The hunting leech
If you're an urban dweller like me, you have probably never seen a leech up close and personal. If you're a dedicated movie fan, you have probably seen them often in old adventure movies, almost invariably latched on to the back of a screaming damsel in distress or a harried hero as he or she emerges from a dismal swamp in some far off country. Leeches, unfortunately, are not well treated by the film industry. It's a real shame, because they are far more interesting than that.
For instance, did you know that leeches hunt down their dinner in very ingenious ways? A leech's diet is very narrow, in a certain sense. They feed on blood, which is why they are frequently used for medicinal purposes. Young leeches always go for fish and amphibians and then, as they grow up, switch to mammals. If you're a fish or an animal, and you go past a leech, it will see your shadow and know exactly where you are. Even if you try not to cast a shadow, the leech will notice you by sensing the disturbances in the water as you move. Either way, you will be dinner. Leeches' hunting methods are quite effective.
Montage of a leech using shadows from waves passing overhead to find its prey; Credit: California Institute of Technology
As it turns out, according to recent findings from a team of Caltech researchers, leeches' hunting methods are also quite thoughtful. Apparently, when young leeches see shadows and sense movement at the same time, they tend to pay attention to both - and I imagine they get somewhat scrambled in the process. Adult leeches, however, do the reasonable thing and concentrate on the disturbances in the water, which are more reliable than the shadows.
According to the scientists, the more mature strategy of adult leeches has nothing to do with physiological changes. Their bodies are just the same as when they were young - at least for the most part. What happens is that adult leeches are more capable to evaluate their different hunting methods and choose the one best suited to their current circumstances. In other words, leeches watch, practice, and learn as they grow up. Good news for the leeches, but maybe not so good for us. More and more, natural sciences are demonstrating that our "superior" intelligence is not unique to humans at all.
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