If you're interested in female friendships, put this post in the simply fascinating category. Few prior studies have looked at the social structures of whales but a recent one made an intriguing observation: After returning from the ocean, the same small groups of female humpback whales meet up each summer to swim alongside each other and feed in the Gulf of St. Lawrence.

Using photographic identification techniques, researchers with the Mingan Island Cetacean Study Group in Canada and their colleagues from Germany and Sweden recorded the movements, from 1997-2005, of five categories of whales: juvenile males, juvenile females, mature males, lactating females and non-lactating females.

The researchers found long-term associations (might we call them friendships?) between mature males and non-lactating females as well as among non-lactating females. However, only the non-lactating females had annual reunions with females of similar ages for as many as six seasons. Additionally, those females with the most stable and long-lasting friendships were more likely to give birth to calves. The researchers suspect that the annual reunions are associated with more cooperative feeding practices. The male-female groups stayed together for about two weeks during the feeding season but didn't meet up again during consecutive years.

Without underwater cell phones or Blackberries, how were the whales able to reconnect? It's not clear how they found each other over the vast ocean but sound, which travels four times faster in water than air, probably plays an important role.

The study was published in the journal Behavioral Ecology and Sociobiology.

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