Among the many “Wow I didn’t know that” moments that confront a new beekeeper is the lesson that a worker bee dies after she delivers a sting. It’s true. Her barbed stinger implants in the skin of her adversary like a harpoon, tears away as she leaves, and its attached poison sac keeps throbbing away under involuntary muscles pumping out every last dreg of venom (Fig. 1). Meanwhile, the mortally wounded worker renews her assault with more determination than ever. She flies recklessly at the intruder, burrowing into fur or hair with a buzzing designed to distract and deter. She flies straight at the eyes. It’s as if she knows she’s a gonner so she might as well give it all she’s got. If she survives the encounter and returns to the nest she has only a few more hours to live before her internal injuries catch up with her and she dies, only to be unceremoniously disposed of by an undertaker bee.
Now I must shamefully admit that more than once I have taken a measure of revengeful comfort in this knowledge, especially at the end of a hot day with my hands sore and swollen and my sweaty shirt “pinned to my back.” But if we can dissociate ourselves a moment we may appreciate the necessity and efficiency of such a repertoire of behaviors. From the perspective of a large predator the honey bee nest is a lottery prize of protein and carbohydrate, and this stationary target requires extraordinary protection. The detachable singer/poison sac essentially doubles the defensive output of every single worker, freeing its possessor to return to the attack with additional intimidations. Multiply this one terrifying performance by tens, scores, or even hundreds of assailants and the result can be effective – the successful deterrence of a predator. And lest we humans get too complacent thinking ourselves somehow detached from the rest of nature, it’s insightful to consider the co-occurrence of Apis mellifera with ancient humans and our immediate ancestors on the African continent. It’s not far-fetched to wonder if the bees’ defensive behaviors we find so effective today are in fact evolutionary adaptations to us, arguably the most dangerous predator they’ve encountered in their natural history.
But for humans of a more modern variety the detachable sting also has the advantage of being convenient for research. What better way to measure defensive behavior than count bee stings on a red leather patch? This is in fact the most common method used by scientists working on defensive behavior, especially in areas with Africanized bees,
(Fig. 2). One season while I was a graduate student some of my lab coworkers were studying defensive behavior of Africanized bees in Venezuela. Things were going quite well until one day the beekeeper who had volunteered his apiary for their experiments angrily ordered them off his property. Come to find out, his bees were responding so well to repeated leather patch tests that my colleagues were killing his entire apiary!
The defense behavior of the honey bee is as firmly engrained in the minds of most people as the fact that they make honey. No greater impediment to recruiting new beekeepers is to be found; in fact, some old-timers celebrate the fact that bees sting, saying – not without reason – that if it weren’t for bee stings then everyone would be a beekeeper.
Whether this is a good thing or bad is fodder for another conversation, but for our present purposes the self-sacrificing worker bee is a bit of a mystery and a clue that bigger issues may be afoot. For not only is the worker bee self-sacrificing, she is reproductively sterile – which makes no sense at all from an evolutionary point of view. In human terms, what the worker bee is saying is, “Not only will I give up reproduction in order to help my mother reproduce, but I will die if necessary to do so.” This is altruism practiced to a pitch rarely seen anywhere. And at the beginning of things it was a serious affront to Charles Darwin’s theory of natural selection. How can one pass on any genes whatsoever, favorable or unfavorable, if one isn’t even capable of reproducing? After all, in Darwin’s economy the only coin of the realm is successful reproduction, that is – getting one’s genes passed on to the next generation.
The awkward problem of the sterile worker was not lost on Darwin himself, and in chapter 7 of his seminal book On the Origin of Species he confronts the problem, and I quote:
“I will . . . confine myself to one special difficulty, which at first appeared to me insuperable, and actually fatal to my whole theory. I allude to the neuters or sterile females in insect-communities: for these neuters often differ widely in instinct and in structure from both the males and fertile females, and yet, from being sterile, they cannot propagate their kind.”
Now it’s interesting at this point to pause and consider one of history’s great What Ifs? and that’s the fact that while Darwin was putting the fine touches on his great theory in the south of England, a contemporary of his, an Augustinian monk Gregor Mendel, was untangling the basic laws of hereditary genetics 1000 miles away in Brno in what is today the Czech Republic. The two never met, and it is unclear to what extent either was aware of the other’s work. As a result, Darwin went to his grave ignorant of the mechanisms of inheritance, the consequences of which he nevertheless understood so profoundly. More practically, this historical near-miss meant that the great neo-Darwinian synthesis between genetics and evolution had to wait to emerge until the 1930s and 1940s. But if Darwin had known even a rudimentary level of Mendelian genetics he might have propelled forward evolutionary biology by decades and solved his “insuperable problem” as well. As it stands, he still hit pretty close to the mark, and again I quote:
“This difficulty, though appearing insuperable, is lessened, or, as I believe, disappears, when it is remembered that selection may be applied to the family, as well as to the individual, and may thus gain the desired end.”
With this sentence Darwin anticipated the neo-Darwinian explanation for social insect altruism that would emerge over 100 years later in the form of two papers published by 26-year old graduate student W.D. Hamilton at University College, London in 1964. It was Hamilton, advantaged with Mendelian genetics, who mathematically showed that the self-sacrificing behavior of an individual can still be adaptive if it promotes the survival and reproduction of near-kin who possess the same genes, or to put it in his own words: “a gene may receive positive selection even though disadvantageous to its bearers if it causes them to confer sufficiently large advantages on relatives.” Hamilton called this “inclusive” fitness, in distinction to “individual” fitness, and in so doing opened up all the resources of Darwin’s theory to biologists working with social insects and their teeming populations of sterile workers. Overnight, Hamilton unloosed a new science onto the world – so-called Kin Selection – the idea that individuals in social units can be predicted to behave in a way that promotes transmittal of their genes, even if those genes are in the bodies of near-kin. A seemingly puzzling behavior like self-sacrifice might make sense if it promotes transmission of that worker’s genes – even if it’s copies of those genes possessed by the worker’s sister.
In the case of honey bees Hamilton’s math is quite simple – 8th-grade level – and I spell it out in the sidebar. Doing the exercise is worthwhile because it really paints the matter in black-and-white.
To begin, let’s remember that natural selection operates on variation in a population – “good” genes vs. less-good vs. bad. Variation in a breeding population is a necessary condition to …