Exploring Life in the Fast Lane

Shrews are voracious, energetic little mammals. So much so that they have even been cast as evil creatures in the 1959 cult classic “The Killer Shrews.” Hollywood depictions aside, shrews do have among the highest basal metabolic rates (BMR) and appear to forage constantly to sustain their high energetic demands. However, there is much variation in activity and energetics even among shrew species.

Shrews in the genus
Sorex (Figure 1) have extraordinarily high BMRs for their size, compared to shrews in the genus Crocidura. Members of the two genera occur sympatrically in some regions, but they differ in life history characteristics. For example, the BMR of Crocidurine shrews is typical for a mammal of their small size, shrews of the genus Sorex have extraordinarily high BMRs. Shorex shrews must eat every few hours to avoid starvation. Not surprisingly, Sorex shrews have short lifespans of just over 1 year. In contrast, Crocidurine shrews live up to 3 years in the wild, have smaller litters, and exhibit a distinctly slower life history strategy.

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Figure 1. A common shrew (Sorex araneus). (From Rudmer Zwerver, 123rf.com)

Could differences in exploratory behavior among shrew species also be related to life history strategies? Two researchers from the Max Planck Institute recently tested the hypothesis that fast-lived animals (high
BMR, short life span) are faster explorers of their environment than slow-lived species. To do so, they captured live shrews of three species and maintained them in captivity. Once acclimated, shrews were individually filmed in an arena as they explored their new environment (Figure 2). Filming was done with infrared cameras to avoid altering shrew behavior.

arena
Figure 2. (a) A drawing of the experimental arena with a shelter (striped), four food boxes (black), bricks separating the arena into five zones (red). Overlaid in green is the track line of one of the experimental Crocidura shrews. (b) Example traces for all three species. (From von Merten and Siemers, 2012)

The fast-lived
Sorex species began exploration quicker and moved at a faster pace than their slower-living counterparts in the genus Crocidura (Figure 3). Sorex shrews also performed more, but shorter exploration bouts than Crocidura.

shrewbouts
Figure 3. Graphs of the results for (a) latency time to find the first food box; (b) number of exploration bouts over the entire 4 h period; (c) mean duration of single bouts; and (d) mean proportion of area covered during single bouts. (From von Merten and Siemers, 2012)

These results suggest that exploratory behavior is related to life history. The authors suggest that the two genera “faced different types of habitat and thus different selection pressures” during their evolution.

References

von Merten, S., Siemers, B. M., Exploratory behaviour in shrews: fast-lived
Sorex versus slow-lived Crocidura, Animal Behaviour (2012), doi:10.1016/j.anbehav.2012.04.002