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Squid near an egg sac.

Male longfin squid swim near an egg sac. Photograph courtesy Roger Hanlon.

Matt Kaplan

for National Geographic News

Published February 11, 2011

More than a few men have fought to the death to win a woman. Now it appears there's a biochemical force driving the duels—in squid, at least.

According to a new study, when male longfin squid touch recently laid eggs, the males rapidly shift from calm swimming to extremely aggressive fighting (see raw video above).

(Related: "Global Warming Making People More Aggressive?")

Researchers first noticed this effect while studying male squid arriving at breeding grounds where females had already started laying egg capsules.

Each egg capsule contains 150 to 200 eggs. A female squid will lay 20 to 30 capsules over a period of up to several weeks, during which time she will mate with multiple males, according to the National Oceanic and Atmospheric Administration.

The scientists noticed that, when male longfin squid saw egg capsules laid on the seafloor, the squid would swim toward the eggs and wrap their arms around them.

"This struck us as a very weird behavior," said study co-author Roger Hanlon, of the Marine Biological Laboratory in Woods Hole, Massachusetts. The team then saw that as soon as the squid contacted the capsules, the males became highly aggressive, violently beating and grappling with each other.

(Related: "Large, 'Glamorous' New Glowing Squid Species Found.")

Keen to take a closer look, Hanlon and colleagues presented 57 pairs of male squid with egg capsules and closely monitored their behavior in the lab.

Upon touching the capsules, the males attacked one another, even in the absence of females, the researchers found.

Squid Discovery to Stimulate Semen Research?

To better understand what was causing the effect, the team chemically analyzed extracts from the surfaces of the capsules. They found a number of proteins and suspected that one of them might be acting as a pheromone that could trigger aggression in squid.

Hanlon and colleagues painted each protein on the outsides of glass containers, placed squid eggs on the inside, and presented the chemical-coated objects to male longfins. Attracted by sight of the eggs, the squid touched the glass containers.

The results showed that contact with a chemical called β-microseminoprotein caused the male squid to become aggressive within seconds.

Hanlon and colleagues think the pheromone helps squid by spurring extreme fights, allowing the fertile females to chose the strongest and fittest males for mating.

(Related: "Mouse Tears Are Aphrodisiacs.")

"We don't know of anything like this that exists in humans," Hanlon added. "But when we researched microseminoproteins in the literature, we found that they occur in mammal semen and, more importantly, that nobody has looked at what functional effect they have.

"We hope that our discovery stimulates research in that direction."

The squid-pheromone study appears in the February 10 issue of the journal Current Biology.

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