Animals grieve for their friends, and other pets in their household, very similarly to humans.
Vet Tim Kirby was speaking after Lunchtime Live presenter Adrian Kennedy told his own story of having two dogs at home: Sparky and Molly.
Sparky became ill a few weeks ago and was sadly put to sleep.
"When we came home, Molly was a completely different dog - she noticed that something was missing," Adrian explained.
"Her brother Sparky was missing, and this was a dog that she would have spent literally every day of her life with.
"From that moment on, when she realised Sparky wasn't around, she went into mourning: it's the only way that I can describe what happened to our surviving dog.
"She wouldn't eat, she wouldn't drink, she wouldn't get up out of bed... she wouldn't even come out to the car.
"This went on for days on end, and I've never experienced anything like it".
Vet Tim Kirby told the show it is normal behaviour.
"Expression of grief is not unique to humans," he said.
"If you look at animals in the wild - such as dolphins, elephants, even birds - it's quite normal for them when one of the group dies that they almost do a little ritual and they actually mourn the death of their friend.
"With domestic pets like dogs and cats it's completely normal as well.
"I think it's only now we're becoming more aware of it; as we see pets are more domesticated, more humanised and essentially a part of the family".
Mr Kirby said an Italian study found behavioural changes in a household with more than one pet where another had died.
"In 90% of those families, the owner said there was significant behavioral changes in the pet that was left," he said.
"That could be a dog or interestingly a cat.
"Attention seeking was probably the most prominent, and the most significant change, that pet owners noticed in the pet that was left behind.
"There's been a lot of research and scientific data that shows the bond people have with their pet.
"It's only now we're looking at the bond between pets, and there's a massive overlap between them".
'Grieving takes time'
Dog behaviourist Suzi Walsh said there is one approach that can help a grieving pet.
"What can help... is if they actually see the another animal that's passed," she said.
"They actually see the dog that's passed on, and they actually see the dead body - which is not the nicest words to say - that can help a lot for animals.
"It's not always possible".
Ms Walsh said the pet left behind is likely looking for some closure.
"In Molly's eyes at the moment, Sparky left - she was probably aware that Sparky wasn't well and you were all emotional - but also he never came back.
"So that level of closure for her possibly makes the grieving part a little bit tougher.
"Grieving takes time, mourning takes time, it's good to get into your routine with your pet as much as possible.
"Just give them time," she added.