Do animals mourn their dead?

DW: I have an aquarium with piranhas at home. Recently, when one of them died, the other six behaved quite strangely. They were unusually calm and refused to eat. Were they grieving for their companion?

Frans de Waal: I don't think so. Piranhas also take bites out of each other; I don't think they are very friendly with each other. In general, grieving is unlikely in fish - unless you have individually bonded fish which might be possible in some species.

Why were they behaving so weird then?

There is something called Schreckstoff - it is a substance that fish release when they are distressed. It is possible that your fish were just influenced by whatever happened to the other fish, in a more physiological way.

What is the difference to 'real' grieving?

Typical grieving happens with mothers and offspring in mammals. Usually, you find grieving with animals who have individual attachments, not just schooling or flying together, but having friends. All mammals have these bonds to some degree, all birds as well, as birds are very often pair-bonded. If the partner dies, they are very affected by it.

What if an animal's companion belongs to a different species? These stories of dogs who mourn for their owner - is that only wishful thinking?

No, I think that is an absolutely true thing. There was the dog Hachiko in Tokyo in Japan. After his boss died, the dog came to the train with which he normally arrived - for about 10 years. Whenever you have attachments, be it between a dog and a human or a cat and a human, you can have grieving.

Do animals mourn like humans do when their companions die?

'Mourn like humans do' is a big statement. They are distressed. With primates like chimpanzees, it is not unusual that if one of the primates in a group dies the others stop eating for a couple of days. They become completely silent, they stare at the body for a long time, they try to revive the body. That is typically human - we don't do that anymore but in the old days people did that.

So grieving in animals lasts for a few days?

If it is a very important partner like your best friend or your offspring then it can last much longer, it may last for years. I knew a female who lost an offspring and for months was sort of crying about it. It has a very long-term effect.

How do we know that those animals mourn? Maybe they just got out of their routine because something is missing in their life?

I remember a story where a baboon mother lost her baby to a predator. Weeks later she came back to the same area where she had lost her offspring and she climbed into a high tree and started calling. That gives an indication that she remembered what had happened there and that she was missing her offspring. With primates we often have the impression that they specifically remember the individual.

Do you think they realize that their companion will never come back?

The one thing about death that primates certainly understand is the permanency. That once an individual is dead, they don't move anymore, they're dead. I think they understand that.

How do you know?

I'll tell you an anecdote about it. Some bonobos found a very dangerous snake in the forest and were very scared of it, poking it with sticks. At some point, the alpha female, which is dominant over the male, took the snake by the tail, hit it against the ground and killed it. From that moment on, the young bonobos picked up the snake, hung it around their necks, walked around with and started to play with it.

That indicates that they know that it's a dangerous animal that you should be very careful of - but once it is dead, you can play with it. So I think they understand that death is a permanent condition.

Do apes also realize that they themselves have to die some day?

It is hard for us to know but there is no indication that they have this kind of understanding.

Do other species like birds mourn in the same ways as primates do?

Some birds who mate for life sometimes even stop eating and die if their partner dies. This is true for geese but also for many songbirds, they have long-term bonds.

Which animals do you think mourn the most strikingly?

I would say elephants, because they go back to the bones of the ones that they have lost. If an elephant dies - and at the moment with poaching that is a typical occurrence - the other elephants inspect the bones of the dead elephant if they can find them. I'm not sure, though, if anyone has done research on whether the elephants return just to any bones or bones of specific individuals that they have known. But that would be my guess. They go back to the bones, a bit like us going to a graveyard.

Do some animals also bury their dead by digging a grave?

No, they don't dig a grave. It is possible that they throw stuff over their dead in order to cover the body. That appears to be like an anti-predator defense, in a sense that a smelling body would attract predators and scavengers. But I don't know if they do that systematically.

Digging a grave is a typically human thing then?

Yes, that's for sure. Recently, there was the discovery of Homo naledi, a human ancestor. The team claimed that they buried their dead which is a true indication of humanity. Even though there are a lot of doubts about that claim now, actually.

Does it help the cause of conservation to know that animals mourn?

Everything we realize about animals in terms of their emotional lives or death cognition helps - in a sense that it makes the animals more complex or human-like and more attractive to people. All this knowledge contributes to the way we look at animals and may change the way we look at the treatment of animals. It has ethical implications, so to speak.

Frans de Waal is a Dutch primatologist and ethologist. He is professor of Primate Behavior in the Emory University psychology department in Atlanta, US, and director of the Living Links Center at the Yerkes National Primate Research Center. He works mostly with chimpanzees and bonobos and is author of several books including "Chimpanzee Politics" and "Our Inner Ape."