Animals can express what seems like a wide range of emotions. A cat angrily protests being served the wrong food. A dog stands dejectedly at the door when left behind. But do members of the animal kingdom outside of Homo sapiens mourn? Evidence shows that not only do animals grieve the loss of a loved one, they express it in many ways.

Animal grief is an issue on the minds of zookeepers at New York’s Central Park Zoo. As the New York Times reports, workers recently euthanized Ida, a popular 25-year-old polar bear who was suffering from liver disease. Now Ida’s companion of 24 years, polar bear Gus, is alone.

In 1994, Gus swam seemingly endless laps in his enclosure — the result, zoo officials believed, of stress and depression. The swimming ceased only after a therapist worked with the animal. Gus is now being closely observed to see how he reacts to Ida's absence. According to The New York Times, “With sticks, toys and other playthings untouched, he [Gus] spent Monday morning swimming between two rock structures, eyes peering out of the shallow waters as he drifted.” So far, officials from the zoo remain optimistic. Dr. Robert Cook, who helps operate the zoo, told the New York Times, “We haven’t decided what we’re going to do next as far as Gus goes. But he seems to be fine.’’

The question remains: do polar bears mourn? Readers weighing in on the question seemed less concerned with the validity of Gus’ emotions and more concerned about how to help him. As Mermaid7seas from Boulder, Colo., wrote in the comments, “I wonder if it would have helped Gus process Ida's death if the keepers/veterinarians had allowed Gus to see her body for a few hours, in their usual environment, and allow him to come to the realization that she had died, and then move on, as animals tend to do, naturally.” If Gus is feeling the loss of his mate, he will receive help from his zookeepers. What do other animals experience when they suffer a loss?

Marc Bekoff addresses this question in a blog on Psychology Today called “Grief in animals: It's arrogant to think we're the only animals who mourn.” Bekoff, a former professor of ecology and evolutionary biology at the University of Colorado, Boulder, cites several examples of animals displaying grief in the wild while mourning lost companions. He quotes expert Cynthia Moss, who tells of an elephant family that tried to rouse a slain member — to the point that one elephant stuffed a trunk-full of grass into her slain companion’s mouth. Bekoff cites Jane Goodall’s observation of Flint, a young chimpanzee who lost the will to live after his mother, Flo, passed away. Flint eventually stopped eating and died near the spot where he lost his mother.

One reader in the United Kingdom wrote to Bekoff about an unnerving explosion of magpie grief that kept him trapped in his barn for an extended period of time. The writer said that 20 magpies emitted loud cackling noises when they discovered their dead comrade. The grief built up until “this was echoed by a similar sympathetic chorus from a nearby wood and within a minute, from all surrounding areas giving the impression that hundreds of magpies were being told of the death and simultaneously expressing their grief.”

Research also supports the idea of animal grief. Karen McComb is an expert on animal communication and cognition at the University of Sussex in Brighton, England. As she told Animal Planet, “African elephants are reported not only to exhibit unusual behaviors on encountering the bodies of dead con-specifics, becoming highly agitated and investigating them with the trunk and feet, but also to pay considerable attention to the skulls, ivory and associated bones of elephants that are long dead.” Studying the elephants living in Amboseli National Park in Kenya, McComb and her team presented the animals with elephant bones mixed in with other large animals. The elephant bones received the most attention, even though the tusks had been removed from the skulls. However, the elephants were not able to differentiate the skulls of close relatives from other elephant skulls. (One could argue most humans could not do this either.)

Ultimately, it seems that some animals do mourn their dead — but not all animals express such concern for lost companions. As Animal Planet reports, lions have been known to briefly lick or sniff their own species ... before eating them.