Cross-Species Tool Use
When studying the evolution of behavior, it is important to consider the departures from and similarities between core features of behavior. As it relates to tool use, we see the possibility for both types of evolution (convergent and divergent). Tools and their use can often look the same across species and arise from vastly different evolutionary histories, or tools and their use can look different across species but be the employed by morphological features that stem from a recent common ancestry. This same logic can be applied to the function of tool use (see Adaptive Value for more information).
Brain changes over time
Two of the best studied classes of animals with regards to tool use are mammals and birds, specifically primates and corvids. Research has shown that the predisposition for tool use and manufacture among these animals is likely to be due to morphological brain changes throughout evolutionary history, with similar changes in mammals and birds producing fundamental similarities in behavior (Emery & Clayton, 2009).
A broad metric for tracing the evolution of tool use is the encephalization quotient, which is the ratio between brain mass and body size, such that animals with larger than expected brain sizes for their body size have higher encephalization quotients (Auersperg, Von Bayern, Gajdon, Huber, & Kacelnik, 2011; Deecke, 2012; Emery & Clayton, 2009). The encephalization quotient is often used as an index for intelligence, but as said before (see Home), tool use does not require intelligence, nor does intelligence predispose an animal for tool use (Emery & Clayton, 2009).
Another measure to predict tool use capacity is the development of the forebrain and neocortex in mammals and birds, which is the part of the brain most commonly associated with higher-level cognitive processing, including executive control and decision making (Emery & Clayton, 2009). It is suspected that with the evolution of these brain structures came a spread of cognitive skills for traits associated with abstract reasoning, social learning, and communication (Kacelnik, 2009).
While some animals are closer in evolutionary history than others, meaning they share a recent common ancestor, this does not necessitate the similarity in all behaviors, or even the capacity for similar behaviors.
Recent common ancestry
A striking example is the comparison between apes and monkeys. Apes are exceptionally well-studied with regards to tool use. Given apes’ flexibility in tool use and manufacture, it would be expected that monkeys, which share a closer evolutionary history to apes than birds, would have similar predispositions towards the sophisticated use of tools, yet the evidence of this is sparse.
While many tool use studies are often selectively targeted towards apes, studies with monkeys demonstrate that different cognitive processes may be employed when using tools and has even given some scientists pause as to whether these different behaviors that are due to different cognitions can both be labeled as tool use. For example, though both apes and monkeys seem to engage in both socially learned and spontaneous use of tools, differences in comprehension result in different levels of flexibility of tool use over time (Visalberghi & Limongelli, 1994).
Capuchin monkeys can use tools quite prolifically, but studies have found that there is little evidence for causal reasoning, learning over time, or sensitivity to consequences (Visalberghi & Limongelli, 1994). On the other hand, apes have been shown to engage in flexible tool use behavior that can require complicated intermediate steps or concurrent behaviors to achieve a desired goal, such as using and balancing up to four tools to crack open one seed (Biro, Inoue-Nakamura, Tonooka, Yamakoshi, Sousu, & Matsuzawa, 2003; Fox, Sitompul, & Van Schaik, 1999; Suzuki, Kuroda, & Nishihara, 1995).
Distant common ancestry
A more impressive evolutionary comparison of tool use may be between elephants and primates. Captive Asian elephants have demonstrated the ability to modify tools if they are expected to be ineffective as “natural” or unmodified tools (Hart, Hart, McCoy, & Sarath, 2001). For example, elephants use tree branches as a fly switch, or a device to swat at flies, in both the wild and captivity (Hart et al., 2001).
While elephants in the wild have not reliably modified tree branches to create more effective switches, lab research with captive elephants shows these animals use their prehensile trunk and feet to create better tools (Hart et al., 2001). The researchers liken this “fine-grained motor neural control of the elephant trunk with that of primate digits” (Hart et al., 2001, p. 840).
It is very possible that the neural circuitry for primate digit dexterity is also present in the elephant for prehensile trunk dexterity, but it is also possible that environmental pressures selected for these neural circuits, morphological features, and behaviors independently, resulting in convergent behaviors.