Evolutionarily, animals that use tools have an leg up on their competition: they can access hard-to-get food items, learn more about their environment, and better protect and defend themselves. But exactly how much of an evolutionary edge does tool use provide? In a new article in Science, a group of researchers set out to answer this question, and were surprised at how much of an advantage tool use can provide.

The scientists studied New Caledonian crows, a bird species that is particularly well known for its tool use. These crows often use sticks to find and extract beetle larvae from holes, much like chimpanzees use sticks to “fish” for termites. This is a very specialized task, because the crows fish for just one beetle species (the wood boring longhorn beetle) in the trunk of a single species of tree (the candlenut tree).

Learning to use these stick tools is costly, since it takes young crows a considerable amount of time and effort for to become proficient at fishing for the larvae; in fact, even experienced adults take a relatively long time to catch each larvae. However, the slow learning curve and long handling time are worthwhile because the grubs that the crows catch in this way are extremely nutritious. The study's authors wanted to determine exactly how advantageous tool use is for these crows.

Since the birds are extremely shy and live in a heavily-forested habitat, New Caledonian crows are notoriously hard to observe, so the scientists had to figure out another way to answer their question. The team captured wild crows and took both feather and blood samples. By analyzing the stable isotope profiles of these samples, they could figure out how much of the omnivorous crows’ diet came from beetle larvae, and how much came from other sources such as lizards, carrion, nuts, and fruit.

The larvae constituted about as much of the birds’ protein as the other food sources; however, the grubs provided far more fat than the other foods did, providing nearly 50 percent of the crows’ total lipid intake. Clearly, spending a little extra time and energy on tool use provides a large nutritional benefit.

The researchers took the question one step further and, using the average nutritional value of the beetle larvae, calculated how many larvae a crow would need to catch to satisfy its total daily nutritional needs. Surprisingly, they found that catching merely three larvae would provide a crow with more than enough energy for an entire day. With this kind of nutritional advantage, it's no wonder tool use spread through the population at some point in the past and is still maintained today.

The authors close the paper with an interesting idea: since these nutritionally-rich beetle larvae come from a single tree species that was introduced to New Caledonia by humans, it’s possible that tool use in these crows is actually at least partly due to anthropogenic influence.

Science, 2010. DOI: 10.1126/science.1192053 (About DOIs).

Listing image by Simon Walker