Modern humans make use of a plethora of tools, ranging from the simple hammer and screwdriver to the sublimely complex space probe, guided missile launch, or magnetic resonance imagery device. Even our ancient ancestors crafted fine arrowheads and spears out of stone, bone, and wood. Until quite recently, we considered tool use to be a hallmark of our species, yet another indication of our superior intelligence and manipulative skill. Then in the mid-1960s, Jane Goodall made the startling discovery that chimpanzees used tools—twigs that were carefully selected and modified appropriately—to fish for termites.
Since then, the study of animal behavior has grown tremendously. and more and more wildly diverse creatures have been discovered to use tools in one way or another: Finches probe for grubs under bark using thorns, sea otters (Enhtjdra lutris) pound open abalone shells using rocks, caddisfly larvae encase themselves in suits of armor made up of bits of debris, and spiders weave webs. The need for a more rigorous definition of tool use became necessary, and Benjamin Beck (1980) devised one that is now widely accepted: “The external employment of an unattached environmental object to alter more efficiently the form, position or condition of another object, another organism or the user itself, when the user holds or carries the tool just prior to use and is responsible for the proper and effective orientation of the tool.”
I. Sponge Carrying
By this definition, the sponge carrying of Indian Ocean bot-tlenose dolphins (Tursiops aduncus) in Shark Bay, West Australia, is a form of tool use (Smolker et al., 1997). Sponge carrying was first observed in 1984. A dolphin, who was later dubbed “Halfluke” because she lacked half of her tail, surfaced near the author’s boat with a large, reddish, knobby “growth” on her face. She took several breaths at the surface and then dove, bringing her damaged tail flukes out of the water as she headed toward the bottom. After watching for some time it became evident that Halfluke was carrying something that she occasionally dropped. It was not a “growth,” but rather a sponge. Halfluke was one of several dolphins who regularly carried sponges, usually within a particular deep-water channel within Shark Bay. These “sponge carriers” are all female (most have had offspring, and some of the offspring have subsequently taken up sponge carrying). The sponge carriers have been observed on numerous occasions during every year since 19S4. almost invariably carrying sponges. They “specialize” in sponge carrying and have been doing so for years. On a few occasions, the author saw other dolphins carry sponges (see Fig. 1), but these were all single observations—usually just a single surfacing with a sponge—by dolphins that otherwise did not carry sponges.
From the surface, sponge carrying looks no different than other forms of foraging. The dolphin surfaces to breathe, traveling slowly along while taking several breaths and then dives, bringing her tail flukes out, an indication that she is diving down toward the bottom. After 2 or 3 min, she resurfaces further along. There is often no predictable travel direction: the dolphin appears to wander around while below the surface. Occasionally the dolphin surfaces without the sponge or trades in one sponge for another. The sponges (Echinodictyum mesen-terinium) are roughly cone shaped and fit over the dolphin’s beak. Some are quite large and flop back over the dolphin’s face, even obstructing vision. Others are small and fit like a yarmulke cap, just covering the beak tip. The shape of the sponge and the backward pressure caused as the dolphin swims forward apparently keep it in place. Sometimes they do fall off and are quickly retrieved by the dolphin.
Figure 1 Sponge carrying was observed first in dolphins of Shark Bay, Western Australia. The practice has been passed from females to female offspring and has an uncertain function.
After years of observing sponge carrying, we have never seen for sure how the sponges are used, as the dolphins apparently make use of the sponges at or close to the bottom in deep and murky water. Given the amount of time that sponge carriers spend with sponges—similar to the time spent foraging by osier dolphins—we suspect that the sponges are used in some way to aid foraging. Also, we sometimes saw sponge carriers surface rapidly without a sponge just prior to surfacing and swallowing some prey item. Thus the sponge is apparently used during search and then dropped during the final pursuit and capture of prey.
All evidence best fits with the hypothesis that the sponges are used as shields, protecting the dolphins’ face and beak. They may be protecting themselves from the stingers or spines of noxious organisms encountered while foraging among rocky ledges or from abrasive sand, broken shell, and rocks while poking their beaks into the bottom in search of burrowing fish. We know that there are spiny lionfish in the area as well as stonefish and various other poisonous organisms. It is also known that the dolphins sometimes poke into the sandy bottom in search of prey.
The use of sponges as shields is not the first example of tool use by dolphins: Tayler and Saayman (1973) reported that a captive bottlenose dolphin used pieces of the tile to rub the tank walls, dislodging pieces of seaweed. Brown and Norris (1956) reported observing two captive common bottlenose dolphins (T. truncatus) attempting to capture a moray eel. After many attempts, one dolphin sought out and killed a scorpi-onfish, carried the fish to the crevice where the eel was hiding, and poked at the eel with the spiny fish. The eel abandoned its hiding place and was captured. These examples are indicative of the resourcefulness and intelligence of dolphins, but they are one-time behaviors. Sponge carrying is engaged in by several dolphins and has been ongoing for many years. It has not spread throughout the population as, for example, washing of sweet potatoes among Japanese macaques, but rather is restricted to a few individuals. Thus, like termite fishing by chimpanzees, sponge carrying is a well-established tradition of tool use by an animal living in a vastly different ecosystem and bearing little relatedness to humans.
Dolphins and sea otters appear to be the only marine mammals that engage in tool use.
