Resource depletion through primate stone technology

Abstract

Tool use has allowed humans to become one of the most successful species. However, tool-assisted foraging has also pushed many of our prey species to extinction or endangerment, a technology-driven process thought to be uniquely human. Here, we demonstrate that tool-assisted foraging on shellfish by long-tailed macaques (Macaca fascicularis) in Khao Sam Roi Yot National Park, Thailand, reduces prey size and prey abundance, with more pronounced effects where the macaque population size is larger. We compared availability, sizes and maturation stages of shellfish between two adjacent islands inhabited by different-sized macaque populations and demonstrate potential effects on the prey reproductive biology. We provide evidence that once technological macaques reach a large enough group size, they enter a feedback loop - driving shellfish prey size down with attendant changes in the tool sizes used by the monkeys. If this pattern continues, prey populations could be reduced to a point where tool-assisted foraging is no longer beneficial to the macaques, which in return may lessen or extinguish the remarkable foraging technology employed by these primates.

Keywords: Macaca fascicularis; Thailand; ecology; shellfish; stone tool use.

Figure 1.. Tool weights of Koram and NomSao Islands.

(A) Comparison between weights of stones used by macaques on Koram and NomSao Islands to open oysters. The plot shows all quantiles and the CIs (grey). (B) Comparison between weights of stones used by macaques on Koram and NomSao Islands to open snails. The plot shows all quantiles and the Cis (grey).

Figure 2.. Location of the two study islands (Koram and NomSao) in Khao Sam Roi Yot National Park, Thailand.

Figure 3.. Stones available on Koram and NomSao Islands.

(A) Weight of stones (with bootstrapped 95% confidence intervals) found on Koram and NomSao Islands, separated for oyster bed and tidal. (B) Average stone availability per island, separated for oyster bed and tidal (with bootstrapped 95% confidence intervals over observed plots).

Figure 4.. Average snail availability on Koram and NomSao Islands for three species (with bootstrapped 95% confidence intervals over observed plots).

Figure 5.. Prey size on Koram and NomSao Islands.

(A) Average size of oysters (with bootstrapped 95% confidence intervals) found on Koram and NomSao Islands. (B) Average snail size (volume) (with bootstrapped 95% confidence intervals) found on Koram and NomSao Islands.

Figure 6.. Long-tailed macaque tool use.

(A) Adult male long-tailed macaque using a stone tool to crack open a snail. (B) Size difference between NomSao and Koram Islands of most commonly harvested snails. (C) Abandoned macaque tool at shellfish cracking site, with prey remains. (D) Recently harvested oysters (white) are clearly distinguishable from older oysters (grey).