Abstract
The circumstances of the evolution of hypsodonty (= high-crowned teeth) are a bone of contention. Hypsodonty is usually linked to diet abrasiveness, either from siliceous phytoliths (monocotyledons) or from grit (dusty environments). However, any empirical quantitative approach testing the relation of ingested silica and hypsodonty is lacking. In this study, faecal silica content was quantified as acid detergent insoluble ash and used as proxy for silica ingested by large African herbivores of different digestive types, feeding strategies and hypsodonty levels. Separate sample sets were used for the dry (n = 15 species) and wet (n = 13 species) season. Average faecal silica contents were 17–46 g kg−1 dry matter (DM) for browsing and 52–163 g kg−1 DM for grazing herbivores. No difference was detected between the wet (97.5 ± 14.4 g kg−1 DM) and dry season (93.5 ± 13.7 g kg−1 DM) faecal silica. In a phylogenetically controlled analysis, a strong positive correlation (dry season r = 0.80, p < 0.0005; wet season r = 0.74, p < 0.005) was found between hypsodonty index and faecal silica levels. While surprisingly our results do not indicate major seasonal changes in silica ingested, the correlation of faecal silica and hypsodonty supports a scenario of a dominant role of abrasive silica in the evolution of high-crowned teeth.
Highlights
Along with the spread of open landscapes and radiation of grasses during the Cenozoic, a striking morphological characteristic of dentitions evolved in different herbivore lineages [1,2,3]: hypsodonty, or high-crowned teeth
There was no overall difference in faecal silica contents between the dry and wet season for all species, and the exclusion of high-browsing and intermediate feeding species resulted in no significant difference
Dietary silica is considered to exhibit negative effects on herbivores [36]; the mechanisms are discussed to work on several levels like diet digestibility [37,38], diet preference [39,40], bite rate [40] and even the development of pathological conditions like urolithiasis [41]; the negative effect of ingested silica is most renowned for a corresponding increase in diet abrasiveness (e.g. [5,8,42])
Summary
Along with the spread of open landscapes and radiation of grasses during the Cenozoic (probably best documented for the Miocene), a striking morphological characteristic of dentitions evolved in different herbivore lineages [1,2,3]: hypsodonty, or high-crowned teeth. The most accepted cause of increased wear is a rise of dietary silica content as a consequence of a higher proportion of grass in diets and/or foraging in open landscapes, respectively. Little data are available from direct comparisons, but the difference between grasses and browse (trees, shrubs, herbs) can generally be considered substantial: for example, in a study on East African vegetation, silica contents have been quantified to be 4.95 per cent dry matter (DM) in grasses compared with only 0.56 – 1.46% DM in browse [11] or in a sample of alpine plants to be 2.66 + 1.60 (grasses) versus 0.20 + 0.23% DM (dicots) [12]. C4 grasses generally seem to have higher values than C3 grasses [13,14]
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