- Aug 19, 2003
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Why Do Dolphins Carry Sponges?
10 Dec 2008
While rare in wild animals, tool use is of widespread interest to researchers because of its relationship to animal cognition, social learning and culture. Measuring the costs and benefits of tool use has been difficult, largely because if tool use occurs, all population members typically exhibit the behavior. However, a new study that examines a subset of Western Australia's Indian Ocean bottlenose dolphin population and their use of marine sponges as tools, provides a unique opportunity to assess costs and benefits of tool use and document patterns of transmission from mother to calf.
Led by Georgetown University's Janet Mann, the researchers are the first to examine the relationship between tool use and fitness in wild animals. Their findings are published in the December 10 edition of the journal PLoS ONE.
"It turns out the brainiacs of the marine world can also be tool-using workaholics, spending more time hunting with tools than any non-human animal," says Mann, professor of biology and psychology, who has been studying the Shark Bay dolphin population for more than 21 years. "This is the first and only clear case of tool-use in a wild dolphin or whale."
Mann, who started systematic data collection on the sponging behavior in these dolphins in 1989, found that 41 dolphins, in a population of thousands, use marine sponges on their beaks as a foraging tool. These dolphins use sponges to find hidden prey in the sandy sea floor and spend more time using their sponge-tool than any non-human tool user documented to date. Spongers are also 'workaholics' spending more time hunting, diving, and diving for longer time periods, than other females in the population. They also tend to be solitary.
Comparing sponge-carrying (sponger) females to non-sponge-carrying (non-sponger) females, the researchers observed that spongers were more solitary, spent more time in deep water channel habitats, dived for longer durations, and devoted more time to foraging than non-spongers. They also found that even with these potential immediate costs, such as less time socializing, calving success of sponger females was not significantly different from non-spongers.
"Despite these costs, they are successful at calving, so their workaholic tendencies pay off," says Mann.
Mann and her co-authors also report a clear female-bias in the development of sponging. Almost all the spongers are females and they transmit this behavior to their offspring.
"While a few males carry sponges, they seem to be slow learners in this regard," notes Mann. Her team found that all female calves started sponging before they were weaned, whereas male calves rarely used sponges, and if they did, it was after weaning.
The authors suggest that while daughters show a strong tendency to adopt the social and foraging behaviors of their mothers, sons appear to be less interested in maternal behavior and are more concerned with finding other male associates. Also, if the mother tends to be solitary, as is the case with spongers, then sons tend to socialize during brief separations from their mothers while daughters will go off and hunt on their own like their mothers.
"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.
Although sponging was discovered in the mid-1980s, the behavior is difficult to observe because spongers hunt for fish primarily in deep channels (8-13 meters). Mann has observed the behavior when water clarity was exceptional, leaving no doubt that the sponge is used to help search for prey that burrow in sand. Although sponging is the predominant foraging technique used in these channels, one family of spongers (grandmother, mother and two daughters) use another deep water area, suggesting that the behavior is not restricted to channels only, and traditional use of other areas can emerge.
While Mann continues to seek answers to new questions in her research on animal behavior, she also works with her students on their own research questions, both on the Shark Bay project and in the classroom. Her students are examining the long-term development and transmission of foraging behavior, social networks, and factors related to female reproductive success. Mann teaches Animal Behavior, Monkeys, Apes and Human Evolution and Behavior at Georgetown. She also takes up to four students with her to Australia each summer, giving them the opportunity to learn the latest field research techniques.
Additional researchers on Mann's team were from Florida International University, Metropolitan State College of Denver and University of Massachusetts at Dartmouth.
Why Do Dolphins Carry Sponges?
Mann J, Sargeant BL, Watson-Capps JJ, Gibson QA, Heithaus MR, et al. (2008)
PLoS ONE 3(12): e3868. doi:10.1371/journal.pone.0003868