Dental calculus evidence of Taï Forest Chimpanzee plant consumption and life history transitions

Abstract

Dental calculus (calcified dental plaque) is a source of multiple types of data on life history. Recent research has targeted the plant microremains preserved in this mineralised deposit as a source of dietary and health information for recent and past populations. However, it is unclear to what extent we can interpret behaviour from microremains. Few studies to date have directly compared the microremain record from dental calculus to dietary records, and none with long-term observation dietary records, thus limiting how we can interpret diet, food acquisition and behaviour. Here we present a high-resolution analysis of calculus microremains from wild chimpanzees (Pan troglodytes verus) of Taï National Park, Côte d'Ivoire. We test microremain assemblages against more than two decades of field behavioural observations to establish the ability of calculus to capture the composition of diet. Our results show that some microremain classes accumulate as long-lived dietary markers. Phytolith abundance in calculus can reflect the proportions of plants in the diet, yet this pattern is not true for starches. We also report microremains can record information about other dietary behaviours, such as the age of weaning and learned food processing techniques like nut-cracking.

Figure 1. Starch and phytolith morphotypes used in the identification model.

Each scale bar represents 10 μm. (a) Aframomum sceptrum seed phytolith, (b) Aframomum excarpum leaf phytolith, (c) Aframomum excarpum seed starch under normal (left) and cross polarized light (right), (d) Laccosperma opacum pith phytolith, (e) Laccosperma secondiflorum seed phytolith, (f) Calpocalyx sp. fruit starch under normal (left) and cross polarized light (right), (g) Cola nitida seed starch under normal (left) and cross polarized light (right), (h) Coula edulis seed starch under normal (left) and cross polarized light (right), (i) Elaeis guineensis fruit phytolith, (j) Elaeis guineensis leaf phytolith, (k) Gilbertiodendron splendidum seed starch under normal (left) and Cross polarized light (right), (l) Eremospatha macrocarpa pith phytolith, (m) Eremospatha macrocarpa pith starch under normal (left) and cross polarized light (right), (n) Napoleona leonensis seed starch under normal (upper right) and cross polarized light (lower left), (o) Panda olesosa seed starch, (p) Piper guineense seed starch under normal (upper right) and cross polarized light (lower left), (q) Sacoglottis gabonensis fruit starch under normal (left) and cross polarized light (right), (r) Sarcophrynium prionogonium fruit phytolith, (s) Sarcophrynium prionogonium fruit starch under normal (left) and cross polarized light (right), (t) Treculia africana seed starch under normal (left) and cross polarized light (right), (u) Xylia evansii seed starch.

Figure 2. Unsilicified microremains, starches (definite and possible) and phytoliths recovered in Taï Chimpanzee dental calculus with chimpanzee age at death (in years) and approximate age of the cessation of weaning highlighted.

(a) Total counts and (b) counts per milligram of calculus. The number of microremains per mg in Ophelia was affected by an unusually small amount of calculus in the sample.

Figure 3. Microremain assemblages recovered in calculus.

(a) Bar chart of the composition of the phytolith assemblage recovered from calculus. (b) Bar chart of the composition of the starch assemblage recovered from calculus. The individuals are ordered by age from youngest to oldest.

Figure 4. Microremain assemblages recovered in calculus.

Microremain counts are normalised by dividing counts by the percent content of dry plant weight of starches and phytoliths among different genera. (a) Phytolith counts compared with feeding records. Outermost ring = proportions of minutes spent consuming each genus averaged across the feeding records of sampled 24 sampled chimpanzees, middle ring = proportions of minutes spent consuming each genus averaged across the feeding records of all 128 chimpanzees, innermost ring = phytolith counts from the sampled 24 chimpanzees. (b) Starch counts compared with feeding records: outermost ring = proportions of minutes spent consuming each genus averaged across the feeding records of sampled 24 chimpanzees, middle ring = proportions of minutes spent consuming each genus averaged across the feeding records of all 128 chimpanzees, innermost ring = starch counts.

Figure 5. Plot of Mixed Poisson regression model.

The number of phytoliths from a genus increased as the minutes spent consuming this plant resource increased. Darker circles reflect overlapping values.

Figure 6. The occurrence of Coula nut starches with chimpanzee age at death (in years).

Coula nut consumption requires nut cracking and its presence implies nut cracking and tool use or food sharing. The individuals are ordered by age from youngest to oldest.