When Jay Parker told his wife Lauren to say cheese, it seems someone else was listening, too.
Otis the hippo pressed his face against the glass of his tank and produced what appears to be a beaming smile.
And when the couple exchanged positions, the three-ton beast duly obliged with another grin. The Parkers, from Little Rock, Arkansas, took their extraordinary photographs during a holiday visit to San Diego Zoo in California.
After posting them on the internet, they soon found them popping up on blogs and chatrooms from Chattanooga to China.
Mr Parker, 5, said Otis and a female named Funani had only recently been put in a tank together and had been battling for supremacy - hence the scars on his back.
'When I look at his face I think he looks pretty tired by it all, or maybe he's having a happy dream - either way it's a great picture.'
'We took the picture on holiday at San Diego Zoo, my wife is a big fan of animals so we always try and visit somewhere when we are away,' said Mr Parker.
'Actually the week we were there the zoo had just put two hippos together for the first time in the tank.
'Hippos are not usually that friendly and the two they had in the tank - the male Otis and the female Funani - had been having a bit of a fight with each other that day.
'The hippo behind me is Otis, when we saw his face we just thought we had to get a picture.'
Mr Parker said he wasn't sure if Otis was happy or not after his day's ordeal with Funani - in fact he might even have been asleep.
He said: 'If you look at Otis's back you can see the scaring from his fight with Funani, this is normal hippo behaviour but I don't think he won.'