The categorical boundaries between humans and other animals continue to disappear the more we study the animal kingdom. Several animals are recognized tool users, such as apes, crows, elephants, dolphins and sea otters, among others. Other creatures, like orcas and songbirds, even have recognized culture.

Now yet another sacred boundary has fallen: the high-five. Researchers studying keas, giant parrots found in New Zealand, have witnessed these highly intelligent birds performing acrobatic, aerial high-fives while playing. (Actually, they perform high-fours, because parrots only have four digits on each foot — but we won't hold that against them.)

Performing high-fives might not seem all that profound — this is only a half-serious anecdote, admittedly — but there's more to it than that. Researchers noticed that the high-fives also coincided with an eerily human-like behavior only ever seen in some primates and rats: spontaneous, contagious laughter, reports New Scientist.

“Our finding further bridges the perceived gap between humans and [other] animals, and shows that [contagious laughter] also happens in birds, which are very distantly related,” said Raoul Schwing of the University of Veterinary Medicine in Vienna, one of the researchers who made the discovery.

Laughter is contagious for these parrots, too

The observation was made while researchers played five-minute recordings of kea calls to gatherings of the wild birds on a mountainside of New Zealand’s Arthur’s Pass National Park. Among those calls were distinctive kea "warbles," sounds that are associated with play. The warbles sound kind of like parrot laughter, and it turns out that comparison might not be far from the truth.

“On hearing the calls, many birds started to spontaneously play with non-playing birds, or with an object close by, or by performing aerial acrobatics,” explained Schwing.

The observations yielded close parallels with the emotional contagion often witnessed in humans, such as the glee-like excitement often seen in children at play. It was clear from the research that the warbles were what prompted the birds to play (which included all those high-fives), but the warbles weren't identified as commands. In other words, the birds weren't using the calls to communicate an idea, such as "hey, come play with me!" Rather, the calls operated like laughs, which made other birds laugh, and so on. The laughter changed their moods as it passed from one bird to the next.

Interestingly, contagious laughter isn't the only human-like trait found in these remarkable birds. Keas have also been witnessed using tools, and they've been shown to be adept at solving logical puzzles.

The study was published in the journal Current Biology.