Studying plant remains at the entrance and the exit of a chimpanzee

Archaeologists and geologists adore teeth. A good tooth can give you information on what an animal ate and its likely size, but a great tooth might have dental calculus (tartar). This gunk can trap some elements of the food an animal ate. So a human tooth could yield insights into past diets. But how well do the remains in calculus match what someone, or something, ate? Robert Power and colleagues have tried to find out by correlating the remains of what goes in with the remains of what comes out. Their paper, The representativeness of the dental calculus dietary record: insights from Taï chimpanzee faecal phytoliths, relies on botanical gems for the answer.

The gems in question are opals. While you won’t get rich selling them to jewellers, the stones are precious if you want to study plants in past environments. The stones exist because plants try to defend themselves against herbivory. They do this, in part, by making tiny silica stones. These stones come in many different shapes, depending on the plant that made them. So if you can identify these stones, called phytoliths (phyt = plant, lith = stone), you can identify what plant material was there.

Power and colleagues took faecal samples from a couple of male chimpanzees in Taï National Park in southwest Côte d’Ivoire. They took the chimpanzee poo, preserved it in alcohol and then freeze-dried it. Once they had finished processing it, they could stick it under a microscope at ×400 magnification and looked to see what they could find.

“The types of eudicot foods that are rare in chimpanzee diet are also rare in the samples, such as wood and sedge. Grasses occur in the samples, although grasses are not consumed at Taï…,” write Power and colleagues. “The presence of grasses plausibly could be linked to accidental inhalation of wind-blown grass phytoliths, or via accumulated dust adhering to foods. This explanation is plausible given that grasses produce very high numbers of phytoliths.”

The team did find a lot of palm phytoliths, many more than you’d expect as a proportion of the diet. However, they note that palms do produce a lot of phytoliths. If a disproportionate number of phytoliths go into the chimpanzee, then there should be a similar number coming out.

Another factor is how the chimpanzees eat. “Taï forest fruit and leaves are thick and leathery, and chimpanzees chew them into wadage, for example, fig fruit…, to absorb leaf nutrients without swallowing the fibrous and phytolith-rich wadage,” write Power and colleagues.

They also found that while calculus is a lifetime deposit, the results were often not as rich as the results from the faecal samples. This difference is why the scientists argue that studying calculus alone would give a distorted view of an animal’s diet.

“Bringing faecal and dental calculus datasets together addresses some of the methodological concerns researchers have raised about inferring diet from dental calculus,” Power and colleagues conclude. “Although this study highlights biases that exist with using dental calculus and faecal samples for reconstructing diet, it also helps to validate that these studies in these regions have produced a signal that is dietary in origin.”