In search of the australopithecines

Evidence from strontium isotope ratios preserved in fossil teeth provides a glimpse into the group dynamics and ranging habits of the australopithecines that can be compared with the patterns for modern primates. See Letter p.76

The australopithecines, human relatives who lived during the Pliocene and early Pleistocene epochs roughly 3.5 million to 1.8 million years ago, remain enigmatic creatures. The trouble is that inferences about their biology and lifestyle have to be drawn from frustratingly little hard information, which means that conjectures about them are plentiful but our knowledge remains scant.

On page 76 of this issue, Copeland and colleagues¹ describe the results of their investigation into a new aspect of australopithecine life. They analysed strontium isotope data from the tooth enamel of two australopithecine species, Australopithecus africanus and Paranthropus robustus, recovered from fossils in two caves in South Africa. The ratio of two isotopes of strontium (⁸⁷Sr/⁸⁶Sr) depends on the type of soils in the environment and is unaltered by biological processes. Australopithecine food items record the same strontium ratio as the soil in which they grew. Significantly, the ratio varies between types of bedrocks around the two cave sites, Sterkfontein and Swartkrans (see Fig. 1 of the paper¹).

On the basis of the ⁸⁷Sr/⁸⁶Sr ratios for 19 individuals, Copeland et al.¹ conclude that, in both species, most of the larger individuals (presumably males) fed primarily near the cave sites, whereas several of the smaller individuals (presumably female) fed elsewhere. The authors suggest that this indicates male philopatry (males remain in their natal group), with female dispersal. The situation in living African apes provides one point of comparative reference: chimpanzees have male philopatry, whereas both sexes of mountain gorillas may disperse².

The isotope ratios imply that males limited their home and day ranges to a circumscribed area. The ⁸⁷Sr/⁸⁶Sr values seen in the male teeth derive from a dolomite-rich region that is only 30 square kilometres in extent, distributed in a northeast–southwest direction. The minimum distances from the caves to bedrock of differing ratios are only 2–3 km to the southeast and 5–6 km to the northwest. There are no barriers, yet the males stayed in the dolomitic areas, apparently because their foods existed only in such regions. Young females apparently fed in other, unspecified areas during tooth mineralization, and moved into the males' region as young adults for reproductive purposes.

These results have implications for understanding australopithecine diet, group size, predator avoidance and home-range size. Living primate species offer insights into these variables². Among modern primate groups, ecology influences the temporal and geographic patterns of exploitation of food items, group size and predator pressure, and all of these factors strongly influence group ranging patterns. For example, leaf-eating primates have smaller home ranges than fruit-eating primates of the same body size. Body size and group size influence when and where individual primates would be vulnerable to predators.

Australopithecines lived in woodland savannahs, which provide tree foods and grass³. The carbon stable isotope ratios (¹³C/¹²C) in tooth enamel suggest that most of them ate both foodstuffs, with some East African members eating only grass^4,5. Their tooth-enamel structure and its microscopic scratches and pits imply that they also ate hard objects (seeds and nuts) or plant tubers and swollen stems⁶.

This diet contrasts with that of extant chimpanzees, which select only tree foods when living in savannah regions⁷. In such habitats, chimpanzees elude predators by avoiding open areas, forming larger than normal groups, and covering large home ranges³. For example, the range of chimpanzees at Ugalla, Tanzania³, is some 600 km² in extent, much larger than the 30 km² proposed by Copeland et al.¹ for the South African australopithecines. The small home ranges suggested by the ⁸⁷Sr/⁸⁶Sr ratios argue against australopithecines having targeted only tree foods and in favour of their including a more ubiquitous food such as grass. Mountain gorillas⁸ feed on ubiquitous terrestrial vegetation in a home range of up to 25 km², similar to that suggested by the strontium isotope data for male australopithecines¹. But male gorillas are very large, thereby providing group protection, and they live in forest habitats without large predators². In contrast to the mountain gorilla, the australopithecines lived in an open habitat, had a smaller body size and faced many species of large carnivores. How did they avoid predation while eating grass in an open savannah?

For diet, the closest equivalent to that of the australopithecines is found in the extant baboon Theropithecus gelada and in its giant fossil ancestor⁹, which had a body size roughly equivalent to that of the australopithecines⁵ and which occupied the same or a similar habitat. Modern T. gelada are much smaller than either fossil group and practise male philopatry; they avoid predation in their open savannah habitat by living in bands up to 60 strong, and have overlapping home ranges of sizes roughly equivalent to that estimated for the australopithecines (see references in ref. 2). But how could two large primate species of similar body size, one quadrupedal (giant baboon) and the other bipedal (australopithecine), have coexisted? They would have been competing with each other, as well as with other fruit-eating animals, and with grass-eaters such as pigs, hippos and zebra ancestors, along with other ungulate species⁵. How the australopithecines balanced predator avoidance and the need to compete for food remains an open question.

In Lewis Carroll's flight of nonsensical whimsy, The Hunting of the Snark¹⁰, there is the warning about the day that “your Snark be a Boojum!” — for the consequence of that is instant disappearance. We are in the midst of a fascinating hunt, but for us a Snark that turns out to be a Boojum would mean success, not failure. What is needed in this field of research is for conjectures about the australopithecines to vanish, instead of arising time and time again^5,9. We must hope that continued investigation along the lines of Copeland and colleagues' study¹ will one day allow us to winnow away some of that speculative thinking.