- Social mammals usually live in well-defined and defended groups.
- Bottlenose dolphins at Shark Bay, Australia, have ranges that overlap with those of other dolphins in the area.
- Although males of this dolphin population do not patrol and defend territories, they have extremely complex relationships.
An unprecedented open society of bottlenose dolphins has just been identified in Western Australia.
Most mammals, including humans, live in areas with boundaries. This population of dolphins, described in the latest Proceedings of the Royal Society B, has no such limits, even though dolphin relationships can be incredibly intense.
"Other mammals with complex social relationships live in a semi-closed group based on one or more reproductive females," co-author Richard Connor told Discovery News, explaining that the groups or territories of these other animals "are defended by one or both sexes."
"An open society is one without such defended boundaries," added Connor, a biology professor at UMass Dartmouth.
He and colleagues Srdan Randic, William Sherwin and Michael Krutzen examined the ranging and behavior of over 120 adult dolphins in a large study area at Shark Bay, Western Australia. They focused on males and their very complex social lives.
Male bottlenose dolphins also were found to engage in extensive bisexuality, combined with periods of exclusive homosexuality. Male pairs, or even trios, cooperate to sequester and herd individual females during the mating season. Most males are also members of second order alliances consisting of 4 to 14 males. Such relationships appear to be long lasting, with one known 7-member group still intact after 17 years.
At first the researchers thought the dolphins lived somewhat like chimpanzees, since male bonding is strong, but in chimps can also lead to patrolling and defending community territory. But because of their open watery range, the Shark Bay dolphins may instead have ranges that overlap with those of other dolphins in the area.
While dolphins can be aggressive, their "make love not war" lifestyle seems to be more peaceful than that of some other mammals, possibly even humans.
"We have seen precious little aggression between females," Connor said. "It does occur and is probably less frequent and more subtle."
As for males, even though "they are capable of serious aggression," he said, "they don't squabble constantly."
Terrestrial mammals may simply be more tied to a home base or hearth, although survival can depend on mutual dependence for both humans and dolphins.
Connor and his team believe that odontocetes (dolphins and sperm whales), humans and elephants form a "big three" group in the animal kingdom, since we all share big brains, have complex social lives, can display overlapping ranges, and have a relatively low physical cost of locomotion. Elephants have the lowest cost of locomotion recorded for any terrestrial animal.
"Elephant babies can't run or hide -- they are just big steaks," Connor explained.
It could then be that certain shared circumstances among the "big three" animals favor alliances, which could have driven big brains, social cognition and more.
Somewhat similar pregnancies might also link the big three. Elephants endure a 22-month pregnancy. Pregnancies clearly slow human moms-to-be. The same holds true for dolphins.
Shawn Noren from the Institute of Marine Science, University of California Santa Cruz, recently donned scuba gear and followed some pregnant female dolphins. Noren found that they "had huge protrusions where the fetus was sitting towards the back end of the body." This caused the females to move less and more slowly, relying on their pod for protection and cooperative feeding.
Despite the open society existence, life isn't all joy for male dolphins either.
"I work on the male dolphins and their social lives are very intense; it seems there is constant drama," Connor said.
"I have often thought, as I watched their complicated alliance relationships, that their social lives would be mentally and physically exhausting, and I'm glad I'm not a dolphin," he said.