Mild-mannered male squid turn into furious fighting machines when their tentacles brush a chemical on the surface of squid eggs, a finding that could give insights into how aggression works.
The discovery also reveals how male squid compete for female mating partners.
"I think that this is really a novel and kind of wonderful addition to our understanding of how aggression might work," said Russell Fernald of Stanford University, who was not a part of the study.
Researchers diving on squid spawning grounds had noticed that not just female squid would hang around the many mop-like clusters of egg capsules.
"We noticed underwater that the males were attracted to the eggs, too. This made no sense. All the eggs are fertilized already. This caught our attention," lead author Roger Hanlon of the Marine Biological Laboratory in Woods Hole, Mass. told Discovery News.
A key to solving the puzzle came during a Cape Cod dive. "There was absolutely nothing that was happening, it was Deadsville." Hanlon said.
So, he fetched some squid eggs from the boat and brought them back to the dive site. "I just put the squid eggs on the bottom. Within a minute or two, one bold male squid went down and touched them. He came back up to the squid above him and started fighting with other squid."
Other males started touching the eggs. "Within minutes, hundreds of male squid were fighting over females," he said. "That was a momentous observation. We realized that the eggs kicked off the entire sexual selection process."
Since then, the team has been working to determine exactly what it is about the eggs that makes the males go berserk.
In a paper published today in Current Biology, the researchers report the answer. It is a single molecule: a pheromone protein incorporated into the outer layer of egg capsules by glands in the female reproductive tract.
The researchers isolated the compound from the eggs, showing that when the molecule was wiped on the outside of a glass jar with squid eggs inside (a visual cue so male squid would come over and take a look), just touching the outside of the jar was enough to trigger the male squid's aggression. The behavior escalates from raising their arms at each other to bumping their fins against each other to grappling and trying to bite each other.
"It's pretty stunning," Hanlon noted. "They're just going from calm to the highest level of aggression they have."
The team also used genetic techniques to produce the molecule in another type of cell and showed that version had the same effect. This was further proof that they had found the molecular culprit.
But why should a molecule found on fertilized eggs release such fury?
"Our explanation is that this is a signal to the males that there are receptive, fertile females in the immediate vicinity," Hanlon said, though he noted that they can't prove that, since the squid aren't talking.
Females mate with many males in the breeding process and have some choice over which sperm they use to fertilize their eggs. "Females likely benefit by obtaining sperm from the most vigorous competitors," the authors wrote.
The findings may have implications beyond squid biology.
"I think this is the kind of study that makes us rethink what is aggression and what are its causations and mechanisms of action," Hanlon said. "I think there might be some simpler, more direct pathways than we envision. You've got this chemical just going, 'whap!'"
The molecule shares some similarity to molecules found in human seminal fluid, and Fernald noted that in some cases, molecules that act as pheromones in some organisms -- carrying signals from one individual to another -- act as hormones in others, carrying internal signals within an individual.
"Now it will be interesting to see if it is used by animals in another context once we know more about it," Fernald said. "We may find something comparable lurking in the human genome or in other vertebrates."