The Sting of the Wild

by Justin O. Schmidt

  AMONG THE SMALL SUBFAMILY OF SLENDER, wasp-like ants, the Pseudomyrmecinae are species with two strikingly different lifestyles. One, represented by Pseudomyrmex gracilis, consists of small colonies of relatively large ants secreted deeply inside protective twigs and stems. They forage mainly for sugary foods and timidly retreat at the least threat. The other, the bullhorn acacia ants, Pseudomyrmex nigrocinctus, are small ants that form large dispersed colonies that live within the large, hollow thorns of the acacia tree. Bullhorn acacia ants live off extrafloral nectar (nectar secreted from areas not within flowers) and protein-rich Beltian bodies (named in honor of their discoverer, Thomas Belt) of the host tree. These ants strongly defend their home and food supply—the acacia tree—from all predators, competitors, and intruders that might harm the tree. A mutualism evolved in which the plant houses and feeds the ants and the ants protect the tree. One prediction based on the hypothesis that the sting of social wasps, ants, and bees evolved in response to predation pressure is that social insects, having much to lose, should have more painful stings than those with less to lose. Pseudomyrmex ants provide an ideal test. Both the large gracilis and the small acacia ant are closely related and are in the same taxonomic genus. Their main difference is their lifestyles: one has little to defend; the other has much to defend. The prediction is that, despite the great size difference between the two species, the stings of the smaller bullhorn acacia ant would hurt more than the stings of the large gracilis ants. Fortunately, I had the opportunity to test this hypothesis in the tropical deciduous dry forests of Guanacaste, Costa Rica, and in Florida. When I touched an acacia plant in Costa Rica, the ants immediately swarmed up my hand and arm, stinging along the way. It was not possible to get the ants off fast enough. And the stings hurt—and in massive numbers, stings really hurt. The rapid multitude of stings tremendously enhanced the pain. Pseudomyrmex gracilis would not even sting; instead, it treated my arm as a tree branch and ran to the far side of the arm attempting to hide. When I grabbed it, it stung me, but the sting was trivial. This difference in painfulness, despite a twofold difference in weight between the two ant species, was clear. The prediction was supported, albeit a bit painfully.

  THE ECONOMY OF LIFE PRODUCES astonishing outcomes. Among these is sting autotomy, the gruesome process in which a stinging insect self-eviscerates, leaving its stinger embedded in the target’s flesh. This suicidal behavior troubled Charles Darwin as he formulated his theory of natural selection. He pondered how killing oneself could promote passing fitness via descendants to future generations. An insect’s self-evisceration could provide strong evidence against his theory. Amazingly, even though Gregor Mendel’s genetics, much less the modern concept of DNA, were unknown to Darwin, he came up with essentially the correct answer. By facilitating the reproduction of your close relatives, mainly nestmates, your lineage would be passed down via relatives, because of your selfless sacrifice. Sting autotomy maximizes the pain and damage of a sting, thereby aiding in the defense of the colony against large predators.

  Life’s economy promoted learning and decision-making in both stinging insect prey and in their vertebrate predators. If an important prey becomes too “spicy,” a predator has two choices: (1) it can abandon predation of the prey and lose meals, or (2) it can learn to avoid the spicy stings and keep its meals. The latter choice would be the obvious favored choice. Intelligence is not simply an accident of evolution. It comes with some cost from increased numbers of brain neurons and energy consumption. Consequently, intelligence must provide some benefit. One benefit is that some form of intelligence is required for learning. Learning, in turn, is valuable for future encounters, or potential encounters, with predators and prey.

  One mid-May morning, as I was sitting in front of my computer writing, I glanced out the window just west of the monitor and noticed an alert western kingbird sitting on a dead branch. The bird was actively looking to the right and then to the left. Beautifully clad with gray caps and golden breasts, kingbirds are master aerial acrobats, snatching flying insects from midair. The bird was sitting above the flight path of an Africanized honey bee colony located on a mesquite tree 10 feet to the south. Periodically, the kingbird would fly to the north and out of sight, quickly returning to its perch. Each time it raised its head and swallowed something that looked like a bee. How could it do that? Bees sting. Bee stings hurt. Asian and African bee-eaters, a colorful family of birds, catch bees in their long bills and then bash their abdomens, where the stinger is located, on a tree branch. The assumption is that this action “defangs” the bee so that it cannot sting and purges it of venom. Could there be more to the story? Humans are generalist feeders derived from a long lineage of entomophagous primates. We are model predators with our own generalist predator sense of taste representative of other general predators. To test the idea that there is more to honey bees than their stings, I captured a batch of bees from the flight path observed by the kingbird and froze them. (I did not want to be stung while eating them.) Next, a bee was divided into its three body parts: head, thorax, and abdomen. I chewed each part in turn to get a good sampling of flavor before spitting out the hard fragments of the body shell. Wow. Heads tasted like nasty, crunchy finger nail polish. The thoraxes were palatable, albeit the wings and legs added an unappealing plastic crunchy texture, and the abdomen flavor resembled a horrible combination of turpentine and a corrosive chemical. These flavors originate from exocrine glands in the worker honey bee. The head produces a mandibular gland pheromone composed of ketones, the thorax lacks large glands, and the abdomen contains the venom and the Nasonov gland that produces lemon oil compounds. No wonder predators might not choose worker honey bees: even if they don’t sting you, honey bees taste terrible.

  Back to the kingbird. Could it be deterred by the noisome taste of honey bees? If so, then what was it eating during these brief sorties in which something went down the hatch? As fortune has it, kingbirds, like owls, lack a grinding gizzard and regurgitate hard fragments from their meals. The fragment pellets are dropped below their roosts. These pellets can be soaked in water and microscopically analyzed to determine the diet composition. The dense swath of bunny-ears cactus (a particularly unpleasant cactus with thousands of nearly invisible, glochid, or prickly, spines) below the kingbird perch was removed and replaced with clear plastic sheeting. Sure enough, within several days, numerous pellets were deposited. The pellets contained head capsules of 147 male bees and not one from a worker (the two are readily distinguishable by the round shape and huge eyes of males and the guitar-pick-shaped, small-eyed heads of workers). Male honey bees cannot sting, lack large exocrine glands, and when eaten have a custard-like taste and texture with some added crunch. Overall, males are quite palatable. The kingbird solved the problem by learning to distinguish male from female bees while on the wing and only preyed on the males.

  Learning and decision-making occurs not only in vertebrates generally recognized for their intelligence but also in prey of vertebrates. Honey bees are well known for their ability to learn to forage at the best times, at the most rewarding flowers, and most efficiently to obtain the nectar from the flower. Could they also learn risks posed by predators and make adaptive decisions based on that learning? To test this, mature honey bee hives were threatened but not harmed. Threats consisted of blowing my breath directly into the hive entrance. I had discovered that mammals’ breath odor is the single strongest stimulus for bee defensive attacks. The hives themselves were never touched or otherwise threatened or harmed. After threatening the colony by breathing into its entrance, I stepped back 6 meters and swept up all attacking bees with an insect net. Each day for two weeks this process was repeated at the same time of day. Enormous numbers of bees attacked during the first two days. By the third day, many fewer attacked. On all subsequent days, very few bees attacked. The number of bees in the colonies was about the same at the end of the two weeks as on the first day of the experiment. The bees had learned that my predatory threats
were harmless and did not deserve a strong defense. Similar learning is routinely observed in South Asia where giant honey bees, Apis dorsata, commonly construct their hives above entrances to religious temples. Although people regularly enter and leave the temples with their heads passing only a few feet from the combs, the bees do not attack. These giant honey bees have also learned to evaluate risks posed by predators. I have never read of anybody reaching up to touch a comb but suspect that would be a most unwise decision.

  4

  THE PAIN TRUTH

  To one accustomed to the flora and fauna of the forests of temperate

  regions, the forms, colors and odors of the tropical forests are a continual

  surprise. Fantastic forms and wonders everywhere meet the eye, until

  presently one becomes like a child wandering in fairyland, accepting

  seemingly impossible things as a matter of course. —Phil Rau,

  Jungle Bees and Wasps of Barro Colorado Island, 1933

  EVERYBODY KNOWS WHAT PAIN IS. It is the sensation felt when a knee is scraped in a fall, the skin is overexposed to the sun, or a bare foot steps on a bee. Pain is familiar, yet mysterious. We know pain when we sense it. Pain is clearly recognizable. Warmth is not pain, though it can become pain if too much heat is experienced. Likewise, chills caused by cold temperatures are clearly unpleasant, but they are not pain. Cold, like warmth, can produce elements of pain but is not classic pain. As the tobogganing kid knows, toes get wet, cold, and unpleasant but not usually truly painful like a stubbed toe. But, oh wait until the true pain comes as the toes warm before a toasty fireplace. Though we might call stomach upset and nausea painful, is it really pain? We call nausea “pain” for lack of a proper descriptive word, but everybody knows nausea pain is different and, I would argue, far more unpleasant than the pain of a runner’s stitch caused by the spleen contracting to squeeze more red blood cells into the bloodstream to provide acutely needed oxygen to muscles.

  We know pain when we feel it. Do we know physiologically and medically what pain really is? The response gets murky. Describing actions that cause pain—slamming one’s finger in a door, for example—is simple. Distinguishing pain from nonpain is generally easy. Hunger, though often called pain, is certainly different from the pain of a stomach ulcer caused as acids eat into and destroy stomach tissues. Again, we lack a common, distinguishing word for hunger pain, though perhaps “hunger pangs” is a proper phrase that when spoken sounds the same as “hunger pains.”

  There may be no universal consensus on what is and what is not pain. We generally recognize pain as a distinct experience, one that presents in a variety of flavors. Common pain is the sensation felt when skin is damaged, a tooth injured, a bone broken, a muscle pulled, the spleen makes a stitch, or a variety of other mostly dermal or skeletal-muscular problems. Another broad category, visceral pain, is experienced when visceral organs signal damage or potential damage. The visceral pain resultant from tonsillectomy in adults, hemorrhoid surgery, childbirth (I am told), or other sources one hopes to rarely experience is distinctly different from common pain and a considerably less pleasant form of pain. Headache pain is another category of pain. The point of this discussion is not to define pain concretely, to make a pain phylogenetic “family” tree, or even to claim that all the separations above are clear-cut (they are not); the point is to illustrate just how complicated and murky pain is.

  How is the nebulous and murky nature of pain explained? What is the cause of this lack of tidiness, both in descriptive language and in real-life sensation? Several poorly defined explanations can be offered. A medical explanation might focus on the separate pathways of nerves and distal structures between the motor and autonomic nervous systems and the sensory nervous system. Action potentials sent from the brain to muscles travel down different nerve axon pathways than pain signals generated when the tongue is bitten. Signals to the brain originate from receptors located throughout the body. Many of these receptors are sensory receptors that detect temperature, pressure, stretch, chemicals, itch, or a variety of other sensations, including pain. The signals from these receptors are transmitted to higher nervous centers in the spinal cord and brain through fine nerves of the separate sensory nervous system. Matters become more subtle. Pain and itch, for example, are separate sensations.1 Are they related, that is, is one just a smaller degree than the other? No, they are not simply degrees of difference, and, unfortunately, how they relate is unclear and an active topic of research. Is the tickle sensation related to the pain and/or itch sensation? Again no tidy answer is forthcoming. Complicating the issue further is the feature that tickling can be a pleasant sensation, especially in social situations, or it can be an excruciatingly unpleasant experience. How are the two tickle responses related? In degrees of stimulus? The difference is unclear.

  Is pain always unpleasant or, as pointed out by my grade 7 science teacher, can it also be pleasurable? A love-hate situation occurs when baby teeth are about to fall out and to be replaced by permanent teeth. The loose tooth hurts, but the urge to wiggle it is irrepressible. We can wiggle the tooth just enough to cause a little pain, an enjoyable sort of pain, but not too much. And we can control this dynamic precisely and entirely ourselves. What is the difference between the two tooth pains? Is it simply strength of the nervous signal emanating from the tooth receptors? Probably not. Here, other important players in the pain system come into play, the higher processing centers of the spinal cord and brain. These centers filter and process the signals to determine the importance of the signals and then send them to our conscious centers of the brain. If the signal indicates a dire situation, as in a hand placed on a hot stove burner, the processing centers convey outside the conscious pathways to the action centers to signal reflexive removal of the hand. The conscious centers are involved in the process of learning for directing future behavior to avoid placing the hand on a hot object.

  Pain serves a higher purpose in the biology of life than revealed by analyses of nerve pathways and processing centers, however interesting these might be. Why should pain, a most universal sensation in living animals, exist at all in nature? Certainly not for pleasure or torture. Only adaptations and sensations of value for promoting the life, survival, and reproduction of an organism stand the test of time. Pain is a basic sensation of life, experienced by all animals. Even the simple single-celled paramecium moves away when it encounters the high acidity from the drop of vinegar placed in its watery bath, just as we jerk our fingers from a hot stove. Does the paramecium experience pain? Certainly not in the way humans do, as it has no brain or self-awareness, but it responds the same as we do to the negative situation, so in practical terms, we can call it a pain response. In biological terms, pain is simply the body’s warning system that damage has occurred, is occurring, or is about to occur. Nothing more. Pain is not damage. It is merely a harbinger of damage. Is pain truth? Perhaps. If damage occurs concurrent with pain, then pain is truthful in sending the honest signal that the body is at risk and has been compromised. A bruised shinbone sends truthful pain.

  What if pain is intense and no meaningful damage occurred? Is that pain truthful? This paradox of the veracity of pain’s role, damage is about to occur, is just what stinging insects exploit. Returning to the bee and the foot, the sting to the sole elicits pain and lifting the foot is a response that benefits the bee (well, maybe no longer to that bee, but to her nestmates). Has meaningful physical damage to the person been done by this sting? Often, the answer is no. Stinging insects are masters at exploiting this weakness to their benefit in the honesty of the pain signal. To stinging insects, we might simply be fools who fall for the trick. To us, it is better to be safe than sure; thus, we believe the signal is true. If the damage were real, the downside cost could far outweigh any benefit obtained by ignoring the pain. Why take a risk? In life’s risk-benefit equation, the risk often dwarfs any potential benefit. Herein lies the psychology of pain. Unless the animal or human can know that
a rainbow of benefit is awaiting on the far side of the pain, natural psychology dictates not to chase the rainbow.

  Pain can be a lie. The insect sting exploits a weakness of the pain signal system to propagate a masterful deception. That deception, the lie, benefits the stinging insect by cheating its adversary out of a meal, perhaps cheating it out of the use of space near the insect or its nest, or even cheating it out of some other resource, such as a feeding site. In most painful circumstances, it is adaptive for the stung individual to accept the lie and ensure its safety. One can lose many small meals and survive. One serious poisoning and the individual might not survive. The math is on the side of caution.

  For every liar and cheater, someone out there is not fooled. For the lying pain of stinging insects, some animals and people have broken the trick: they ignore the pain and reap the rewards offered by the stinging insect. In much of North America, skunks are common denizens of the rural countryside. Beautifully adorned in tuxedo black with brilliant white stripes or spots, skunks are known mainly for their aromatic properties, but they are also efficient predators of insects and other small game. Skunks have a fondness for stinging insects and avidly dig out and consume the contents of yellowjacket wasp nests. They also enjoy honey bees, another spicy dietary staple, and have learned to discount the pain. Bears are another trick-breaker, famously known for their love of honey. Bears tear apart beehives in hollow trees or beekeepers’ boxes, relishing the sweet honey and rich brood, all with apparent impunity to the bee stings. Common wisdom dictates that the dense fur of bears protects them from stings, but this half-truth wisdom mainly protects our empathy for the bear and its potential pain. In real life, the bear suffers many stings, especially around its sensitive eyes, nose, ears, tongue, lips, and mouth. It has learned that a certain number of bee stings can be endured without injury and that the reward is worth the pain. Likewise, for the fabled African ratel, or honey badger. Ratels, relatives of wolverines, are medium-sized black-and-white, tough-skinned intrepid animals that routinely feed on all sorts of prey, including poisonous snakes (reputed to be unable to bite through the tough skin), chase lions and other carnivores from their prey to claim the kill, and are best known for their love of honey and bee brood. Ratels, like bears, learned that a certain number of stings cause no meaningful damage, and thereby they have learned to overcome the pain. This is a tricky game for ratels. Bee stings are truthful as well as painful. Enough stings, about 4 for a mouse, or an estimated 140 stings for a ratel, can kill. Until around a hundred stings, the ratel is safe. No one knows how well ratels can count in the ratel–bee brinkmanship game, but they likely can sense when a dangerous level of envenomation is near. The game can be tricky, however, for some ratels have misjudged and paid the ultimate price of being stung to death.2