Differentiating siliceous particulate matter in the diets of mammalian herbivores

Silica is crucial to terrestrial plant life and geochemical cycling on Earth. It is also implicated in the evolution of mammalian teeth, but there is debate over which type of siliceous particle has exerted the strongest selective pressure on tooth morphology. Debate revolves around the amorphous silica bodies (phytoliths) present in plants and the various forms of siliceous grit-that is, crystalline quartz (sand, soil, dust)-on plant surfaces. The problem is that conventional measures of silica often quantify both particle types simultaneously. Here we describe a protocol that relies on heavy-liquid flotation to separate and quantify siliceous particulate matter in the diets of herbivores. The method is reproducible and well suited to detecting species- or population-level differences in silica ingestion. In addition, we detected meaningful variation within the digestive tracts of cows, an outcome that supports the premise of ruminal fluid 'washing' of siliceous grit. We used bootstrap resampling to estimate the sample sizes needed to compare species, populations or individuals in space and time. We found that a minimum sample of 12 individuals is necessary if the species is a browser or as many as 55 if the species is a grazer, which are more variable. But a sample size of 20 is adequate for detecting statistical differences. We conclude by suggesting that our protocol for differentiating and quantifying silica holds promise for testing competing hypotheses on the evolution of dental traits.

Automated summary

Silica is important for terrestrial plant life and geochemical cycling. It is also relevant to the evolution of mammalian teeth, but there is debate over the type of siliceous particle that has had the strongest selective pressure on tooth morphology. This study presents a protocol for separating and quantifying siliceous matter in the diets of herbivores, which can detect differences in silica ingestion at species or population level. The study also found meaningful variation within the digestive tracts of cows, supporting the hypothesis of ruminal fluid 'washing' of siliceous grit. The protocol shows promise for testing competing hypotheses on dental traits evolution.