BOSTON -- Two Atlantic white-sided dolphins that stranded in Wellfleet on Thursday have died, eight remain stuck and six more dolphins were saved after rescuers battled against time and hip-deep mud in a desperate effort.

The International Fund for Animal Welfare Marine Mammal Rescue Team received a report at about noon Thursday that 10 dolphins were stranded in an area of Wellfleet called Drummers Cove.

About two hours later, IFAW learned that six more dolphins stranded on nearby Lieutenant Island. One of those animals was dead, and one had to be euthanized. The other four were rescued.

Katie Moore, of the Cape Cod Stranding Network, said Atlantic white-sided dolphins are social animals, which usually helps in their survival. But in this case, it may have contributed to the problem.

"They stick together, and if you think about it, (social) animals do well in the wild -- there is safety in numbers, sort of speak. But that can actually backfire on them when one of them comes to shore, or comes too close to shore," Moore said.

Deep mud made rescuing some of the animals difficult.

"Our rescuers were hip-deep in mud, and it was too dangerous to even get to some of the animals at all," Moore said.

The six surviving animals were released off Provincetown.

New England Aquarium workers and dozens of volunteers helped in the rescue effort.

A.J. Cady, deputy program director for IFAW, said it was possible the two groups of dolphins were part of the same pod that became separated.

Atlantic white-sided dolphins can measure up to eight feet in length and are common in the waters off the Cape. Mass strandings of the species are not uncommon, said Cady.

"They are extremely intelligent and social animals, so they will stay together," Cady said.

Scientists are not certain what causes marine mammals to become stranded, though one leading theory is that the animals become confused after chasing prey into shallow waters.