When humans think `fast predator', we typically think of charismatic megafauna (e.g. the cheetah, the shark, the eagle). Often times though, tiny predators are proportionally more impressive. One of the fastest mammalian predators is actually not the cheetah, but the very small and common American water shrew, Sorex palustris. Inhabiting streams across North America, these ferocious predators wake after sunset to go on the hunt, eating virtually any animal that they can overpower. Importantly, a big part of shrew cuisine consists of fish and other things that move really fast underwater. This is particularly impressive given the sorry state of the shrew visual system. Water shrews have terrible eyesight. So how does a virtually blind animal hunt so well in such a dark place? Ken Catania and colleagues recently addressed this question by bringing water shrews into the lab and filming them while they hunted.
In their first set of experiments, Catania and coworkers provided sound evidence that shrews hunt the same regardless of whether it's light or dark. The authors noticed that shrews appeared to respond to water currents generated by fish movements. To study this further, the team put individual animals in chambers ringed with controllable water jets and tested how the aquatic hunters oriented to puffs of water coming from different directions. Indeed, shrews launched predatory lunges with their `mouths agape' in response to small pulses of water. These attacks were not random; the targets were clearly the offending water jets. Understandably, the shrews then interrogated the nozzles using a behavior (underwater sniffing) known to be involved in olfactory investigation. These observations argue that the little rodents cue in on water movements and smells while hunting.
Catania and colleagues next tested the idea that shrews also cue in on the shape of a potential prey item, not just how it moves or smells. To test this,they let hungry shrews loose in a tank with silicone `prey' of various shapes. In amongst all the shapes was a fish replica cast in silicone. The little hunters were not fooled. After a brief search, they clearly attacked the `fish shape' more often than the `non-fish shape.' In addition, when presented with stationary objects of different shapes, shrews showed more interest in objects shaped like common prey items, and they did all of this in the dark. These results imply that shape detection by touch is a key component of underwater hunting behavior in water shrews.
The team found no evidence that shrews use other more sophisticated remote hunting techniques. Tests for electroreception or echolocation abilities came up negative. It appears the animals have evolved a foraging strategy that is based, instead, on detecting form and movement while continuously sampling odors under water.
It is important to note just how stunningly fast these shrews move while foraging. Seeing them in action leaves one breathless. These animals are going from raw sensory input to coordinated motor output in ∼20 ms. This kind of speed is very rare among other vertebrates. We know very little about how the nervous system does this, but one thing is for sure: this animal's nervous system has evolved for speed.
This study highlights the fact that fast, fearsome predators come in all shapes and sizes and many of them live right in our backyards. This work is also notable because it started with a very simple natural history observation(shrews catch fish). Many of the great stories in animal behavior research have started out with just that kind of observation. It is inspiring to see that new work on a unique and beautiful animal can still be started in the same way.