A couple posts ago, I talked about a self-aware fish, and how the list of self-aware animals makes humans seem “less special.” What separates us from animals? If it isn’t our self-awareness, is it our intelligence? That is a tricky one, because intelligence is a difficult concept to quantify, and it is all too easy to separate human an animal intelligence as a matter of degree. Without a firmly defined, special type of intelligence that humans possess that animals do not, the best we can do is claim an arbitrary level of intelligence (such as cranial capacity) that separates humans from animals. What about an upright posture? While uncommon, some animals can assume an upright posture (such as bears) or habitually walk in an upright posture (such as penguins). About the only thing we can say is that we do it more frequently than animals or that we have a better design for walking upright. What about an opposable thumb for manipulating objects? We do have the best hand for manipulating small, fine objects, as opposed to simply having a strong power grip, but there is such a variety of animals that have opposable thumbs, the best we can do is claim that ours is “better” by some arbitrary definition.
Now, you may have noticed that I have deliberately skipped one of the classic differences between humans and animals. That difference is tool use. It used to be thought that humans can build and use tools and animals cannot. However, that idea has been discarded for a while, since it is now well known that many types of animals use tools. The woodpecker finch uses cactus needles to pick grubs out of wood. Sea otters use rocks to smash shellfish to get at the meat inside. Alligators use sticks as bait to catch egrets (no really, they do, and using bait can be considered a form of tool use). Perhaps most impressive of all are chimpanzees. They use “sponges” made of leaves to soak up water, insert sticks or grass into ant hives to fish out and eat the ants, crack open nuts using heavy branches or stones, and other types of tool use. Chimpanzees even appear to have different cultures: different populations of chimpanzees will use one kind of tool but another population of chimpanzees will not use that particular tool, but use a different type for a different purpose.
Since tool use itself is not a uniquely human trait, some have suggested that using a tool to shape another tool might be a uniquely human activity. To put this in context, when an animal uses a tool, it often uses a tool that has not been shaped in any way. When a sea otter uses a rock to crack open a shellfish, it does not shape the rock, chip it to make a specific shape, or do any other kinds of modifications. It just picks up a rock, holds it on its chest, and smashes a shellfish against the rock. Some animals do modify tools before using them. Some chimpanzees shape branches into spears and use these to stab bushbabies that are curled up deep in a hollow tree. However, this modification of tools is usually done with the animal’s own body. In the spear making chimpanzees, for example, to make a spear, a branch is torn off of a tree, the twigs are pulled off of the branch, and sometimes, the tip is chewed to a point. Notice that all of this modification is done with the chimpanzees’s own hands and teeth: they do not use a tool to modify the spear.
Initially, this may seem like a good separation between humans and animals: humans use tools to make tools while animals only modify tools using their own bodies. However, if we define “tool” broadly enough, there is at least one example of animals using a tool to modify another tool. Some chimpanzees use heavy branches as hammers to crack open nuts. Typically, they modify a branch simply by breaking it with their hands, but they have been observed to smack a heavy branch against a tree root in order to break the branch down to a more appropriate size. In this latter case, a tool (the tree root) is used to modify another tool (the heavy branch).
Now, I realize that we have to be very generous with our definition of “tool” to say that a tree root and a branch are both tools. In fact, biologists are not in complete agreement as to what exactly constitutes a true tool. Some say that a true tool has to be something that has been modified. Using this definition, the sea otter does not use tools, since it does not modify the rock it uses to smash shellfish. Chimpanzees, however, do use tools since they modify their spears and hammers to make them more appropriate for their function. However, there are other biologists who define “tool” more broadly so that sea otters do in fact use tools. The point is, the definition of “tool” is vague enough that it is actually difficult to use “tool use” as a clear distinction between humans and animals.
We seem to be back at square one. Tool use can be a vague idea and there are a number of animals that make and use tools. However, there is an aspect of tool use that I believe can more assuredly separate use from animals. That will be discussed in the next post.
Thoughts from Steven
Cranial capacity measures the volume of the cranium, which is proportional to the volume of the brain. The idea is that a larger brain indicates higher intelligence, but it is far from certain that such a correlation exists.
Dinets, V.; J. C. Brueggen; and J. D. Brueggen (2013) “Crocodilians use tools for hunting” Ethology, Ecology & Evolution 27(1): 74-78
Lycett, Stephen; Mark Collard; William McGrew (2009) “Cladistic analyses of behavioural variation in wild Pan troglodytes: exploring the chimpanzee culture hypothesis” Journal of Human Evolution 57: 337-349
In case this behavior seems strange, understand that chimpanzees are omnivores and they can, and do, hunt prey. The bushbabies are one such animal that chimpanzees prey upon.
Pruetz, Jill; Paco Bertolani (2007) “Savanna Chimpanzees, Pan troglodytes verus, Hunt with Tools” Current Biology 17: 412-417
Boesch, Christophe and Hedwige Boesch (1990) “Tool Use and Tool Making in Wild Chimpanzees” Folia Primatol 54:86-99
Seen, Amanda and Richard Byrne (2010) “Animal Tool-Use” Current Biology 20: R1032-1039