Shelling Out On Defense

Defense budgets are always controversial, but usually get voted through if there is a perceived threat. The military/industrial complex can consume as much resource as we can throw at it. Guns before butter can be ruinous.

“Perceived” is the problem and we humans seem very good at using our big brains to imagine all sorts of things – and lots of money can be made.  The question is how do other species respond to perceived threats? The ultimate threat is that something bigger is going to come and eat you, so what is the cost/benefit of avoiding being predated upon?

This issue interested Brönmark et al from Lund U who chose snails as a sensible phlegmatic species who have evolved defense strategies (1). They wanted to see whether the snails grew and reproduced as effectively if there were predators about, or did they put more resources into defense.

They set up eight large tanks with five smaller snail habitats in each containing hiding places. Half of the tanks were stocked with tench. Tench like to eat snail, so snails in tench tanks had to cope with a significant threat, which was totally absent in the controls.

The threatened snails came out to feed less and also grew thicker shells in the hope that heavy armor would offer some protection. As a consequence they grew less, showing that there was a significant cost coming along with their defense strategy.  Being busy defending themselves, they produced fewer offspring. The experimenters do not appear to have removed the predators to see if a baby-boom resulted.

With the tanks that were more heavily stocked with snails at the beginning of the experiment, less fish induced defense effects were observed. Clearly if you are in a large crowd (do snails have herds?) the perceived threat is less. Just like people.


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