Why Use Tools?

Many animals interact with fixed objects without the goal of altering them, such as rubbing an itchy body part against a tree. Tool use, in contrast, involves steering a freely maneuverable object toward a target (another object, organism or medium) with the goal of altering its position, form, or condition (3, 4). The majority of reported tool use cases in the wild come from foraging contexts. Fewer cases of tool use are seen in other contexts, such as courtship, defense, or body care—the latter exemplified by puffins using sticks to scratch themselves. Another example of wild birds using stick tools for a nonforaging goal is palm cockatoos (Probosciger aterrimus) rhythmically drumming on trees with sticks to enhance their vocal and visual displays (5). Next to the puffins, the only other observation to our knowledge of a wild seabird using a tool outside a foraging context was a double-crested cormorant (Phalacrocorax auritus) using a detached feather as a brush to spread preen gland oil over its plumage (6). Some seabirds and waterbirds use tools for foraging, in both captivity and the wild, such as gulls and herons using bait or lure for fishing (4, 7).

In an updated phylogeny, cormorants and puffins fall under the same clade containing all waterbirds (Aequorlitornithes) (8). This raises the question of possible shared evolutionary origins of tool use for self-maintenance in (some) waterbirds and urges further research on this understudied group. For both species, the body areas targeted by the tools were within bill reach, suggesting that improved access was not the main function. This stands in contrast to wild primates and elephants, and captive parrots, which often use tools to scratch body parts otherwise hard to reach (4).

In general, it is essential for birds to keep their feathers in prime condition, so perhaps finding cases of avian tool use for body care is not so surprising. One common way of using tools for body care in birds may not intuitively be understood as such by the casual observer: the practice of throwing substrates over the body while sand bathing. Several species use their beaks to rake or pick up and throw sand on their own bodies (9), which is a behavior that qualifies as tool use. This shows not only that body care is important for birds but also the difficulty in defining and categorizing tool use when it is camouflaged as seemingly plain behavior. Another peculiar example is the so-called anting, i.e., placing insects in the plumage in an attempt to combat parasites and reduce discomfort (4, 9).

The puffins (1) and the cormorant (6) used sticks and a feather as tools—objects commonly found in their environment—which lends support to the hypothesis that opportunity rather than necessity prompted their tool use (3). The finding that two seabird species use stick-like objects for body care invites further research to examine the adaptive value and ecological drivers of tool use for body maintenance.

Does Tool Use Require Intelligence?

A central question is whether tool use is associated with particular cognitive abilities and whether it plays a role in the evolution of intelligence (2⇓–4, 10⇓⇓⇓–14). It is not a straightforward question, because it is hard to agree upon a definition of intelligence. At least, most would agree that flexible behavior is a sign of a weighty cognitive repertoire. Flexible behavior is, put simply, when an animal goes beyond purely innate behaviors or the simplest forms of associative learning to solve an old or novel problem in a new way. The complexity of tool behavior can also be revealing (15); compared to a chicken throwing sand over its plumage, the puffins’ self-scratching with a tool is more complex as it requires more precision and prolonged control.

As Fayet et al. (1) point out, tool use can be divided into two broad categories, namely “genetically based” inflexible behavioral specializations adapted to one specific context and flexible “behavioral innovations” whose development may be partially governed genetically too but which can be employed in other contexts flexibly (14). This distinction is oversimplified but serves to point out that not all complex behaviors are underpinned by complex cognition. Highly complex behavioral patterns can be largely innate, rigid, and regulated by simple cues. While the evolution of inflexible tool use is interesting to understand, flexible tool use is most relevant for examining links between the evolution of advanced cognition and tool use.

In any case, what type of tool user the puffin is remains open. When a new tool-using species is discovered in the wild, it is often impossible to tell how its tool use first arose. It is equally conceivable that the puffins’ self-scratching represents a rare but species-wide, genetically programmed behavior, or an innovation by a single individual in each population that may spread through social learning.

Some scientists have rightly questioned why tool use should be considered as more cognitively challenging than other interactions with the physical or social world. Nest building, for example, is not categorized as tool use but arguably needs comparable fine motor control and possibly the same technical understanding speculated to underlie many forms of tool use (11). Does the tool use of the puffin require a more intricate cognitive machinery than that of, say, a bird weaving an elaborate nest? Can tool users reasonably be assumed to be more intelligent than nontool users?

In PNAS, Fayet et al. report on two cases of tool use in a seabird. In two distant populations they recorded Arctic puffins (Fratercula arctica) using sticks to scratch themselves.

Comparative studies have investigated whether birds that are habitual tool users (i.e., exhibiting species-wide tool use) have more advanced cognition than their nontool-using cousins. Comparisons of Darwin finches have not shown any discrepancies in general cognition between species that do and do not use tools (12, 13). Within corvids (i.e., the crow family) there are two known habitual tool users: New Caledonian crows (Corvus moneduloides) and Hawaiian crows (Corvus hawaiiensis). While the latter still have hardly been studied (16), New Caledonian crows do not appear to exhibit higher levels of general cognition than other corvids (13, 17).

The other side of the coin reveals the ability of cognitively advanced birds that are not habitual tool users to use and even make tools. Corvids such as rooks (Corvus frugilegus) and ravens (Corvus corax) are able to quickly learn or spontaneously innovate tool use (10, 18). Similarly, Goffin’s cockatoos (Cacatua goffini) and kea (Nestor notabilis) are not habitual tool users but have shown remarkable tool-using skills in captivity (19, 20).

New Pieces in the Puzzle?

Thus, an observation of tool use in a species does not prima facie tell us much about its cognitive levels but prompts further research. The puffins and cormorants offer an interesting new angle because tool use in a body-care context is rare. Controlled studies on puffins are necessary to understand the origins of this behavior: Is it innate, or innovated and potentially socially learned? Future research should examine how their tool use develops from a young age and whether it can be learned from others. Second, the flexibility of this behavior should be tested, including whether other forms of tool use are within their capacity if exposed to situations where tools are needed or after training. Finally, we need to study the cognitive and innovative skills of puffins and cormorants alongside nontool-using seabirds for direct comparisons.

Fayet et al. (1) conclude by suggesting that one should include species that are traditionally not considered good candidates for tool use and to report “unusual behaviors” as markers of behavioral flexibility. We agree. If unraveling human cognitive evolution and its link to flexible, creative tool use is the goal, it may be more fruitful to shift our focus from tool use per se to behavioral flexibility and creativity in general. There may be plenty of species that can use novel tools spontaneously when exposed to problem-solving tasks, thus exhibiting a latent capacity for tool use, which rarely ever shows in nature. Studying them will inform us about the evolution of intelligence and the prerequisites of tool use.