A dolphin with a severe spinal deformity was adopted by a group of sperm whales. Friendly interactions between whales and dolphins had not been observed prior to this discovery in 2011. Researchers speculate that the dolphin was separated its original group either by an inability to keep up or it may have been ostracized.
Dolphins are social creatures, so it makes sense why one who is alone would seek out a new slower-moving group. It is unclear what the whales gained from the relationship, though many of the whales did return affectionate rubs from the dolphin.
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Photo credit: Alexander Wilson and Aquatic Mammals