In the fields of Northern Europe, a tundra vole (Microtus oeconomus) population explosion could be going on right now. As they hit the reproductive jackpot, their numbers rocket. But with most booms there comes a bust, and the same is true for vole populations, which can collapse suddenly and unexpectedly. Ivan Calandra, from the University of Reims Champagne-Ardenne, France, explains that the cause of these dramatic population crashes was not clear, although there was one candidate that could account for the dramatic fall from grace: the vegetation upon which the voles feast. Plants are not as vulnerable as they may at first appear. ‘It has been known for at least 30 years that plants build up silica bodies (phytoliths) in response to intense grazing,’ says Calandra. But it was not clear whether these plant defences were directly responsible for the vole's demise, and if they were, how? Were the abrasive silica structures that accumulate in plant cells wrecking the rodents' teeth or damaging their intestines, leading to starvation and population collapse?
Karol Zub, Andrzej Zalewski and Paulina Szafrańska, from the Polish Academy of Sciences, decided to feed voles on two diets – one composed of pellets containing high levels of sedge, which is reinforced with silica, and another in which the sedge was replace with lower-silica grasses – to find out how different levels of silica affected the rodents’ teeth. Then Zub and Szafrańska shipped the voles’ teeth to Gildas Merceron, a CNRS Researcher at the University of Poitiers, France, where he and Calandra could look for any damage using one of only four profilometers – confocal microscopes that scan surfaces to produce 3D representations of structures – that can analyse tooth wear.
However, when the duo compared the damage on the surface of teeth that the voles had used to grind their food, they were surprised to see little difference between the patterns of wear in animals on the different diets, possibly because the lab diets were not optimal. However, Calandra's Polish colleagues also knew that voles switch diet from season to season, dining on high-silica sedges in the winter and spring before supplementing the diet with softer, low-silica leafy vegetation in the summer and autumn. In addition, Calandra explains that sedges retaliate as the vole population expands by arming themselves with increased silica levels. Could the team find differences in tooth wear if they investigated the wear patterns in different populations of voles across the years and seasons as the silica levels increased in response to the expanding populations?
This time the team investigated vole teeth from animals that had been trapped in the wild from the late 1970s to 2001, including a period when the population crashed in 2000 after peaking the previous year. And this time, having scanned the surface of 47 teeth, they found evidence of changes in wear pattern as the voles shifted their diet through the seasons and as the sedges became more impregnated with silica.
However, the team warns that this evidence of changes in tooth wear may not account for vole population crashes, saying, ‘Tooth wear rates might seem unimportant in voles since their molars are ever-growing’. They also add that there is no clear mechanism linking the effects of tooth wear to dramatic reductions in vole population, although they suspect that if the teeth are worn unevenly, the animals may have difficulties feeding and face starvation.
