Shrews are famously active predators, constantly on the move and searching for new foraging opportunities. Along with scurrying, they spend a great deal of time `twittering' – producing brief, high-pitched squeaks. And, for some reason, these sounds become more frequent as shrews investigate novel surroundings. What are these twitters used for?
One theory is that they may be used for `staying connected in real time'like online human tweeters. Shrews may twitter to determine, connect with or indeed avoid other shrews in the environment. If this is the case, then one might expect more excitement – and twittering – if they had reason to believe that other shrews were at large. In order to test this, Björn Siemers, Grit Schauermann, Hendrik Turni and Sophie von Marten caught common and white-toothed shrews and recorded each animal's squeaks as they wandered around cages, either with or without their own species' scents. As signs of a nearby neighbour did not elicit a significant change in squeak rate, the team concluded that shrews were not using twitters to get in touch with residents.
By contrast, the insectivores appeared to use their squeaks for some form of echolocation. The rate of squeaks related strongly to the flooring in the cages: the squeak rate was low in cages with only a thin layer of hay and dramatically higher in thick hay. This echoes the phenomenon in bats, where call rates are relatively high in cluttered environments; the shrew results may suggest that echolocation allows them to explore their environment or identify routes. But how effective are shrew squeaks for echolocation?
In order to approach that question, the team made an `artificial shrew',consisting of a small shrew squeak emitting speaker and a very sensitive microphone to record the squeaks' reflections. And then this pseudo shrew was squeaked in a range of shrew-friendly habitats. The echoes returned were of very different intensities, with moss returning very little sound and leaf litter a great deal. So, echoes from shrew squeaks might carry some information concerning general habitat type.
A couple of issues make bat-like use of echolocation to search for prey very unlikely. One is termed `forward masking', which is the closest that an object can be and produce a clear reflection without interference from the outgoing squeak. In the case of the shrews, that distance is 1.7 m for a 10 ms squeak. Another is `backward masking', where the reflections from a prey item's surroundings make picking out a target tricky – the leaf on which the prey is sitting sends back a very similar signal to the prey.
While the properties of the shrew twitter share some of the characteristics of the echolocation squeaks of bats, it appears likely that shrews are unable to `disentangle echo scenes' and hunt in the same way that bats do.
However, the echolocation abilities of shrews may not be limited to the identification of habitat types; further experiments with the artificial shrew suggested useable information may be available from echoes returning from relatively large and sturdy objects, such as a brick, even when hidden by`moss' or `meadow'. While not as impressive as pinpointing a flying moth, this capacity could be most useful. But whether such a signal is ever actually used to determine escape routes or find safe places to hide will take further studies.
