Dolphin 'sponging' spans centuries
Bottlenose dolphins using sponges to protect their noses while foraging is a technique that the animals discovered in the 19th century, a study has found.
Scientists analysed data on the dolphins of Shark Bay, Australia, to model the appearance and transmission of the skill over generations.
The study found that "sponging" could have begun with a single "innovation event" between 120 and 180 years ago.
It suggested that mothers passed on the skill by teaching their offspring.
The analysis is published in the journal Animal Behaviour, and used previous field studies to investigate how sponging was established and maintained.
"It has been thought that behaviours which are exclusively learnt from one parent are not very stable. With our model we could now show that sponging can be a stable behaviour," said Dr Anna Kopps, a biologist at the University of New South Wales.
Dolphin see, dolphin do?
The study created a new technique to calculate the likelihood that the offspring of a "sponger" would learn the ability and pass the skill on.
By modelling the emergence of "sponger" dolphins in a computer simulation, the team could see different scenarios in which the skill could have spread among the dolphin population over the years.
They then compared the results of these simulations with field data on the genetic relationship between the spongers, to estimate the role of mothers teaching their offspring in transmitting the skill.
They found that if the likelihood of a sponger's offspring learning the ability was less than certain, the dolphins that did pick up the technique needed to gain a survival advantage from the skill, in order for the ability to pass on to the next generation.
The model also allowed them to attempt to calculate the date that the behaviour was likely to have originated.
"The results suggested that sponging was innovated at least 120 to 180 years ago - it is only a best estimate," said Dr Kopps.
"Unfortunately, the model did not give us a maximum time span," she said.
The dolphins wear the sponges on their rostrum when foraging on the sea floor, apparently to protect themselves from sharp rocks or shells.
The famous dolphin society of Shark Bay has been a focal point for scientists studying dolphin society for decades.
"Bottlenose dolphins in Shark Bay, Western Australia, show a wide array of behaviours. For example, 13 different foraging types have been described," said Dr Kopps.
"Sponging is the best-studied... because it is unusual in that it includes the use of a marine sponge as a tool and it is transmitted from mother to offspring," she said.
Attempts to calculate the date of "innovation events" have also been made with chimpanzees.
The excavation of stone nutcracking tools allowed scientists to date chimpanzee tool use back 3,400 years, suggesting it had been transmitted for over 200 generations, but Dr Kopps said that on the whole it has proven difficult to date these innovations.
"If the behaviour involves tool use, artefacts may be found," explained Dr Kopps.
"However, the aquatic habitat makes that very unlikely."
She hopes that the method in her study could now be applied to a wide range of species.
"It would be interesting to use the model we built but based on life history data of other species than dolphins, great apes for example."
"It is possible that dolphin life history characteristics make learning from a single parent more likely and stable than in other species."