Why Do Dolphins Use Sponges? Hint: Not For Baths
One of the coolest examples of tool use by animals (the non-human kind) isn’t chimps that twist sticks into termite mounds to haul out a nice insect dinner, or that use rocks to crush nuts—impressive, to be sure, but I think many of us have come to take for granted that our close cousins are clever enough to use tools. To my mind, the most startling example of tool use by animals is bottlenose dolphins that use sea sponges: they spear the sponges with their noses and then poke around in rocky crevasses to scare out the fish hiding inside or swab the sandy sea floor to expose hidden, burrowing prey. The sponges protect their noses, and the custom has persisted for generations, passed down from mother to offspring.
But the behavior, first discovered in the mid-1980s in Shark Bay, in the Indian Ocean off western Australia, has always puzzled scientists. Only a few of the bottlenose dolphins in Shark Bay are “spongers.” If the behavior helps them survive and reproduce, it should be more widespread. If it carries too high a cost, it should die out within a generation or so, but it hasn’t. So is sponging adaptive for the dolphins or not?
In a fascinating paper being published this evening in the open-access journal (that means it posts all its content free) PLoS ONE, scientists led by Janet Mann of Georgetown University ask, “Why Do Dolphins Carry Sponges?” They compared sponger females to non-sponger females, finding that spongers are “more solitary, spent more time in deep water channel habitats, dived for longer durations, and devoted more time to foraging than non-spongers.” Each of these is a “cost”—that is, if you have to spend more time looking for food that’s time away from other evolutionary-desirable activities, such as mating and raising offspring. Yet Mann finds that even with these costs, “calving success of sponger females was not significantly different from non-spongers."
Bottlenose dolphins “spend more time hunting with tools than any non-human animal,” says Mann, who has been studying the dolphin population in Shark Bay for more than 21 years. “This is the first and only clear case of tool-use in a wild dolphin or whale.”
Yet out of thousands of dolphins in this population, she has found only 41 that use sponges. She also finds that almost all the spongers are females. They teach sponging to both male and female offspring, and “while a few males carry sponges, they seem to be slow learners in this regard,” says Mann. All female calves started sponging before they were weaned, but male calves rarely used sponges, and if they did, it was after weaning. That suggests that daughters adopt the social and foraging behaviors of their mothers, but sons are less interested in what mom is doing and more concerned with palling around with other males. “We believe these early sex differences foreshadow the long-term reproductive interests of males and females, with males being focused on alliance formation, necessary for successful mating, and females focused on foraging skills, necessary to meet the demands of three to eight years of nursing each calf,” says Mann.