Division Of Labor

The relatively large biomass of ants in many ecosystems can be attributed not just to the way in which the ants interact with other organisms but to the way in which they interact with their nestmates in general and, in particular, to efficiencies that accrue from divisions of labor. One of the most dramatic traits associated with the division of labor among the workers is physical polymorphism, which is the presence of different physical worker forms within the same colony. In the African army ant, Dorylus wilverthi, for example, the smallest workers at 0.12 mg dry weight are only 1% of the dry weight of the largest workers (soldiers), and this relatively great size range is exceeded in certain other species (e.g., in Pheidologeton diversus, the smallest workers have a dry weight that is about 0.2% that of the largest majors). It is not just the size range that is impressive in such species but also the degree of polymorphism among the workers. Darwin, writing in The Origin of Species, seemed well aware not only of the phenomenon but also of its implications. Indeed, one of Darwin's most penetrating insights in his 1859 masterpiece was his suggestion that sterile forms evolved in social insects because they are "profitable to the community" and that "selection may be applied to the family, as well as to the individual." He further suggested that once such colony-level selection had begun, the sterile forms could be molded into distinct castes "Thus in [the army ant] Eciton, there are working and soldier neuters, with jaws and instincts extraordinarily different" (Fig. 1a, b).

Such worker polymorphism is now known to be associated with the differential growth rates of different putative tissues and body parts during the preadult stages. Indeed, the study of ants made a major contribution to the development of the concept of allometric growth (Fig. 1c, d). Notably polymorphic genera include the army ants Eciton and Dorylus, leafcutter ants (Atta), carpenter ants (Camponotus), and members of the genera Pheidole and Pheidologeton. Indeed, Camponotus and Pheidole are the two most species-rich ant genera.

However, genera with polymorphic workers are in the minority. Approximately 80% of ant genera consist entirely of species with monomorphic workers, most of the remaining genera consist of species in which there are at most only two easily recognizable worker morphs, and only about 1% of genera have species in which three or more worker morphs can be relatively easily recognized within colonies.

FIGURE 1 The army ant, Eciton burchelli. (a) Head of major worker. (b) Head of minor worker. (c) Head width vs ponotum width allometry for workers. (d) Frequency—dry weight histogram for a large sample of workers. The allometrical relationship has a slope greater than 1, so larger workers (such as majors) have disproportionately large heads. The size frequency distribution is skewed to the right so relatively few of these very large majors are produced. (Drawings © Nigel R. Franks.)

FIGURE 1 The army ant, Eciton burchelli. (a) Head of major worker. (b) Head of minor worker. (c) Head width vs ponotum width allometry for workers. (d) Frequency—dry weight histogram for a large sample of workers. The allometrical relationship has a slope greater than 1, so larger workers (such as majors) have disproportionately large heads. The size frequency distribution is skewed to the right so relatively few of these very large majors are produced. (Drawings © Nigel R. Franks.)

Polymorphism among the workers is mostly associated with extreme physical specialization. Thus, Eciton majors have ice-tong-like mandibles and are specialist defenders of the colony against would-be vertebrate predators or thieves (Fig. 1a). It has been shown that colonies of Pheidole pallidula can produce more defensive majors in response to stresses induced by conspecific competitors. Majors are not always for defense: large-headed majors in Pheidole and Messor serve as specialist grinders of harvested seeds. Even among such polymorphic species, however, the majority of workers belong to castes of generalists, which give their colonies an ability to respond rapidly to changes in the environment. Such generalists show behavioral flexibility not possible with the extreme morphological specialization of certain physical castes. Nevertheless, divisions of labor also occur within the majority generalist caste. Such workers typically specialize in different tasks at different times during their lives. This is known as temporal polyethism, in contrast to physical polyethism.

The sophisticated divisions of labor in monomorphic ants are being investigated. In Leptothorax albipennis, the workers show very little size variation, and colonies consist of, at most, a few hundred such workers living in flat crevices between rocks. Such crevices can be only 2 or 3 cm wide and deep and may have an internal cavity height of only 1 or 2 mm. Individual workers could easily roam all around such nests within a minute, but instead they have spatial fidelity zones; that is, they remain faithful to certain parts of the nest and the segregated tasks within such areas for months on end. The workers can even reconstruct their own spatial fidelity zones relative to one another if, and when, their colony is forced to emigrate to a new nest site because of the destruction of the old site. In this (and many if not all) ant species, younger workers tend to work deep within the nest at its safe center, tending the queen and the eggs. As they get older, workers tend to move progressively out from the center of the nest, and toward the end of their lives they eventually engage in the most dangerous task of foraging in the outside world, where they are likely to meet predators and other hazards. However, the correlation between age and task is often very weak, and in an increasing number of species it has been shown that the division of labor among monomorphic workers is extremely flexible. Workers can respond to the removal of other workers by reverting to tasks that they did earlier in their lives or, if need be, they may begin foraging even when they are very young. Thus, though age may influence what workers do, it is unlikely to be the organizing principle of the division of labor in many species. Rather, it seems that workers are continuously monitoring their workloads and the delays they experience while waiting to interact with their nestmates and will flexibly change their tasks accordingly to maximize their productivity.

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