The major side effect observed with Xarelto is bleeding, similar to other anti-clotting drugs.

FORBES: Rivaroxaban Gains FDA Indications For The Treatment And Prevention Of DVT And PE

This is sometimes observed to have the same effect as artificial pruning.

ECONOMIST: Elk saliva stimulates plant growth

As the rowers had been teammates for nearly a year it is possible that the mere presence of friends explains the observed effect.

ECONOMIST: Exercise and company

"Sympathetic concern" has also been observed in gorillas, bonobos, dogs and even rooks - but it is the calming effect that it had on the Chester Zoo chimps which is said to be a new observation.

BBC: Hugging benefits fractious chimps