Nature is loaded with odd traits and behaviors. There are elephant trunks, the widely separated eyes of hammerhead sharks, and the wacky, effervescent mating dances that sandhill cranes do. But nothing is quite as strange as human crying.

It does not seem odd to us, of course. We do it often enough ourselves and witness someone else doing it nearly every day. According to one study of more than 300 men and women conducted in 1980s at the University of Minnesota, women cry five times a month or so and men about once every four weeks. And the first thing a baby does when it enters the world is bawl to let everyone know it has arrived healthy and whole. It is not the howling itself that makes our crying unusual; it is the tears that go along with it. Other animals may whimper, moan and wail, but none sheds tears of emotion--not even our closest primate cousins. Apes do have tear ducts, as do other animals, but their job extends only to ocular housecleaning, to bathe and heal the eyes. But in our case, at some point long ago, one of our ancestors evolved a neuronal connection between the gland that generates tears and the parts of the brain that feel, sense and express deep emotion.

Like all genetic mutations, the one that led to tears was a mistake. But it was a mistake that worked. If the wayward gene had not enhanced the survival of the creatures that inherited it, natural selection would have long ago kicked it to the curb. The question is: What advantages come with our special brand of teary-eyed crying? Recently researchers have begun to piece together some answers, and along the way they are uncovering some surprising insights into what makes us tick.

The reasons we cry are many. They range from the primitive--a simple signal of pain or distress--to the mysterious, a sophisticated and highly developed form of communication that bonds humans in ways no other creature can experience. Ultimately this type of bond helped our ancestors survive and thrive and in time allowed our species to emerge as the most successful and cognitively complex of all the creatures on the planet.

Hoots and Howls
Complex behaviors often have simple roots. Crying is one of them. Like other animals, we humans yowl to signal distress, and we start in infancy. During their first three or four months, before babies learn to smile or laugh or gesture, they cry often and with ear-piercing effectiveness. Later, as they edge closer to the first year of life, they cry less often, and they work out other ways to express what they want, such as pointing, grunting, or tossing spoons and food around. (Some babies cannot cry emotional tears until they reach three to six months of age or so.)

Infants develop different cries that send specific messages as they grow older--shrieks and screams of pain, or cries of separation, discomfort or hunger. Each serves as a kind of rudimentary vocabulary that precedes a baby's first words. They all trace their origins to the hoots and howls that other animals, including primates, still use as their primary way of communicating. This fact probably explains why electromyographic studies, which record the electrical activity of skeletal muscles, show that the nerves that operate the mentalis muscle (the one that makes our chins quiver when we are on the verge of tears) or put the lump in our throats or depress the corners of our lips (with the depressor anguli oris muscle) when we are upset are nearly impossible to control consciously. Scientists have also found that babies born without structures above the midbrain can cry, an indication that the roots of crying run deep into our evolutionary past to a time long before the apparatuses of speech and conscious thought emerged.

Cocktails for Crying
Our reasons for crying grow more varied as we enter adulthood. The deeper emotions that maturity brings seep into the mix, and the messages communicated by our cries extend beyond simple physical discomfort or the basics of survival. This transition does not mean physiology is no longer at work. It is, and it now has become more deeply tied to higher brain function and our increasingly subtle emotional needs. And the change means that tears themselves play a larger role as a signal to others that the emotions we are feeling are strong and genuine.

Emotional tears are one of three kinds of tears we produce. The other two share a similar chemistry, although they perform different functions. Basal tears bathe our eyes each time we blink. Reflex tears well up when we get poked in the eyes or when the fumes from the onions we are cutting irritate them. But emotional tears have a makeup all their own--one that provides some clues about their function. William H. Frey II, a biochemist at the University of Minnesota, has found that they carry 20 to 25 percent more types of protein and have four times the amount of potassium than reflex tears, as well as 30 times the concentration of manganese than human blood serum. They are also loaded with hormones, such as adrenocorticotropin (ACTH), which humans produce when under stress, and prolactin, which controls the neurotransmitter receptors in the lacrimal glands that release tears.

Frey believes these chemical cocktails are linked to the moods and emotions associated with crying. High concentrations of manganese, for example, occur in the brains of people suffering from chronic depression. Excessive ACTH indicates increased anxiety and stress. And higher levels of prolactin in women's bodies may explain why they cry more often than men do, especially after puberty.

Because so many hormones exist in emotional tears, Frey has speculated that crying is the body's way of flushing out the chemicals that are present when we are experiencing strong feelings. This is why, he says, we sometimes counsel one another, "Go ahead. Have a good cry."

But not all scientists agree. It is difficult to prove that tears alone can flush enough hormones from our bodies to provide the sense of relief we often feel after crying. Our tear ducts simply are not that big or that efficient. Even a good, long, heaving bout of sobbing produces only a thimble full of hormone-laden tears. So is some other mechanism at work that leads to the relief we feel when we cry?

Maybe, and it may not be all that mysterious. You might call it the Goldilocks principle. All natural systems struggle to maintain a state of equilibrium in the face of the forces around them. They work to remain neither too hot nor too cold, neither too active nor too lethargic. If the environment swings them in one direction, they counter by pulling back to the middle, "normal" ground as quickly as possible. Rain forests, guppies and humans all seek out their comfort zones. The very same primal need to maintain a middle ground may help explain why we cry.

The autonomic nervous system controls so-called mindless operations such as breathing and heartbeat as well as the basic functioning of organs such as the kidneys and brain. The autonomic nervous system itself is divided into two subsystems, the sympathetic and the parasympathetic. The role of both in crying is controversial but intriguing. The sympathetic nervous system prepares us for fight or flight--physically, mentally and emotionally. When we are scared, the sympathetic nervous system fires off messages that prepare our body to stand its ground and do battle--or to skedaddle. The parasympathetic nervous system then pulls us back to normalcy afterward.

Since the 1960s researchers have theorized that we cry because we are upset, not because we are seeking relief, and that our sympathetic nervous system must therefore govern weeping. But just as many scientists have held the opposite view. They argue that crying is an involuntary way of calming down. There have been plenty of studies, but none has been conclusive because it is difficult to induce and measure genuine grief and crying in a laboratory. Nevertheless, researchers such as James J. Gross of Stanford University have tried and subsequently have speculated that even though crying does seem to upset us, and those around us, it may ultimately have a calming effect. Other studies have shown that if the nerves central to the sympathetic system are paralyzed, patients cry more; when important parasympathetic nerves are damaged, they cry less. Those findings suggest that we do not cry because we are upset but because we are trying to get over being upset. In other words, crying resets the breaker on our emotional circuit.

If that is the case, then crying exemplifies the Goldilocks principle, physiologically at least. After all, following every fight or flight, every close call or every tense situation, we have to settle down. If we did not, we would blow an aorta or have a stroke and that would be the end of that. Given the dangers our ancestors coped with, a means for calming down would have been not only useful but also downright necessary; otherwise they might have been wiped out in a series of cerebrovascular accidents or a rash of coronary thromboses.

Jungle Truth
None of these findings precisely explains why we cry tears. Why should crying "hot tears" of emotion, as Shakespeare's Lear put it, make good evolutionary sense? They blur our vision and add to the vulnerability that our scrambled emotions have already created. Our social nature may provide a clue. No primate is more deeply bonded to other primates than humans are to one another. Our kind grew up on the savanna, not in the jungle, and had no shortage of dangers to encourage cooperation for survival. But we also compete with one another. Anyone who has been involved in office politics or high school cliques knows that. Our higher intelligence has only made our coalitions and competitions more complex. So our affairs, as they evolved from early hominid to human, must have favored traits that improve communication, from subtle body language and facial expressions to speech and... tears.

In 1975 Amotz Zahavi, a biologist at Tel Aviv University, conceived an interesting theory about how animal behaviors and traits that seem detrimental to survival often turn out to be perfectly useful. Why, for example, does a peacock have an enormous and colorful tail when the tail slows the bird down, draws the attention of predators and interferes with flying? Or why does a gazelle, when it senses a lion is about to attack, bound straight up into the air like a pogo stick before making its exit?

These traits and behaviors are examples of what Zahavi called the "handicap principle." On the surface they come at a high price--they require energy and resources and attract dangerous amounts of attention. But, Zahavi speculated, they also send powerful messages. Take the gazelle's first vertical bound, which puts it at an immediate disadvantage: it has lost precious moments that it could have used to distance itself from the predator that intends to make a meal of it. But such a leap also says, "I am so fast and can jump so high, you will never catch me. So don't waste your energy." Often the lion or cheetah poised for the kill absorbs the message, performs a quick, primal cost-benefit analysis and walks away in search of less vigorous prey.

Tears may serve a similar purpose in a species as intensely social as ours. They are noticeable, and the blurred vision they cause is a hindrance. That makes them costly. Because tears appear only when a person feels very deep emotions, they are not easy to fake. They send an unmistakable, Zahavian signal that the feelings behind them are absolutely real and, therefore, should be taken seriously. Tears, after all, reveal us at our most vulnerable. When we have reached the point where we are crying, the walls are down and our defenses have been breached. The intense emotional bonds forged partly by the binding ties of crying may have helped human communities band together more successfully than they would have otherwise.

Crying Wolf
Mothers tend to respond quickly to the tearless crying of their infants, who are so clearly helpless. But later, in toddlerhood, the situation changes. Crying, like all forms of communication, can be (and is) used to manipulate. Children, even as they grow older, want the attention of their parents. Because crying has been their most effective way of getting it, they continue to do it, even when they do not absolutely need help for basic survival.

Dario Maestripieri, a primatologist at the University of Chicago, has found that infant rhesus macaques share this behavior. They cry out to their mothers in infancy and tend to howl and whimper even more around the time their mothers wean them. At first, macaque mothers come running, but as the cries increase they respond less, because so many of the alarms turn out to be false. Eventually the macaque moms grow more skeptical, and the infants cry less because it does not bring the reassuring attention they seek. As a result, the young monkeys also grow more independent, which in the long run improves their chances of survival.

In the case of people, tears give mothers an extra tool for detecting if a toddler is crying wolf. Every parent has experienced the tearless crying (a.k.a. whining) of a child who is unhappy and wants attention but is not really in trouble. Parents quickly learn to look for real tears if a child cries, a sure sign that their toddler truly needs help.

Randolph R. Cornelius, a psychology professor at Vassar College and an expert on human crying, has done some of the most interesting research on tears as a kind of instinctual soothsayer. Since 2000 Cornelius and his students have been gathering still photographs and video images from news magazines and television programs recorded all over the world, all of them of people crying real and visible tears. When they find a particularly appropriate image, they prepare two versions: one, the original, with tears, and another with the tears digitally erased.

Cornelius and his colleagues then sit down with volunteers one at a time in front of a computer monitor to watch a slide show. Each slide presents two pictures: one tearful, the other a different picture with the tears secretly erased. No participants are allowed to see the same picture with and without tears. The investigators then ask each participant to explain what emotion the person in each photograph is experiencing and how he or she would respond to a person with that particular expression.

The test's observers universally registered that people in pictures with wet eyes or tears rolling down their cheeks were feeling and expressing deeper emotions--mostly sadness--than those who were tearless. But when participants looked at pictures in which the tears had been digitally removed, they were confused about what people were feeling and guessed everything from grief to awe to boredom. Cornelius's conclusion: tears append a crucial communicative dimension to our crying. They add one more true and powerful arrow to the quiver from which we draw our many forms of human communication.

Raw Emotion and High Intelligence
During the past six million years, enormous changes have taken place in our ancestral lineage, much of it from the neck up. Our brains doubled in size and then doubled again. Our faces also have changed, and with them so have our ways of conveying emotion. The rich, expressive musculature evolved by chance but remained with us because it helped us more precisely communicate with, and sometimes manipulate, one another. The parts of the brain associated with the experience and expression of emotion somehow became connected, quite literally, to the lacrimal gland that sits above each of our eyes.

Complex relationships beg for similarly complex forms of communication. For our kind, language was one mighty adaptation that served that purpose. Tears, with the strong, highly visible messages they send, became another. They married raw emotion with a human brain capable of reflecting on those howling, primal feelings. They help us express overwhelming emotions that well up from the primal side of us and linger beyond the reach of words. We all know the feeling, whether it is profound sadness, frustration, joy, pride or pain. Tears take us where syntax and syllables cannot. Without them, we would not be human.