Please don't feed the baboons - They eat toxic foods by themselves

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Please don't feed the baboons - They eat toxic foods by themselves

Daryl Codron

What do baboons really eat and why? Codron provides an insight into the diverse diets of baboons, suggesting that in some instances, baboons, like humans may even select certain plants for mind altering purposes.

Baboon Feeding. Photo by Thomas P Peschak

Baboons are ecological generalists, having amongst the most diverse diets of all African mammals. The wide range of food items they consume has made it difficult for researchers to explain the environmental variables that underlie their feeding behaviour. To quote DeVore and Hall (1965), "…it is simpler to list the items not eaten by baboons than to describe the items which they do…", a statement which to this day remains a realistic portrayal of our embryonic understanding of baboon diet.

Carbon isotoping: grasses or plants

A useful tool to overcome such research problems is analysis of the stable carbon isotope ratios (¹³C/¹²C, or d¹³C) in animal tissues and excreta. In African savannas, animal d¹³C values reflect proportions of browse to grass intake, because the d¹³C values of C₃-photosynthesizing plants (browse foods such as trees and shrubs) are consistently lower than those of C₄ plants (grasses).

Recently, a study was undertaken to resolve the carbon isotopic composition of chacma baboon diets. The focus of this study was different populations of baboons living in two savanna areas of South Africa: the Kruger National Park, and the Welgevonden Private Game Reserve in the Waterberg. The results, based on carbon isotopic analysis of over 200 faecal samples, revealed a marked difference between the diets of these two populations. In Kruger National Park, baboon diets were almost entirely C₃-based (this may have included tree leaves, fruits, flowers, and bark), while in Welgevonden C₃ foods constituted between only 30 and 50% of the diet of different individuals. In other words, a large portion of the diet of Welgevonden's baboons was comprised of grasses (and perhaps other ¹³C-enriched foods).

This result is indeed surprising; the Kruger National Park vegetation, especially on the nutrient-rich basaltic soils in the eastern regions, consists of a much greater abundance and diversity of palatable grasses than does the Waterberg. Welgevonden does have sufficient grasses to support an array of grazing ungulates such as Burchell's zebra, square-lipped rhinoceros, blue wildebeest, and buffalo, but one does not expect animals such as baboons, with the behavioural and anatomical capacity to exploit any number of food resources, to select such a high proportion of grasses in this area as opposed to Kruger National Park.

This presents a conundrum - do baboons truly favour the 'sourveld' grass of the Waterberg, or does the Welgevonden population have some unusual dietary adaptation?

A subset of faecal specimens from Welgevonden were collected beneath a Euphorbia ingens (candelabra tree), on which local staff had observed baboons feeding, and on which numerous bite marks were visible. E. ingens is a succulent plant that is neither C₃ nor C₄, but follows a third photosynthetic pathway known as Crassulacean Acid Metabolism (CAM). d¹³C values of CAM plants are often indistinguishable from that of C₄ grasses, and analysis of this particular tree confirmed that trend. While baboons certainly utilize some proportion of grass on Welgevonden, it seems that E. ingens (and perhaps other succulents) are a commonly selected food item - another unexpected result.

E. ingens contains a milky latex notorious for its toxic properties, yet baboons seem to be unaffected by this plant. This behaviour is, however, not limited to Welgevonden; baboons from the Eastern Cape and the Namib Desert also consume Euphorbia species. In Queen Elizabeth National Park, Uganda, intensive foraging on E. ingens by baboons is believed to be responsible for the emergence of two morphologically distinct forms of the tree!

Concurrent analysis of faecal nitrogen content provided some indication of the nutritional quality of baboon diets, at least in terms of crude protein (CP) intake. Two interesting patterns emerged: Firstly, no differences were observed in the nitrogen content of baboons from areas in the Kruger National Park where human foods were freely available (around the Skukuza and Lower Sabie restcamps) compared to more remote areas of the Park. Secondly, baboon nitrogen content was consistently higher than that of sympatric ungulate species in both areas, advocating intense selection of high-protein diets by this savanna primate.

What is clear is that baboons optimize their nutritional uptake in a given environment by choosing foods rich in protein (they are also quite capable of obtaining highly nutritious diets without supplements from humans…). It may be that in the nutrient-poor, recovering savanna of the Waterberg baboons are forced to supplement their diets even with poisonous plants such as E. ingens. However, the nitrogen content of E. ingens is not particularly high, which suggests that food selection extends beyond protein composition.

Addiction across the species barrier

Why then do baboons eat E. ingens? The tree is reported to have a high water and lipid content. Baboons are known to feed on other poisonous plants such as Datura during periods of water shortage. But water is not a limiting resource on Welgevonden. Lipids are believed to be an important macronutrient for baboon diet selection, especially during the winter months, which may explain the use of such unlikely resources as E. ingens. In The Soul of the Ape (Human & Rousseau Publishers Ltd., Cape Town; 1969), Eugene Marais argued that intoxicating, mind-altering chemicals are addictive not only to humans, but that addiction persists across the species barrier to other members of the animal kingdom. Perhaps, as with so many modern teenagers, our closest South African relatives make use of toxic substances simply to "stimulate their creative instincts".

More information:

Daryl Codron, Quaternary Research Centre, University of Cape Town

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