Animals Wide-open dolphin snout signals playful behavior

It's all just a game: it can be vital to convey this message to a defensive conspecific. Dolphins use an expression known from primates and wolves.

Dolphins may appear as if they are always smiling - in fact, very little can be read from their facial expressions. However, there is at least one clear signal: according to a study, the marine mammals like to open their mouths when playing - which looks as if they are openly smiling at their counterpart.

The expression is mainly used when the respective playmate can see it, reports the research team in the journal "iScience". It is then often reciprocated by them.

To humans, it looks as if dolphins are always smiling. In fact, the facial expression is an innate anatomical characteristic and says nothing about the animal's emotional state. They supposedly smile even when they are under severe stress or in pain.

The scientists led by Elisabetta Palagi from the University of Pisa have now observed bottlenose dolphins (Tursiops truncatus), which really enjoy playing. They filmed and analyzed how animals living in captivity played with each other, alone or with their trainers.

According to the study, the bottlenose dolphins mainly show an expression called "open mouth" during social play with other dolphins or - more rarely - humans, almost never during individual play.

If the playmate could see the open mouth, it often opened its mouth as well - which gives the impression to humans that the animals are laughing happily at each other.

The results showed that the open-mouth expression also occurs in dolphins and that bottlenose dolphins are probably able to mirror the behavior of conspecifics, according to the researchers.

Playtime is training time

Many animals like to play extensively, especially during their childhood. This helps them to train the movement sequences they will need later on - for hunting or fighting with conspecifics, for example. Social interactions and hierarchies are learned, and courtship and mating behavior can also be tested through play.

In some animal groups, such as wolves, primates and dolphins, adult group members also often play. Communication is of central importance here: after all, the behaviors and facial expressions used sometimes have patterns typical of aggression - friendly play can unintentionally degenerate into a real fight.

One example of such communication is the open-mouth expression, which is probably derived from biting: a wide-open mouth, but where the teeth are not shown threateningly and the overall facial expression remains relaxed. It signals to the other person that all wrangling and anger is to be understood as fun. The expression can also be seen in humans, the researchers explain: when laughing.

Lifelong jokers

Bottlenose dolphins are also in the mood for fun throughout their lives. They use natural waves for "surfing" or swim in front of ships in their bow wave, jump several meters high out of the water and hit the water surface with their tail fin.

Seaweed, coral and other marine animals are also popular play objects. The dolphins also create air bubbles under water, which they play with. They prefer to do all this together with others rather than alone.

Dolphins have developed one of the most complex vocal communication systems in the animal kingdom: they communicate using various underwater sounds such as clicks and whistles. This enables them to communicate even in murky waters where visual signals would be difficult to recognize.

So why do dolphins use open-mouth expression at all when playing? The researchers assume that they risk attracting the attention of predators if they make sounds. This is particularly unfavorable during play, when less attention is paid to the wider environment than usual.