Beavers are known for their monogamy and mating for life.
But when two pairs were introduced to the wild west coast of Scotland, they let standards slip and promptly indulged in a partner-swap.
The five-year Scottish Beaver Trial was held in Knapdale Forest in mid-Argyll, and beavers remain in the area.
Field operations manager for the trial, Roisin Campbell-Palmer, said: “During the monitoring period one of the interesting observations was that two pairs swapped partners.
“Beavers usually mate for life, however these were all young adults encouraged to form pairs either in captivity prior to release or at the release site after the loss of an original mate.
“When given the freedom to choose a mate on their own, they obviously preferred a different arrangement.
“This is actually a natural and positive outcome as animals selecting their own partners are shown to have strong pair bonds and higher reproductive rates.”
The beavers – who came from Norway – were obviously happy with their new companions because soon one of the pairs bred for the first time, and the other female was suspected of being pregnant, although no kits were ever seen.
Darren Dobson, owner of the Cairnbaan Hotel near to the beaver trial area, was amused to hear of the partner-swapping in the wilds of Knapdale.
He said: “The beaver trial has been a godsend to the local tourism industry.
“It more or less carried the industry and this hotel. So many people are interested in the beaver.
“Obviously the wildlife in this area is amazing, but to have something unique like that has been a life-saver for this area.”
The findings of the trial will help decide what happens next.
The future of beaver reintroduction is ultimately up to the Scottish Government with a ministerial decision expected in the second half of 2015.
The project was the first official trial mammal reintroduction in the UK.