Other animals cannot achieve an emotional state as complex as human depression. Right? Human depression involves states such as sadness or guilt. It is unknowable whether other animals can feel sadness or guilt because they cannot tell us they are sad or guilty. The difficulty of getting inside the animal mind, and the pride of human uniqueness, has made many resist the idea of depression in other species (despite the fact that we test antidepressant drugs on other animals).
Papers just published in Current Biology provide new evidence concerning chimp grief that show remarkable parallels between chimpanzee and human behavioral reactions to the end of life. The most riveting is a painstakingly detailed month-long observation of three captive chimpanzees (Blossom, Rosie, and Chippy) as a fourth chimp, Pansy, became gravely ill and died. The zookeepers decided to allow the other chimps to stay with her as she died, while a research team, led by psychologist James Anderson, observed their reactions. The fact that the chimpanzees were continuously, systematically, and meticulously observed over a thirty-day period lends the paper greater credibility than previous reports, which were largely anecdotal.
There is a striking passage that describes the reactions of the chimpanzees after Pansy died.
The next day the three surviving chimpanzees were profoundly subdued. From the day area they watched silently as two keepers lowered Pansy from the platform, carried her into the exit corridor, placed her in a body bag, and loaded her into a vehicle that was then driven away. They remained subdued the following day as the night area was cleaned and disinfected, and new straw provided. When the connecting doors were opened Blossom and Rosie entered hesitantly, but Chippy refused; instead he showed fear grins and made loud alarm calls, causing the two females to quickly return to him. The doors were left open, but the chimpanzees slept in the day area, and Chippy again refused to enter the night area the following day. For five consecutive nights no chimpanzee nested on the platform where Pansy died, yet this platform had been used for nesting on every evening of the 29-night study. Rosie was the first to resume nesting there.
As the report continues, it can be easy to forget that these are chimpanzees and not a human family touched by death.
During Pansy's final days the others were quiet and attentive to her, and they altered their nesting arrangements (respect, care, anticipatory grief). When Pansy died they appeared to test for signs of life by closely inspecting her mouth and manipulating her limbs (test for pulse or breath). Shortly afterwards, the adult male attacked the dead female, possibly attempting to rouse her (attempted resuscitation); attacks may also have expressed anger or frustration (denial, feelings of anger towards the deceased). The adult daughter remained near the mother's corpse throughout the night (night-time vigil), while Blossom groomed Chippy for an extraordinary amount of time (consolation, social support). All three chimpanzees changed posture frequently during the night (disturbed sleep). They removed straw from Pansy's body the next morning (cleaning the body). For weeks post-death, the survivors remained lethargic and quiet, and they ate less than normal (grief, mourning). They avoided sleeping on the deathbed platform for several days (leaving objects or places associated with the deceased untouched).
If a chimpanzee behaves as a human would in a situation (death of a relative) that would be a strong trigger for human grief, how much of a leap is it to infer that the animal may also experience an internal state akin to the human experience of mourning (sadness)?
I think the leap is very small.
Can these controlled observations of grieving chimps tell us about the building blocks of human sadness and depression? Lest one forget, while life is billions of years old, mammals have only been around for 300 million years, and humans only for a few hundred thousand. Given the conservatism of evolution (we share nearly 99 percent of our genes with chimps), most of our adaptations developed in species that did not have language or culture. Mood is a prime example of an adaptation that antedated language (which explains why it's not so easy to reason one's way out of a depression!).
Moods have stuck around because they have function. High moods lead us to more efficiently pursue rewards. Low moods focus our attention on obstacles in our path. Largely because we equate mood with the language that is used to describe mood, we tend to balk at the idea that other species have mood as well. Yet language is not necessary to benefit from the organizing activity of moods. Surely, the capacity for mood helped our mammalian ancestors pursue survival-related goals even before there were labels for misery or elation—they got a mood boost when they were on track to find a mate, food, or a new ally, and a mood drop when their efforts weren’t paying off, when they were in situations where there was nothing to be done.
In Pansy's survivors, we see reactions to the profound loss of a close relative, a situation that could in the wild imperil survival for the entire group. It makes sense that such a loss would trigger the suite of behaviors so nicely documented in the Current Biology article. Is it too much to consider this sadness? Without being able to communicate with Blossom and Chippy, the evidence will always be presumptive, but everything about the chimps' behavior suggests that they are inhabiting a low mood state in which their normal activity is interrupted and they are compelled to draw together to analyze and cope with a loss. Sadness wraps us humans in a cocoon in which we pause and analyze what has gone wrong. It makes good sense that chimpanzees would be equipped with the same cocoon.
Does it matter that the chimps cannot report that they are sad? In this respect they are like a nine month old infant human, or a human adult with advanced Alzheimer's who lack the capacity to describe their internal state like a linguistically competent human adult could. If we deny these chimpanzees sadness, we must deny sadness to the millions of humans who cannot report on their feelings.
Does it matter that the precipitants of human sadness are more idiosyncratic than the precipitants of chimpanzee sadness? From the bride left at the altar to an unexpected layoff to a world series defeat in extra innings, yes, the antecedents of human sadness are almost ridiculously multifarious. And yet there is a core theme of loss that cuts across species. In this sense, we are all Pansy's survivors.