First, 'proof' is the wrong word to use here. That aside... Sure, let me read the paper for you, why not. >Given the wide range of inter- and intra-species va

First, 'proof' is the wrong word to use here. That aside... Sure, let me read the paper for you, why not. >Given the wide range of inter- and intra-species variation of plant structural and mechanical properties, and that such physical properties could influence the functionality of a constructed tool, we hypothesize that the structural and mechanical properties of plant material may be a selection criterion used by chimpanzees when making their tools. >Our results indicate that the mechanics of plant tissues are a factor in selecting materials for the construction of termite fishing probes by Gombe chimpanzees, notably low EI and reduced E. The selection of tool materials based on specific physical properties has already been described in the use of stone tools by nonhuman primates.7,8,11,12,61 Our research therefore extends the technological knowhow of wild chimpanzees to their toolmaking behavior. You'll forgive me if I don't paste the whole paper in here, it's pretty long, and full of math formulas.