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EVOLUTION: ON THE POLICING OF INSECT SOCIETIES

The following points are made by F.L Ratnieks and T. Wenseleers (Science 2005 307:54):

1) Insect societies, like human societies, experience internal conflicts [1,2], and research increasingly shows that policing is important to resolve these. Within both human and insect societies, conflicts arise because the interests of individuals differ. In insect societies, conflict revolves around reproduction. Reproducing individuals gain by being more closely related to the young males and queens reared in their colony. By reproducing, society members also exploit the colony and this can be costly. First, uncontrolled reproduction upsets the division of labor between queen and workers and results in a less efficient colony. Second, the offspring reared are often genetically less related and so are less valuable to other society members.

2) To prevent exploitation, social insects have evolved several methods of policing. The best known is "worker policing", whereby workers kill eggs laid by other workers. This phenomenon was first discovered in the honeybee 15 years ago [3]. Since then, it has been discovered in more than 15 species of bee, wasp, and ant. This past year alone, five more insect species -- two species of British wasp [4] and three ant species from Florida [5], Brazil, and Finland -- have been added to the list.

3) In addition to reducing reproduction by workers, policing also acts to regulate the development of females into distinct queen and worker castes, and to prevent excess females from developing into queens. When different species are compared, one important overall conclusion emerges: More effective policing results in fewer individuals acting selfishly. There are other striking parallels to human society: Insect policing relies on both detection and prevention, and individuals sometimes attempt to evade policing.

4) In the life of any female bee, wasp, or ant, there are two points at which she may try to reproduce. The first is when, as a larva, she starts developing into either a queen or a worker. In most species, queens are morphologically specialized for egg laying and are often incapable of working. The second is when, as an adult worker, she decides whether to activate her ovaries to lay eggs. In most species, workers cannot mate yet retain ovaries. Therefore, they can lay unfertilized eggs, which develop into males if reared.

5) Young female larvae of bees, wasps, and ants are usually totipotent, that is, they have the potential to develop into either a queen or a worker. A larva is often better off developing into a queen, yet policing ensures that most are prevented from doing so. Because queens are generally larger than workers and need more food, adult workers can control whether a larva will develop into a queen by controlling her food supply. Consider the honeybee (8). A colony reproduces by swarming, dividing into several colonies each headed by the mother queen or a newly reared daughter queen. Prior to swarming, a few queens and thousands of workers are reared. Thus, although a few larvae are reared into queens, with a good chance of heading a colony, thousands of their less fortunate sisters are reared into workers. The fitness advantage of developing into a queen is large.

References (abridged):

1. The Proceedings of the Old Bailey. Policing in London Before the Bobbies. www.oldbaileyonline.org/history/crime/policing.html

2. A. F. G. Bourke, N. R. Franks, Social Evolution in Ants (Princeton University, New Jersey, 1995)

3. F. L. W. Ratnieks, P. K. Visscher, Nature 342, 796 (1989)

4. T. Wenseleers et al., Evolution, in press

5. A. Endler et al., Proc. Natl. Acad. Sci. U.S.A. 101, 2945 (2004)

Science http://www.sciencemag.org

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ON ALTRUISM OF INDIVIDUALS IN INSECT SOCIETIES

The following points are made by Edward O. Wilson (citation below):

1) Altruism is self-destructive behavior performed for the benefit of others. The use of the word altruism in biology has been faulted by Williams and Williams (1957), who suggest that the alternative expression "social donorism" is preferable because it has less gratuitous emotional flavor. Even so, altruism has been used as a term in connection with evolutionary argumentation by Haldane (1932) and rigorous genetic theory by Hamilton (1964), and it has the great advantage of being instantly familiar. The self-destruction can range in intensity all the way from total bodily sacrifice to a slight diminishment of reproductive powers. Altruistic behavior is of course commonplace in the responses of parents toward their young. It is far less frequent, and for our purposes much more interesting, when displayed by young toward their parents or by individuals toward siblings or other, more distantly related members of the same species. Altruism is a subject of importance in evolution theory because it implies the existence of group selection, and its extreme development in the social insects is therefore of more than ordinary interest. The great scope and variety of the phenomenon in the social insects is best indicated by citing a few concrete examples:

a) The soldier caste of most species of termites and ants is virtually limited in function to colony defense. Soldiers are often slow to respond to stimuli that arouse the rest of the colony, but, when they do, they normally place themselves in the position of maximum danger. When nest walls of higher termites such as Nasutitermes are broken open, for example, the white, defenseless nymphs and workers rush inward toward the concealed depths of the nest, while the soldiers press outward and mill aggressively on the outside of the nest. Nutting (personal communication) witnessed soldiers of Amitermes emersoni in Arizona emerge from the nest well in advance of the nuptial flights, wander widely around the nest vicinity, and effectively tie up in combat all foraging ants that could have endangered the emerging winged reproductives.

b) I have observed that injured workers of the fire ant Solenopsis saevissima leave the nest more readily and are more aggressive on the average than their uninjured sisters. Dying workers of the harvesting ant Pogonomyrmex badius tend to leave the nest altogether. Both effects may be no more than meaningless epiphenomena, but it is also likely that the responses are altruistic. To be specific, injured workers are useless for most functions other than defense, while dying workers pose a sanitary problem.

c) Alarm communication, which is employed in one form or other throughout the higher social groups, has the effect of drawing workers toward sources of danger while protecting the queens, the brood, and the unmated sexual forms.

d) Honeybee workers possess barbed stings that tend to remain embedded when the insects pull away from their victims, causing part of their viscera to be torn out and the bees to be fatally injured. A similar defensive maneuver occurs in many polybiine wasps, including Synoeca surinama and at least some species of Polybia and Stelopolybia and the ant Pogonomyrmex badius. The fearsome reputation of social bees and wasps in comparison with other insects is due to their general readiness to throw their lives away upon slight provocation.

e) When fed exclusively on sugar water, honeybee workers can still raise larvae -- but only by metabolizing and donating their own tissue proteins. That this donation to their sisters actually shortens their own lives is indicated by the finding of de Groot (1953) that longevity in workers is a function of protein intake.

f) Female workers of most social insects curtail their own egg laying in the presence of a queen, either through submissive behavior or through biochemical inhibition. The workers of many ant and stingless bee species lay special trophic eggs that are fed principally to the larvae and queen.

g) The "communal stomach", or distensible crop, together with a specially modified proventriculus, forms a complex storage and pumping system that functions in the exchange of liquid food among members of the same colony in the higher ants. In both honeybees and ants, newly fed workers often press offerings of ingluvial food on nestmates without being begged, and they may go so far as to expend their supply to a level below the colony average.

2) These diverse physiological and behavioral responses are difficult to interpret in any way except as altruistic adaptations that have evolved through the agency of natural selection operating at the colony level. The list by no means exhausts the phenomena that could be placed in the same category.

Adapted from: Edward O. Wilson: The Insect Societies. Harvard University Press 1971, p.321.

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EVOLUTIONARY BIOLOGY: ON EUSOCIALITY

The following points are made by D.C. Queller and J.E. Strassmann (Current Biology 2003 13:R861):

1) "Eusociality" is a term coined to cover ants, bees, wasps, and termites that have three properties: overlap of generations, cooperative rearing of young, and non-reproducing worker castes. Other organisms that have these traits have since been added: some aphids and thrips, a beetle, some snapping shrimp, and the naked mole rat. In eusocial species, non-reproductive workers care for the young of the reproductive queens (and sometimes kings). As such, workers are analogous to the somatic cells of an organism, which work for the transmission of their genes by proxy via the germ line cells. Like the cells of an organism, the members of a eusocial colony have evolved elaborate mechanisms to enhance the survival and reproduction of the larger unit. The colony consisting of one or more queens and workers has been called a superorganism, essentially a new kind of organism built up of organisms of the old kind.

2) Consider the famous honeybee waggle dance. This dance, performed by returning foragers, tells other workers the direction and distance of rich food sources. The colony benefits by exploiting the hard-won knowledge of those foragers that find food bonanzas. The dance is celebrated as a rare example of symbolic communication between individual organisms, but it can also be viewed as a part of a signaling cascade of the larger superorganism that regulates work according to the supply and demand. If the supply of food is great, there will be more waggle dancers stimulating more foraging to harvest it. But that is not the only adjustment necessary. Foragers, with their knowledge of valuable food sources, do not waste time processing the food, but hand it off to another set of bees inside the hive. If a forager has trouble finding a processor bee, she begins a different dance, the tremble dance, which both activates bees to become processors and inhibits waggle dancing. The result is a negative feedback system that allocates workers to foraging and processing tasks according to need. Additional links in the system include the needs of the brood and the degree to which storage capacity is filled. Such regulatory feedback systems operate in nearly every aspect of social insect colony functioning, just as they do in other organisms.

3) Besides the clear similarities between organisms and superorganismal colonies, there are some differences that show us that entities with organism-like functionality and integration can operate in unfamiliar ways. For example, the cells of organisms terminally differentiate into numerous specialized types, while social insect colonies have at most only a few terminally differentiated castes. Instead, much of the division of labor is carried out by means of a temporal specialization, often with the youngest adults tending the brood, older ones carrying out other activities in the nest, and the oldest ones foraging outside the nest. Just as cells are more fixed in function, so are they more fixed in space. Social insects, in contrast, are not physically connected, and their colonies give us examples of organismal entities that are dispersed in space. A final important difference is the lack of centralized control in social insect colonies. Despite the controlling image conveyed by use of the term "queen", there is nothing like a colonial brain. No individual perceives the state of the entire colony and sends out instructions. Instead, actions are usually self organized by simple rules. Different individuals each have small pieces of information, which are integrated by the colony as a whole. A returning forager doesn't know how many foragers and processors are at work. Instead, she just experiences an indirect effect of those numbers -- the time required to offload her nectar or pollen -- and acts accordingly.(1-5)

References (abridged):

1. Bourke, A.F.G. and Franks, N.R. (1995). Social Evolution in Ants. (Princeton University Press)

2. Holldobler, B. and Wilson, E.O. (1990). The Ants. (Cambridge: Harvard University Press)

3. Queller, D.C. and Strassmann, J.E. (1998). Kin selection and social insects. Bioscience 48, 165-175

4. Seeley, T.D. (1995). The Wisdom of the Hive. (Cambridge: Harvard University Press)

5. Szathmary, E. and Maynard Smith, J. (1995). The major evolutionary transitions. Nature 374, 227-232

Current Biology http://www.current-biology.com

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