Why Are Social Insects Social?

The social insects have long posed a challenge to biologists, mostly because they exhibit altruism: individuals engage in seemingly selfless sacrifice for the benefit of others. At its most extreme, altruism can involve the actual suicide of an individual member of the colony—a worker bee dying, for example, after stinging an attacker threatening the colony. Altruism is usually more mundane: most workers in a social insect colony are sterile and exhibit their altruism by forgoing the option to reproduce, a type of "reproductive suicide."

This behavior posed great difficulty to the early Darwinian evolutionists, who could not explain why natural selection would allow such a scheme to evolve. Imagine that there is an "altruism gene," which causes individuals that carry it to give up their own efforts to reproduce and to help other individuals reproduce. By not reproducing, those organisms with the altruism gene would not pass it on; that is, the gene would be selected against. One would expect any altruism gene that appears soon to be driven from the population. So great was this difficulty that Charles Darwin considered the social insects a dagger pointed at the heart of his theory of natural selection. If some reasonable explanation could not be found for them, then his whole theory would have to be dismissed.

The explanation Darwin was looking for finally emerged with the advent of the synthetic theory of evolution. One of the fundamental insights of the synthetic theory was to treat organisms as mere vessels for genes in a population. From that insight, it is a short leap to realize that there is no significant difference between an individual passing on a gene and an entirely different individual passing on an identical copy of the gene. Therefore, one's own evolutionary fitness (the likelihood you will reproduce and pass your genes on) is not just a matter of your own likelihood of reproducing; it must also include the likelihood that every individual that carries a copy of your genes will reproduce.

This concept of inclusive fitness provides the explanation for sociality that Darwin so avidly wished for. The like lihood that another individual carries a copy of your genes follows some well-behaved rules of inheritance. For example, you and your siblings have, on average, about a 50 percent chance of sharing identical copies of a gene. The more distantly related you are, the smaller this likelihood would be. Half-siblings share your genes only about a fourth of the time, and first cousins would share a gene with you only about an eighth of the time. Evolutionarily, your fitness would be the same if you reproduced yourself or if you altruistically decided not to reproduce and at least doubled the chance that your siblings would reproduce. Or if you quadrupled the chance your half-siblings would reproduce or octupled the chance your first cousins would reproduce. Thus, altruism should be reasonably common in family units, and it should become rarer among more distantly related individuals.

Among the Hymenoptera (the bees, ants, and wasps), the rules of inheritance are somewhat peculiar, and this peculiar mode of inheritance seems to predispose these insects to altruistic sociality. The mode of inheritance is called haplodiploidy. The queen of the colony (the sole reproductive member) produces haploid eggs, that is, eggs that contain one copy of her diploid genome. In this sense, she is like any other sexually reproducing animal. In the normal course of events, the haploid egg would unite with a haploid sperm, carrying one copy of a male's genome, and at fertilization the two sets of genes would form a normal diploid individual, carrying genes of both the father and mother. This also occurs in hymenopterans, and when it does, the offspring is inevitably a diploid female. The queen also produces eggs that are never fertilized by a sperm, however, and these go on to develop into a haploid male. This unusual means of reproduction skews the patterns of inheritance that determine inclusive fitness. I don't wish to bother with the details of the calculation here, but to put it simply, haplodiploid reproduction makes a female bee more closely related to her sisters than she would be to her own daughters. For a worker bee faced with the "choice" either of reproducing herself or forcing her mother to produce more sisters, she will tend to go with whatever increases her inclusive fitness the most. This means forcing her mother to produce sisters. From this reproductive fact follows the peculiar aspects of social insects, like the legions of sterile workers, the single reproductive individual, the high degree of altruistic behavior, and so forth.

Like most beautiful explanations, the haplodiploid theory of sociality only goes so far. Altruism and social behavior are found among animals that do not reproduce by haplodiploidy, and it obviously cannot explain sociality among them. Human beings are an obvious example, but since our concern here is with insects, the most dramatic exception among them has to be the termites. These insects are only distantly related to the Hymenoptera, the termites being descended relatively recently from the cockroaches and the Hymenoptera descended from an as yet unknown "protowasp." Termites reproduce in the conventional way, that is, male and female produce hap-loid sperm and egg, which combine to form diploid offspring that are either male or female. Yet these insects, too, form large social colonies, with one or a few reproductive females tended by an enormous number of sterile workers, an organization remarkably similar to that of the bees and ants. In the case of termites, the sterile workers are both male and female.

The probable explanation for sociality among termites rests with their unusual digestive physiology. As everyone knows, termites eat wood (cellulose, actually). This is a peculiar thing for an animal to do because animals generally do not produce cellulase, an enzyme required to cleave sugar off the cellulose molecule. If an animal is to digest cellulose, it must enter into an alliance with an organism that possesses cellulase, which can be found among many bacteria, some flagellate and ciliate protozoa, and fungi. For most termites, these organisms are cultivated in the termites' guts to form a rich intestinal flora. Among others, including the macrotermitines described in the text, cellulose digestion has been "outsourced" to fungi cultivated outside the gut. The termite provides its symbionts with a rich source of cellulose for food, and the termites get in return glucose cleaved off the cellulose, as well as protein, vitamins, and essential amino acids produced by the symbionts.

The problem for termites is that they hatch from their eggs without this essential intestinal flora. To acquire it, they must be inoculated with it, which the colony's inhab itants do by feeding the newly hatched termites with feces and drops of regurgitate that contain the symbionts. Thus, the digestive physiology of termites forces them into a social interaction right from the get-go; it is the foundation of their very strongly developed social lives.

enon of homeostasis to organisms—we could objectively look for social homeostasis in assemblages of organisms.

What might these signs be? Let us look for them in the context of temperature regulation. Suppose there is an organism with a stable body temperature in an environment with a fluctuating temperature. This disparity means that body and environmental temperatures will frequently differ. Any difference in temperature between the body (Tb) and the environment (Te) establishes a potential energy difference that can drive a thermodynamically favored flux of heat (a TFF, in the jargon introduced in Chapter 3) across the boundary separating the animal from its environment:

where QTFF is the TFF of heat. Temperature, being a measure of heat content, would be affected by this flux: as the TFF drives heat into or out of the body, the body's temperature must change—the antithesis of homeostasis. Homeostasis arises only if the organism has some way of actively heating or cooling the body,3 which means driving an opposing flux of heat, designated in Chapter 3 as a physiological flux (PF). The PF must match the TFF in magnitude, but not, obviously, in sign:

3. Among animals, active heating involves the generation of heat directly from the hydrolysis of ATP, the so-called metabolic heat production, or thermogenesis. Active cooling of the body can only be accomplished by evaporation. See Box 11B.

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