Bottlenose Dolphins Form Cliques Based On Sponges

By Mo Mozuch on August 1, 2012 4:38 PM EDT

photo: Eric M. Patterson
photo: Eric M. Patterson

Georgetown University researchers have discovered that bottlenose dolphins form cliques based on behavior, the only other species besides humans to do so, according to a paper published in the journal Nature Communications. Biologists studied dolphins in Australia's Shark Bay and noticed dolphins that hunt with the aid of sea sponges associated with other dolphins that did the same.  The dolphins wear the sponges to protect their beaks and startle fish while they poke around rocks on the ocean floor. It appears to be a socially learned behavior, a significant find, according to researcher Janet Mann.

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"Hunting with sponges is a solitary behavior, so it is all the more remarkable that dolphins prefer to spend time with others that use sponges even though they don't use them together," Mann said on a Georgetown University website. "This suggests that spongers not only identify who is a sponger and who is not, but also have some type of group identity."

Researchers studied 22 years worth of recorded interactions between 36 spongers and 69 non-spongers in the Shark Bay region. Homophily, the tendency to associate with similar animals, was evident in every analysis, researchers said. Although other factors such as maternal kinship, sex and location also contributed to the dolphins' social preferences.

Female dolphins that use sponges were the most cliquish of all the dolphins in the study. Male dolphins did not tend to base their social interactions around spongers vs. non-spongers. Researchers are unsure as to the reason behind the gender divide, but suspect it is due to the general differences in the way male and female dolphins participate in social groups.

Tool use is not unique to dolphins. Other species have been observed using tools, but the patterns are different. Other animals learn a behavior because they're part of a group, but the dolphins are forming groups based on behavior. Since sponging is a solitary behavior affiliation among spongers is not based on collective foraging, but on identifying other individuals as spongers.

"These patterns are remarkable," researchers wrote, "because spongers lead a relatively solitary lifestyle and have weaker ties with other dolphins than non-spongers do."

Sponging was first observed in Shark Bay dolphins in 1984. Scientists initially believed sponging simply protected the dolphins' beaks from jagged rocks. However, research published in 2011 revealed that there several other benefits to sponging. Bottom-dwelling fish are more nutritious because they tend to have a higher lipid content. Many also lack a swim bladder, a gas-filled organ that allows fish to control their buoyancy. Dolphins that hunt solely by echolocation can't locate fish that lack a swim bladder, thereby increasing the available prey for spongers. And plenty of prey there is, according to researcher Eric Patterson, who replicated the sponge-hunting technique for the study. By attaching a marine basket sponge to a pole and probing the same rocks the dolphins hunt from, the researchers averaged a fish every nine minutes.

"The prey are numerous and reliable," Patterson said, "and their behavior is so predictable-they always dart out of the sands-that they make this hunting behavior worthwhile for the dolphins."

The only time spongers hunt with other spongers is when sponger mothers are teaching their calves the technique. Those calves, in turn, taught the behavior to others in the group. The researchers say this is the first demonstration that a behavior transmitted by single mother dolphins to their calves serves a grouping function as well.

"Recently, many biologists are moving beyond genetic inheritance to examine the processes involved in inclusive heritability, which includes culture," Mann notes. "We sometimes think that traits such as culture are exclusively human, but a growing body of literature proves otherwise."

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