A team of biologists from four US universities have discovered that use of tools among a group of bottlenose dolphins varies from individual females in the population. Dolphins appear to be the only mammal in the sea to regularly use a sponge as a tool to help catch fish.
Writing in the journal PLoS One, the authors note that tool use is rare in wild animals but is of widespread interest because of its relationship to animal cognition, social learning and culture.
Despite such attention, however, they say that quantifying the costs and benefits of tool use has been difficult, largely because if tool use occurs, all members of the population typically exhibit the behaviour.
But in Shark Bay, off the coast of Western Australia, only a subset of the dolphin population uses marine sponges as tools to hunt fish hiding in the sand at the bottom of the sea. This provided an opportunity to assess proximate and ultimate costs and benefits while documenting patterns of transmission.
The researchers compared sponge-carrying females to non-sponge-carrying females and found the former were more solitary, spent more time in deep water channel habitats, dived for longer durations and devoted more time to foraging than the non-spongers.
They say the female dolphins wore the sponges while searching for fish but typically not while chasing prey. They found the dolphins swam slowly along sand-bottom habitats with a sponge on, slightly and intermittently disturbing the seafloor. When prey was apparently detected, the dolphins dropped the sponge, accelerated about five to 10 metres and then probed the seafloor with their beaks.
"Occasionally rapid single breaths or leaps without the sponge were observed before returning to the same spot, indicating that prey may burrow in the sand. Subsequently, the dolphins retrieved sponges and began the search process again."
The technique of using a tool to search out fish is passed along from female dolphins to their female offspring. But the biologists found that even with the potential costs of using tools to catch their prey, the calving success of sponger females was not significantly different from non-spongers. They write that with a solitary lifestyle, specialisation and high foraging demands, spongers used tools more than any non-human animal.
The researchers say the findings suggest the ecological, social and developmental mechanisms involved may help explain the high intra-population variation in female behaviour, indicate trade-offs such as time allocation between ecological and social factors and limit the spread of this innovation to "primarily vertical transmission" - that is, from a female dolphin to its daughter.
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