This is an interesting question and one that is very well phrased. Humans are primates and it is important to consider our behaviour within the evolutionary context of other primates. But it is also necessary to define what we mean by crying. If crying is defined as the act of tears coming from the eyes, then simply, the answer is yes: tears appear to be unique to humans among the primates. The more interesting and complex answer, however, concerns what crying can mean in terms of emotion, emotional expression, and/or feelings.
We can think of crying in two ways: as an emotional expression, with or without feelings (for example, of sadness, distress, or pain), or as a communicative signal (for example, of vigorous health of babies, or with communicative intent to invite caregiving and/or solace). In terms of emotional expression, crying might include any or all behavioral indices of distress (such as vocalizations, body movements, and facial expressions), or indices of sadness (for example, depressed body postures including slumping shoulders), or of pain. Crying as an emotional expression has been used to describe the vocalizations of many primates, including the coo vocalizations of squirrel monkeys and the whimpers and screams of chimpanzees. Crying has also been used to describe the vocalizations of monkey and ape infants when they are being weaned, and when they are separated from their mothers (either temporarily due to losing sight of the mother or permanently due to maternal death).
Scientists commonly try to avoid attributing interpretations of human behavior when trying to understand the behavior of other primates. This is especially true when it comes to emotions. Indeed there are researchers who will not use any anthropomorphic words for other primates, reserving the emotional terms for humans alone. Some deny that other primates have feelings. Most, however, take a conservative stance and say that it is too difficult to tell whether they do have feelings. Moreover, if they do have feelings, it is difficult to discern whether they are feeling anything similar to our feelings. Many scientists thus refrain from using the word crying when referring to nonhumans, preferring to talk about distress vocalizations, or describe the acoustical properties of the sound.
In sum, if we define crying as tearful sobbing, then we know that humans are the only primates that cry. If we define crying as emitting vocalizations that co-occur with distressing situations, then we can conclude that most monkeys and apes cry, especially as infants. If we are really asking whether humans are the only primates that feel sorrow, then we do not know the answer. Yet.
Answer originally posted on June 16, 2003.