The stronger animal will win the contest, but is it really a win? If the dominant male sea lion loses a pint of blood to defend his harem and the loser loses a gallon of blood, does anybody actually win? Both would have won more—or lost less—if the battle had never occurred. Threat displays evolved to obviate such hopeless battles. Likewise, a cow killer or blister beetle benefits if it is never assailed by the lizard and need not waste its valuable defensive resources or risk injury or death. Nature has evolved truthful advertising slogans for stinging insects to tell predators to “stay away, don’t mess with me.” These messages can be the eye-popping aposematic color patterns of red and black, orange and black, yellow and black, white and black, or simply any of those colors richly displayed alone. Or the message can scream by way of a rattle, a snap, a squeak, or a rasp. These acoustic warnings are broad ranged, usually low frequency, and generalized so that any predator capable of hearing not only detects the signal but also recognizes that the signal is not a specialized communication among individuals of the same species. These messages are designed not to be confused with a bird’s courtship song or a lustful katydid’s chirping. Some predators, toads and frogs for example, might not obviously respond to color or sound signals. In these cases, the most basic of all sensory systems—taste—can be targeted for messaging. Nasty tastes in food signify nasty consequences if eaten. Giant velvet mites, squat, red, furry balls on stubby legs, emerge during the first summer rains and wander around looking for winged termites for dinner. Toads, horned lizards, and other known potential predators fastidiously avoid these bulky honey bee–sized mites. They convey their message through their bright red color and, especially, their nasty taste. Lizards will lick a mite and reject it. One sampling seems sufficient for a lifetime. Toads sometimes eat a single mite; even these slow learners get the message and will not eat more.
My curiosity demanded answers to the mystery of why velvet mites were so noisome. From a biological perspective, humans are simply big generalist predators, scavengers, and herbivores that eat almost anything, incorporating a wide variety of animals, plants, and fungi, live or dead, into their menu. Our taste reflects this generality and is tuned to respond to a host of gustatory chemicals that might convey “food” versus “poison.” Our taste response approximates the taste responses of other generalist predators, such as birds and lizards, wishful predators of big, juicy velvet mites. If lizards and toads can taste and respond to (reject) velvet mites, should not I likewise detect and respond to their taste? I delicately approached the problem, remembering the childhood admonishment not to eat anything I knew to be unsafe. Velvet mites might be toxic. They might cause blisters. Therefore, they should not be eaten outright. I placed a fat velvet mite on the tip of my tongue, the safest and most distant part from my throat, smashed it against my incisors, and chewed as best one can with incisors. The taste was truly amazing, stunning is perhaps a better word. After the 2-second analysis, I spat out the red juice like chewed tobacco. However, the flavor did not spit out with the crimson fluid. It was bitter, more bitter than quinine or any medicine I had ever tasted. It was also hot and burning, like a habanero pepper. Worse, it attacked the back of my throat and lingered there, that combination of bitter and corrosive heat. I was used to most nasty tastes leaving shortly after being spat out. Not this one. It lingered. The lingering seemed to last forever, an hour at least, before finally releasing its grip.
A stinging insect benefits from communicating its unsuitability as dinner to any predator and in any possible manner. Strutting is a time-tested way of saying, “I am tough. I know you are watching, and you don’t want to mess with me.” Tarantula hawks and many other spider wasps strut on the ground while frequently flipping their wings. The message is clear, “I want to be seen, and you want to remember how I move so you don’t make a mistake and misidentify me.”
Visual systems of humans and other vertebrates are geared to recognizing walking gaits of other humans or potential prey or predators. Recognizing walking gaits is an ancient ability of the brain that relies on peripheral vision and does not require clear or focused vision. I frequently search sandy areas of Florida or Arizona deserts where harvester ants are abundant, on the surface searching for seeds to cache. They are about the same size and same color as many velvet ants. Many small velvet ants, diminutive relatives of cow killers, are orangish and about the same size as large ants. Harvester ants typically are a thousand times more abundant on the soil. Yet, out of the corner of my eye, I spot the motion of the one velvet ant mixed with the hundreds of harvester ants. The gait of the velvet ant catches my attention, not its size, color, or slightly different body shape, features my peripheral vision cannot distinguish. They simply move differently than harvester ants, a motion I subconsciously recognize. Harvester ants, powerful stingers in their own right, walk in a jerking fashion. They move a few steps, abruptly stop, move again, halt or slow again, repeating these random movements. This gait might be a warning that the ant is “spicy.”
Warnings and gaits all serve the purpose of reducing attacks on defended and stinging insects. Ignoring the special situation of Batesian mimicry, these warning signals are only possible and effective because they advertise real defensive ability. Combined, the artillery of the sting and the sensory trumpeting of warning allowed some stinging insects to exploit otherwise forbidden areas such as desert surfaces, open fields, and even our picnics. Without its sting, the common yellowjacket would be unable to steal ham from our sandwich or imbibe the sweet juice from our peach. Without the sting, the evolution of sociality, especially higher eusociality in ants, wasps, and bees, would not be possible.
Evolution of Sociality
WOE TO THE ANIMAL with poor defenses that casually aggregates with others of its kind. An even greater woe would be to form a society of individuals cooperating to perform community tasks and raise their young together. Predators would quickly devour the delectable, defenseless society, and the story would end with few potentially surviving individuals, no societal reproduction, and no society. Evolution of societies would be prevented, and we would see no social insects or other social animals.
But we do see many social insects and some social vertebrate animals, including humans and naked mole rats. How can this be? All social animals have effective defenses to blunt the attacks of predators. Humans lack claws, horns, or long, sharp canine teeth and cannot run that fast for our size, but we do have big brains and agile hands and arms. Our brains allowed us to tame fire, a defense no other animal uses and that predators of humans fear. Our brains also allowed us to develop tools and weapons. Our hands and arms then allowed us to accurately throw objects or spears at potential predators (something even chimpanzees cannot do), thereby providing defense from a distance. Defense from a distance gave us an unusual and highly effective defense, something lacking in antelope, elephants, and even lions. In essence, we evolved defenses that rendered us nearly invincible and allowed us to become obligatorily social.
Like people, all other social animals have evolved some effective defense(s) against predators. Some defenses are structural, as in the case of mole rats that live in tunnels they dig in rock-hard African ground. Predators simply cannot dig them out of their underground fortresses. Termites use similar structural defenses; they inhabit the soil, wood, or hardened self-made nests aboveground or in trees. The effectiveness of these hardened termite nests, found in Australia and in Africa, is apparent to anyone who has ever kicked one.
The other way social insects defend themselves is by active physical defenses that can cause harm. Some aphids, whose soft bodies are pathetically hopeless for defense, evolved sociality through the evolution of a special caste of warrior aphids. These tiny aphid warriors effectively use their sharp beaks to puncture and inject venom into predators that breach the protective plant gall they live within. Most social wasps, ants, and bees evolved sociality largely because they also evolved effective stinging defenses. Granted, some social wasps,
bees, and ants lack effective stings, but, in all cases, these species form tiny colonies and live underground or within small hidden nests, or they secondarily evolved the loss of the sting after evolving sociality. The prime examples of secondary loss of the sting are many ants and the stingless bees. The nature of predation pressure against nonstinging ants changed from primarily large predators to other small predators, mainly other ants. Agility, sharp mandibles, and chemical defenses, such as formic acid, proved better defenses than stings against other attacking ants. Stingless bees also possess powerful biting mandibles in association with noisome defensive chemicals and produce wax and resinous structural and chemical defenses against small predators. The sting is not essential in nonstinging ants and bees. Their mandibles, chemicals, and agility provide effective defenses against large predators, as anyone who has messed with a large carpenter ant or stingless bee colony can attest.
The issue is not why highly social wasps, ants, and bees don’t sting but how sociality evolved in these groups in the first place. To evolve sociality, a species must evolve good defenses against predators hell-bent on making a meal of the “wishful society.” Why do we not see social grasshoppers, beetles, or flies? Yet we see myriads of social Hymenoptera? The answer lies in the lack within grasshoppers, beetles, and flies of a meaningful defense against large predators. Ants, social wasps, and bees, in contrast, evolved from ancestral sting-bearing wasps preadapted for defense against large predators. As I argued in 2014 in the Journal of Human Evolution, a key component for the evolution of sociality in ants, wasps, and bees was the evolution of venom, and some modification of the stinger and behavior.4 This sting-venom evolution then allowed social evolution to progress despite powerful predators working against that social evolution.
6
SWEAT BEES AND FIRE ANTS
There may be many surprises in store for us when the
life-histories of these seemingly monotonous and
uninteresting bees [sweat bees] have been subjected to
more careful scrutiny. —William Morton Wheeler,
The Social Insects, 1928
The ferocious little pests. —Edward O. Wilson,
foreword, The Fire Ants, 2006 (in reference to the
fire ant Solenopsis invicta)
SWEAT BEES, THOSE RASCALLY DENIZENS that appear during the peak of hot, humid, sticky summers in eastern and central North America, are familiar visitors to backyards and social gatherings. Why “sweat bees,” a peculiar name, for sure? Do they sweat? No. Do they make us sweat in fear? No. Are they even bees? Yes. OK, that part of the name makes sense, but where did “sweat” come from? It originated from the unusual habit of some species of these bees: they land on and lap sweat from human skin. Most kinds of bees do not collect sweat, which makes the sweat bee’s habit oddly noticeable.
Bees are a collective of 20,000 species, outnumbering all warm-blooded animals on Earth.1 Sweat bees are members of the enormous bee family Halictidae, which contains 4,387 species, more than all mammals, excluding bats. Sweat bees live throughout the world on all continents except Antarctica and display a greater diversity of social behaviors and life histories than any other group of insects. Many are strictly solitary, that is, lone female bees work in isolation, collecting food, building a nest, and otherwise providing for the young. Others remain solitary, but nest in aggregations of many individuals, each working on its own within the close aggregation of nests. Some species are semisocial, with two or more females working together in the same nest but performing different tasks. Finally, some are truly social, with several individuals living in the same nest, including an egg-laying queen and workers. To complicate matters further, some sweat bee species are solitary at one time or in one location and social at other times or locations.
Most sweat bees are small, 3–12 millimeters (⅛–½ inch), and black, grayish, or metallic green or blue, sometimes with splashes of yellow or red. They usually nest in the ground. Nests consist of a tunnel excavated from the soil surface, descending into the earth, usually with side branches leading to individual cells where the young are reared (only females do the work; males sit around and, at most, guard nest entrances). Once a cell is prepared, the female collects pollen and nectar from flowers to form into pollen “balls,” or “loaves,” the sole food for the young. She then lays a single egg in each cell on its pollen provisions and seals the cell before working on the next cell. Curiously, female sweat bees, as are most other bees, wasps, and ants in the order Hymenoptera, are haplodiploid organisms, in which fertilized eggs become female young and unfertilized eggs become males. This allows females to choose the sex of each young, something people cannot naturally do (perhaps a good thing overall). In practice, this female choice often translates into males getting the short end of the loaf in the form of smaller food provisions; hence, males are often smaller and scrawnier than females.
Sweat bee life begins as an egg laid on or near its doughy pollen mass. In a few days, the egg hatches into a tiny, almost transparent whitish larva that feeds on the pollen. In the process, it grows larger, molting apparently four times into successively larger grub-like larvae. Sweat bee larvae lack a connection between their midgut (stomach) and their hindgut, rendering them incapable of defecating, which is likely a good thing because each is confined to a small cell, feeding on a rich, potentially spoilable food source. At the end of larval feeding, the connection forms between the mid- and hindguts, allowing the now enormous larva to make its only, and probably much-appreciated, defecation. Unlike some bees, the larva does not spin a silken cocoon; instead, it molts within its cozy, wax-lined cell into a pupa, the resting stage in which the adult bee develops. Pupae are delicate creatures, and usually this stage is short. The pupa molts into an adult that remains in the cell during unfavorable seasons until the winter or adverse periods are over, then digs out to become a free-flying adult. Adults often live a comparatively long time for small insects, giving them opportunities to visit many flowers, typically a succession of different floral types. Some sweat bees have two or more generations per year, others simply one generation. In any case, males and females mate, and females are able to store sperm for later use after the season with males has passed.
The insides of sweat bee cells are lined with a waxy protective coating secreted by the mother bee.2 This impermeable coating within the cell protects against problems, including excess wetness from rain, desiccation during dry periods, and fungal or other pathogen problems. The lining is produced in the Dufour’s gland, a curious gland associated with the sting, and is applied to the insides of cells with a combination of the abdominal tip and the tongue. The gland name, itself, also has a curious history. It is named after Léon Dufour, a prominent French physician, scientist, and scholar. Dufour mentioned in 1835 that a “plastique” cell lining in some bees appeared to be derived from a large abdominal gland.3 In 1841, he elaborated that the glandular secretion also was used by females to coat eggs. Along the way, the gland acquired the strange name “alkaline,” or “basic” gland. It was believed that its fluid was alkaline in nature. Even though this characterization was disproved, the term “alkaline gland” persists. When the gland became known as Dufour’s gland remains a mystery. Dufour never named it, and after a few brief forays into the subject moved on to other topics. With numerous synonyms (e.g., sebifique gland) in use, somehow the name Dufour’s gland arose, perhaps shortly after 1841, and stuck, apparently in honor of the great man. Odd, how today he might be best known for this obscure gland named after him.
Even though more than 4,000 species of sweat bees exist and that some species lap human and animal sweat, we still don’t know why they collect sweat. Surprisingly, little research has been conducted to address this question, and most of what is known was performed in 1974 by Edward Barrows, at the time a graduate student at the University of Kansas. Ed showed in a series of choice tests that sweat bees are attracted to and prefer table salt solutions over controls that lacked s
alt.4 Because salt does not evaporate or have any odor, something else must attract the bees. Likely candidates might include lactic acid, carbon dioxide, or the mosquito attractant 1-octen-3-ol, all released from skin surfaces. We simply don’t know. We also are left wondering whether bees are seeking salt, water, or some other sweat component as a nutritional requirement; lactic acid, octenol, or carbon dioxide seem unlikely. An additional complication is that not just sweat bees collect sweat. Africanized honey bees on occasion collect sweat. In Asia, stingless bees in the genus Trigona collect sweat and are sometimes called sweat bees. In parts of Africa, stingless bees that collect sweat are often called sweat bees and are sometimes called “mopane flies,” even within academic circles.5 Yet none of these insects are either true sweat bees or flies. These African stingless bees are actually stingless; that is, they lack a functional sting. Not that stingless bees are defenseless. They have sharp mandibles and attack in mobs, biting eyelids, nose, and ears and crawl into ears, nose, and mouth, a most definitely unpleasant experience.
Although some Australians call them “sweet bees,” a much more pleasant moniker than sweat bees, we appear to be stuck with the term “sweat bee” for these little bees that visit our summer activities. These visitors, unlike stingless bees, do not bite, but they do sting. In a typical scenario, a person is leisurely enjoying a pleasant July afternoon, relaxing outdoors with a favorite drink in hand. A few flies are buzzing around, and the occasional honey bee visits a nearby flower; otherwise, the kids are playing, and the afternoon is perfect. Up to one’s mouth comes the drink and, ouch, something stung me! Tranquility is ruined by a little dark bee in the crook of the elbow. The little bee meant no harm; it was just enjoying one of its favorite beverages, the sweat accumulated in the fold at the elbow between the forearm and upper arm. When the glass was raised, the bee became pinched between the skin. The threatened bee responds defensively by stinging in an attempt to stop the pinch. Often that strategy works. Other times the unsympathetic recipient of the sting responds by smashing the poor bee.
The Sting of the Wild Page 7