The giant pandas at Edinburgh Zoo could be getting ready to mate, according to the experts observing them.
Hormone tests and behavioural observations enable experts to predict when breeding is likely to occur, but until now, no changes had been observed in either panda.
But the behaviour of both animals has attracted attention in recent days, with female panda Tian Tian and male Yang Guang showing important changes that may indicate their readiness to mate.
Yang Guang, meaning 'Sunshine', has been seen performing handstands against trees, walls and rocks, and has taken to marking out his scent high up on trees - known in the wild as a display of virility.
Meanwhile Tian Tian, or 'Sweetie', has started calling to the male, a common sign of readiness among females in the breeding season.
The pandas were first introduced last April, having arrived in Scotland from Ya'an reserve in Chengdu, China, in December 2011. However, they failed to mate.
Female pandas ovulate just once a year, leaving a window of only 36 hours in which they can conceive.
The pandas are the first to come to the UK for 17 years, and are being rented at $1m per year from China, where fewer than 2,000 survive in the wild.
The zoo has put in place measures to synchronise the animals' breeding cycles, including controlled lighting, urine testing for hormone levels and swapping enclosures.
Iain Valentine, of the Royal Zoological Society of Scotland, said: "We're delighted that both Tian Tian and Yang Guang have started to scent mark and call, both clear behavioural indicators of courtship and mating.
"Although both giant pandas are showing these changes in their behaviour, it is way too early to give any accurate prediction on timings.
"However, early indicators do suggest the breeding season will probably fall in March this year.
"We could be as little as four weeks away, although equally the big day could still be as far off as eight weeks."
The animals spend their lives apart, keeping to their own enclosures and coming together only for the brief mating season.
Last year, the two pandas were brought together in a specially created "tunnel of love" between their enclosures. The pair showed interest in each other and wrestled "a lot", said observers, but ultimately failed to mate.