Red squirrels fed on peanuts have weaker jaws – here’s why that matters for conservation

Red squirrels have been replaced over a large part of Great Britain by the non-native grey squirrel, introduced from North America in the late 19th century. Today, the British population of red squirrels (approximately 287,000) is mostly restricted to Scotland, northern England and some offshore islands.

This fragmented distribution is bad news for conserving red squirrels, but provides an interesting case study for scientists. Given the different climate and habitats the remaining red squirrels live in, and the duration of their isolation from one another (many decades in some cases), might these squirrels have developed anatomical differences as other separated populations of the same species elsewhere?

In a similar case, Charles Darwin famously noted how the finches of different Galápagos islands had different beak shapes that reflected the things they were eating.

My colleagues and I compared the shape of the skull and lower jaw in red squirrels from four populations – north Scotland, the Scottish-English border region, the Formby red squirrel reserve in Merseyside, and Jersey in the Channel Islands – and found that there were indeed significant differences between the populations.

You are what you eat

Red squirrels from the Formby reserve near Liverpool in north-west England had skulls with flatter braincases and longer faces. These squirrels also had less robust lower jaws, with muscle attachment sites closer to the jaw joint compared with their relatives elsewhere.

These differences all implied that the Formby squirrels had smaller and less efficient jaw muscles, and less forceful bites than red squirrels from other populations.

What’s more, within the Formby population, we found changes in anatomy over a short period of time. Red squirrels from the 1990s had less efficient muscles than Formby squirrels from the 2010s.

These differences could be the result of random genetic mutations, or inbreeding as the red squirrels at Formby have been through population crashes owing to disease. But the impact of the differences in skull and jaw shape on the strength and efficiency of the squirrels’ bite suggests that diet could be an underlying cause.

Red squirrels in the UK eat a variety of food, depending on where they live. In northern Scotland, red squirrels almost exclusively eat pine cone seeds which they extract by gnawing away the cone scales one by one. In northern England’s mixed deciduous and coniferous forests, red squirrels eat a more varied diet of hazelnuts, beechnuts and yew seeds, alongside pine and spruce seeds, and on the Channel Island of Jersey, red squirrels eat acorns, hazelnuts and sweet chestnuts.

At the Formby reserve, red squirrels live among pine trees. However, throughout the 1990s and 2000s they were provided with supplementary food all year round in the form of peanuts in the shell (or monkey nuts). Workers at the reserve cut back on this type of feeding from 2007 onwards and encouraged members of the public to do likewise.

Peanuts are much easier to prise open than hazelnuts or pine cones – we can crack peanut shells with our bare hands, but few of us can break a hazelnut shell without nutcrackers. So, we think that the less mechanically demanding diet at Formby led to a change in the skull and jaw anatomy of the red squirrels. This change was then partially reversed when supplementary peanut feeding was stopped.

Evolution or development?

The question that remains over our research is whether the anatomical changes seen at Formby were a product of evolution over several generations or a developmental response within the lifetime of each individual squirrel. The latter mechanism is known as bone modelling and occurs in all vertebrates such that bone mass decreases with reduced use of the muscles (as seen in astronauts who don’t have to work their muscles against gravity).

The two scenarios are difficult to tease apart, although the rapid change seen in the Formby squirrels following the removal of supplementary peanuts suggests it might be developmental.

Whatever the mechanism, our research may have important implications for the conservation of red squirrels and other species, particularly where animals are being bred in captivity for later release into the wild.

It seems that, based on our results, it would be wise to match any supplementary food to what the animals are most likely to find in the wild in order to give them the best chance of survival.

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Philip Cox has received funding from the Royal Society.