Yang Guang (Sunshine) recently began doing handstands against trees, walls and rocks, and scent-marking as high up as possible - known as displays of virility in the wild.
At the zoo today, Yang Guang could be seen looking through a closed gate linking his enclosure with Tian Tian's.
He was also reclining on his rock eating bamboo, increasing his daily intake from 80lb per day to 110lb. This will rise to 220lb as breeding gets closer as in the wild he would have to travel to a female's territory and possibly face other males.
Meanwhile, Tian Tian has started calling out to him, which is common during the breeding season and taken as another good sign.
Female pandas ovulate just once a year, giving a narrow window of less than 48 hours in which they can conceive.
Keepers are able to predict when both giant pandas are ready to breed by a combination of behavioural observation and hormone testing, but so far no hormonal changes have been seen in either panda.
The pandas were introduced to each other last April but they did not end up fully mating.