methods, the authors isolated a single protein, a beta-microseminoprotein (loligo β-MSP), that females embed within eggs as they are being deposited in long strands – known as egg mops – glued to the near-shore ocean bottom. In nature, males approach egg mops using visual cues and then, after touching them with their chemosensitive legs, become agitated. Most impressively, the authors found that even in the lab where most natural cues (including females) were removed, exposure to recombinant loligo β-MSP smeared on the outside of an Ehrlenmeyer flask induced aggression.
But why does this happen? What is there to gain? The authors propose that loligo β-MSP is a reliable cue that males use to time their aggressive encounters to appropriate moments when females are present to notice. Males and females would both gain were this explanation true: males by outcompeting their rivals and therefore increasing access to females, and females by ensuring that they mate with the most robust males. A more one-sided alternative may also be plausible. By increasing aggressive encounters among males, females would be left alone to complete egg laying using stored sperm and males would be kept from eating eggs (they are eager cannibals as adults); males might also be encouraged to attack egg predators of other species. Here, females gain while males are pawns in their coercive game!
Signals between animals can convey unidirectional or bidirectional messages, with mutual or one-sided interests being served. In this case, it remains unclear whether loligo β-MSP is akin to a chemical come-hither or whether it is more similar to the robot bunny at the dog races. Fortunately, Hanlon and his team can put all this rampant speculation to the test. Is loligo β-MSP species specific or will it also rile sympatric squid? Does male aggression scale with the presence or absence of females? Finally, why does a highly visual predator require a chemical signal to tell it something that it can already see? The longfin squid still has some explaining to do, and this fascinating story is far from over.