Just a touch of a female pheromone can send male longfin squids into a frenzied rage, potentially giving wimpy squid males a chance to fight for the ladies.

Whether there exists a human analogue to the pheromone, called Loligo beta-microseminoprotein, is a matter of premature speculation. But the findings do reveal a potentially fascinating subject for further research.

“It’s like Popeye’s spinach. When they touch it, they say ‘let’s go’ and start to kick ass,” said biologist Roger Hanlon of the Marine Biological Laboratory, who reported the findings Feb. 10 in Current Biology. “It’s a beautiful, robust response. It may be a mechanism for smaller males who have trouble being dominant to mate with females.”

Pheromones are compounds produced by animals that trigger behaviors and physiological cycles in their brethren, including aggression, alarm, ovulation and even sex. As their mating season peaks in the spring, female longfin squids secrete Loligo beta-microseminoprotein onto their capsules of eggs.

During a 1997 dive to investigate the longfin squid’s mating behavior, Hanlon and others placed one female egg capsule in a school of about 1,000 squids. All of a sudden, the males — which typically show no signs of aggression — went crazy.

“Males are visually attracted to egg capsules. So one bold male squid wiggled his arms in there and immediately started fighting with other males,” Hanlon said. “Another came down and started fighting, then another, then another.” Within five minutes, the entire school had spawned.

Hanlon and his team went on to search for compounds that could trigger the behavior. They brought wild squids into the lab, then isolated compounds secreted by female genitals and in eggs. Males were exposed to each compound until the researchers found one that drove them nuts: Loligo beta-microseminoprotein (video below).

The pheromone resembles a class of poorly understood compounds secreted in reproductive cells and fluids across the animal kingdom, including mammals. However, Hanlon cautioned against thinking it would have an effect on people.

“It’s easy to take a molecule that turns on aggression in squid out of context, though. The NFL and Army shouldn’t be calling for this stuff. It’s so far removed from that,” he said. “There is no evidence at all that it would cause aggression in any vertebrate animal, let alone humans.”

Neurobiologist Edward Kravitz of Harvard University, who studies aggression in flies, called the finding compelling and is curious to see where it leads.

“People think these may be signaling molecules. It’s possible this is a molecule used throughout evolutionary history for a similar purpose,” Kravitz said.

Many other questions remain about the pheromone’s function in squids.

“We don’t know how it gets into the suckers and the blood stream, what receptors it affects, and how it influences the nervous system,” Hanlon said. “This is really just the beginning, and we hope to inspire other folks to start looking closely at how this class of proteins function.”

Images: Longfin squids, also known as Loligo pealei (WHOI/Tom Kleindinst)

Video: Roger T. Hanlon/The Marine Biological Laboratory.

Citation: “Extreme Aggression in Male Squid Induced by a ?-MSP-like Pheromone.” 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. Current Biology, Feb. 22, 2011. DOI: 10.1016/j.cub.2011.01.038

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