It’s not mealtime for certain bottlenose dolphins living off Australia’s coast unless they sport cone-shaped sea sponges on their beaks. These mammals are not following a strange, marine-based dress code. Their behavior has been identified as the first clear case of tool use by wild dolphins or whales, a new study concludes.
These dolphins dive to the bottom of deep channels and poke their sponge-covered beaks into the sandy ocean floor to flush out small fish that dwell there, says a team led by biologist Janet Mann of Georgetown University in Washington, D.C. Foragers then drop their sponges, gobble up available fish and retrieve the implements for another sweep, the scientists report online December 10 in PLoS ONE. Dolphins hold the sponge with the bottom of their beaks and can sweep away much more sand than they could otherwise.
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Mann’s team documented this behavior among 41 bottlenose dolphins, most of them female, out of a population of several thousand that inhabits Australia’s Shark Bay. The researchers estimate that sponge-carrying dolphins, or spongers for short, devote at least 17 percent of their time to ferreting out bottom-dwelling fish using these beak-borne prods.
“It turns out the brainiacs of the marine world can also be tool-using workaholics, spending more time hunting with tools than any nonhuman animal,” Mann says. Chimpanzees and other nonhuman primates spend a small amount of time using tools. One population of woodpecker finches spends an estimated 10 percent of its time using twigs and cactus spines to pry insects and spiders out of tree holes.
Mann has studied sponge-carrying among Shark Bay dolphins since 1989. Until now, it was unclear what, if anything, sponges were used for. Most of the female bottlenose dolphins Mann studies use various specialized tactics to obtain fish, but without using sponges. Only about one in 10 of the females are spongers, most of them members of two maternal lines.
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Evidence of tool use for foraging also exists for killer whales and sea otters, remarks biologist Hal Whitehead of Dalhousie University in Halifax, Canada. “In the marine environment, individuals may heavily concentrate on particular cultural practices, especially when foraging,” he says.
Not everyone regards such food-gathering tactics as purely the products of social learning or culture. Genetic traits and habitat characteristics may influence how animals forage as much as or more than any cultural traditions, argued Kevin Laland and Vincent Janik, both biologists at the University of St. Andrews in Scotland, in a 2006 paper.
Further research needs to tease out the effects of genes, environments and culture on sponge-assisted foraging by bottlenose dolphins, comments evolutionary geneticist Michael Krützen of the University of Zurich, who studies the social behavior of bottlenose dolphins and orangutans. “Dolphins are highly intelligent and are prime candidates for social learning in the wild,” Krützen says.
Mann’s team used small boats and GPS tracking of dolphins to monitor approximately 550 Shark Bay dolphins. The researchers analyzed records of foraging behavior from the past five field seasons.
Of 41 spongers studied, 29 were females, six were males and six were of undetermined sex. In all 17 cases where maternity was known, the researchers had also observed the mother carrying sponges.
All the daughters of female spongers took up sponging within the first few years of life, while still weaning. Male calves of these females rarely used sponges. If males did, sponging began after weaning had ended.
A comparison of sponge-carrying females to other adult females showed that spongers spent much more time alone or alone with their calves. Spongers also spent more time in deep-water channels, dived for longer periods and spent more time foraging than nonspongers did.
Despite their lack of socializing, spongers gave birth to calves at a rate equal to that of nonspongers, Mann says.
Early sex differences in sponging reflect the contrasting reproductive roads traveled by male and female bottlenose dolphins, Mann suspects. Grown males focus on forming a network of alliances that provides access to fertile females, rendering the isolated sponging lifestyle unappetizing. Females hone specialized foraging skills, including sponging, to satisfy the nutritional demands of nursing each calf for three to eight years.
It’s not clear why few males have taken up sponging and whether such behavior helps or hurts their reproductive prospects, Mann notes.
Because sponging occurs on the floor of 8- to 13-meter-deep channels, it has been hard to observe. Mann viewed several instances of sponging on days of exceptional water clarity, confirming that the animals used sponges to jostle fish out of the sand. The researchers also scuba dived to the ocean floor to confirm that sponges can be used to ferret out fish from the sand, and observed female spongers on occasion coming to the surface holding fish in their mouths.
“We think our evidence points toward sponging in this dolphin population as a socially learned tradition,” she says.