Rats giggle. Dogs wag their tail. How do other animals express joy? You can't ask them, so researchers have to find other sneaky ways of figuring out animal emotions. Professor Ximena Nelson is studying how curious and intelligent kea, New Zealand's alpine parrots, might show positive feeling.

Some rats giggle when their bellies are tickled. Researchers have known it for some time now, though it requires special microphones to detect this - they giggle at an ultrasonic frequency.

It's just one way that one animal expresses joy. How might others be doing it?

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Negative emotions are a bit easier to study, says Professor Ximena Nelson from the University of Canterbury. The animal might yelp or flinch, retract a limb, or shy away. But positive emotions are trickier. You can't ask the animal, and you can't look for those telltale signs in humans - a smile or crinkle around the eye, especially if the animal has completely different facial muscles, as is the case for kea.

It's a multi-step puzzle. Starting with: is the animal even likely to experience positive emotions? For kea, Ximena had her own anecdotal observations of them playing in the mountains as the first clues that they can be joyful. From there, she and her team investigated the different calls that kea made, and then analysed the different behaviours of the kea as they made these calls, to try to discern their functions.

This proved extremely difficult, but one call did seem to be strongly linked to play - the warble call, or kea laughter as it is called. To confirm this, the next step was a series of playback experiments. Design a kea-proof speaker, bring it high into Arthur's Pass, and then investigate what the kea do when you play different calls and other sounds to them.

Turns out the warble call is like a play contagion. Kea were more likely to start playing after hearing the call, and to warble themselves.

Ximena is now focused on trying to determine if this is a reflection of positive emotion such as joy, and if so, does it influence the bird's decision making, as it does in humans? This trickier second step will have to be done using captive kea that can be trained to take part in a glass half-empty or half-full experiment.

Listen to the episode to learn about the how and the why of studying positive emotions in animals.

To learn more:

Listen to this episode from Alison Ballance, who visited Ximena's spider lab to learn about jumping spiders

Listen to 'Kea get a helping hand' which covers citizen science efforts to help curious kea

Go to this episode on rnz.co.nz for more details