Brian Langerhans and Thomas deWitt of the department of Wildlife and Fisheries Sciences at Texas A&M University examined the specificity with which freshwater snails use environmental cues to induce defensive phenotypes such as shrinking. In one environment they introduced a molluscivorous sunfishspecies. In the other a non-molluscivorous, plant-eating, sunfish . Perhaps surprisingly snails in both environments developed predator avoidance behaviour, either by developing more rotund shells that are harder to crack, or by becoming smaller, making it more difficult for the sunfish to hunt them.
Apparently for freshwater snails a reduction in size can also be induced without exposure to real danger. But people are not snails. Who or what is our vegetarian sunfish? Is there a friendly danger signal to help us shrink before the real collapse of the ecosystem?