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June 25, 2010
Security Trade-Offs in Crayfish
The experiments offered the crayfish stark decisions -- a choice between finding their next meal and becoming a meal for an apparent predator. In deciding on a course of action, they carefully weighed the risk of attack against the expected reward, Herberholz says.
Using a non-invasive method that allowed the crustaceans to freely move, the researchers offered juvenile Louisiana Red Swamp crayfish a simultaneous threat and reward: ahead lay the scent of food, but also the apparent approach of a predator.
In some cases, the "predator" (actually a shadow) appeared to be moving swiftly, in others slowly. To up the ante, the researchers also varied the intensity of the odor of food.
How would the animals react? Did the risk of being eaten outweigh their desire to feed? Should they "freeze" -- in effect, play dead, hoping the predator would pass by, while the crayfish remained close to its meal -- or move away from both the predator and food?
To make a quick escape, the crayfish flip their tails and swim backwards, an action preceded by a strong, measurable electric neural impulse. The specially designed tanks could non-invasively pick up and record these electrical signals. This allowed the researchers to identify the activation patterns of specific neurons during the decision-making process.
Although tail-flipping is a very effective escape strategy against natural predators, it adds critical distance between a foraging animal and its next meal.
The crayfish took decisive action in a matter of milliseconds. When faced with very fast shadows, they were significantly more likely to freeze than tail-flip away.
The researchers conclude that there is little incentive for retreat when the predator appears to be moving too rapidly for escape, and the crayfish would lose its own opportunity to eat. This was also true when the food odor was the strongest, raising the benefit of staying close to the expected reward. A strong predator stimulus, however, was able to override an attractive food signal, and crayfish decided to flip away under these conditions.
It's not that this surprises anyone, it's that researchers can now try and figure out the exact brain processes that enable the crayfish to make these decisions.
Posted on June 25, 2010 at 6:53 AM
• 15 Comments
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The distance to food/speed of predator tradeoff is a hogwash... there's a much simpler and realistic explanation for the difference in reaction between fast moving and slow moving predator stimuli:
- if predator is slow making escape movement has a good chance of taking the crayfish out of danger, even if the predator has noticed the crayfish.
- if predator is fast, trying to escape is futile, but there's a chance that the predator may fail to notice the frozen-up crayfish.
It's just the good old playing possum strategy of defence from superior adversary, not the distance from food consideration.
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