Author Spotlight: How soil features are shaping the bite force and skull morphology in subterranean rodents

17 01 2017

The role of soil features in shaping the bite force and related skull and mandible morphology in the subterranean rodents of genus Ctenomys (Hystricognathi: Ctenomyidae)

L.R. BorgesR. MaestriB.B. KubiakD. GalianoR. Fornel4 and T.R.O. Freitas

Tuco-tucos (genus Ctenomys) are subterranean rodents widespread in the southern cone of South America. They are members of the caviomorph lineage (e.g. the guinea pig and their relatives), which arrived in South America ~50 million years ago via transoceanic dispersal from Africa. Today, more than 60 species of Ctenomys are described.

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Ctenomys minutus, a species of tuco-tuco. Photo by Daniel Galiano

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Another species of tuco-tuco, Ctenomys ibicuiensis. Photo by Daniel Galiano

To live underground, the tuco-tucos must be able to excavate the soil. They use both claws and teeth to break up the soil, cut roots, tubers and other plant material, therefore opening their way to live underground. Adaptations to excavate with the incisors are particularly well demonstrated in this group. As soil features change, it is possible that distinct adaptations arise – the more compact the soil, the harder it is for digging. The question we asked ourselves was: does species living in more compact (harder) soils have a stronger bite force than species living in less compact (soft) soils?

In our paper published in Journal of Zoology we tried to answer this question. We used Freeman and Lemen’s bite force index (a useful formula to estimate bite force of rodents when direct measurements are not available) to estimate bite force values for 24 Ctenomys species, and we used a bulk density variable for soil compaction at each species’ distribution. Morphometric geometric techniques were applied on 1,122 specimens of the same species to investigate skull and mandible features correlated with bite force.

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Figure 1 from the article by Borges et al.: Landmarks used for measuring the shape of the skull and the mandible of the tuco-tucos.

We found that the species with strong bite force values do tend to occur in highly compact soils, while species with low bite force values tend to occur in less compacted soils. However, it turns out that species with low bite force values are also found in highly compacted soils. This makes us believe that different species developed different strategies to manage the excavation process. While some species probably rely mostly on their teeth (those species with high bite forces values occupying harder soils), others may rely on other distinct strategies to excavate, using their claws for instance, making these species able to have weaker bite force even though inhabiting highly compacted soils. We also discovered that a wider skull and a robust mandible are associated with the strongest bites, while an elongated skull and mandible are correlated with the weakest bite forces, in what seems to be a recurrent pattern for rodents and other mammal species.

Our next step is to figure out how appendages (limbs, the shoulder blade) contribute to excavation in species of tuco-tucos.

Renan Maestri