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Female squid pheromones trigger fighting among males

A newly-identified pheromone produced by female squid – the first of its kind found in marine creatures – is thought to induce aggression in males.

A newly-discovered pheromone secreted by female squid has been found to cause dramatic duels and fierce posturing among male squid, according to a recent paper in Current Biology. The pheromone, the first of its kind found in marine creatures, is thought to induce aggression in males to increase their chances of mating with females. The chemical make-up of the pheromone bears some similarity to a family of proteins that are found in other animals, including humans.

Aggressive behavior between males is often seen in animals competing for shelter, food, and mates. In a press release, Dr. Roger Hanlon, a senior scientist at the Marine Biological Laboratory (MBL) at Woods Hole, Massachusetts, said,

The identification of this pheromone as a key component of this signaling system is highly unusual because the male squids need only to come into contact with these protein molecules to initiate the complex cascade of behaviors that we term aggressive fighting.

Longfin squid (Loligo pealeii) are highly advanced marine invertebrates that inhabit deep waters off the northeast and mid-Atlantic coastlines of North America. In spring, the squid move to shallower waters to breed. Mating is complex and frenzied, with females mating with multiple males, and fierce competition for females among the males. The females then lay fertilized eggs on the sea floor.

Longfin squid (Loligo pealeii) spawning activity. Image Credit: Roger Hanlon

While studying the squid during the breeding season, Dr. Hanlon and his colleagues observed that male squid were first visually drawn to the eggs, but when they came in physical contact with them, their calm demeanor abruptly switched to extreme aggressiveness. This behavior occurred regardless of whether females were in the vicinity or not.

What caused this dramatic change in behavior? In examining the eggs, the scientists found a protein pheromone embedded in the eggs’ outer surfaces. The pheromone had originated from within the females’ reproductive tracts, and was long-lasting enough to remain on the eggs for an extended period.

In a laboratory tank, the scientists presented extracts of the pheromone to male squid. It triggered the same violent male-on-male aggression they had observed in the wild when the male squid came in contact with the eggs.

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Click on the player to view video of laboratory experiments demonstrating squid response to aggression-producing pheromone. Video Credit: Roger Hanlon, Kendra Buresch, Chelsea Bennice, and Charlie Fry, MBL.

What role does this pheromone play in squid reproduction? Said Hanlon,

Our lab experiments show that the male squid that touches the eggs first becomes aggressive faster than other males who have not yet touched the eggs. This leads to dominance by the males that encounter the pheromone. Dominant males pair with the females and mate more often, and they gain greater fertilization success, so the extremely competitive aggression has a payoff.

He continued,

Squid may have revealed a more direct way of stimulating aggression. We doubt that many researchers have thought that contact with molecules in the external world could stimulate such complex and extreme aggressive behavior.

So, the reproductive success of male squid could depend on them coming in contact with pheromones on the surface of eggs laid by females. Those pheromones bear similarities to a family of proteins found in vertebrates (including humans), and they increase the aggressiveness of the affected males, which increases their chances of mating with more females. Such aggression-causing pheromones have never before been observed in marine animals.

Loligo squids approaching and touching the egg capsules, which harbor the contact pheromone that initiates high-level aggression among males. Image Credit: Roger Hanlon

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