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ScienceWeek
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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HONEY-BEES, BUMBLE-BEES, AND SOLITARY BEES
The following points are made by Karl von Frisch (citation below):
1) The idea that all forms of life on earth today were created together at the beginning of the world was abandoned some time ago, when scientists found out that animals of comparatively simple structure have, in gradual transition, developed into more and more highly organized forms. What is more, even within the short span of our own life, we can watch this process gradually taking place.
2) Like other existing animals and insects, the community of bees must have reached its present high degree of organization at some definite period in the past. But we have no idea how things happened; nor do we know anything about the ancestors of our present-day bees; they no longer exist and our curiosity about their earthly appearance will probably never be satisfied. However, it is interesting to consider how a community like that of the bumble-bee, which shows a much simpler organization than that of the honey-bee, in spite of the close relation between the two species, may actually represent a stage in its development. For example, the bumble-bees already make some use of their wax secretions in building nests, but they have not reached the stage of building pure wax combs like the honey-bees. Again, although they have learned how to build cells for accommodating their grubs, they have not yet discovered the most economical way of doing so. Consequently, their building material is quickly exhausted and, as a result, numbers of grubs have to be herded together in each of the narrow cells, a state of affairs which leads to the production of those females with stunted ovaries generally known as workers. Though possessing the feminine instinct for tending and nursing their brood, they have lost all capacity for egg laying. We may well imagine that the first workers ever to appear inside an insect community owed their existence to very similar circumstances.
3) Furthermore, like the honey-bee, the bumble-bees instinctively collect honey and pollen for storage, but their stores do not last them through the winter, so that a female that survives until the following spring will then have to lay and tend her eggs entirely on her own.
4) Among the members of the bee tribe proper, we encounter forms that show the first signs of social life, alongside with others that are completely lacking in social instinct. It will surprise most readers to learn that the community life seems to be the exception rather than the rule even within the bee family: as many as several thousands of species are known to lead a solitary life. They, also, collect honey and pollen for their brood, and can build cells to house their own grubs; but each female toils for herself and her own particular brood alone with no worker-bees to help her. Each one of these insects strictly obeys a law of nature, but the law which governs the way it has to tend its brood varies greatly from species to species. It is this variety in the behavior of the solitary bee that makes its history so fascinating.
Adapted from: Karl von Frisch: The Dancing Bees: An Account of the Life and Senses of the Honey-Bee. Harcourt Brace & World 1953, p.173. More information at: http://www.amazon.com/exec/obidos/ASIN/0156238071/scienceweek
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ON HONEYBEE SOCIAL BEHAVIOR, GENES, AND THE ENVIRONMENT
The so-called social insects live in societies that rival human societies in complexity and internal cohesion. Honey bees, for example, apparently always follow 3 rules: a) they live in colonies with overlapping generations; b) they care cooperatively for offspring other than their own; and, c) they maintain a reproductive division of labor.
The following points are made by Gene E. Robinson (American Scientist 1998 86:456):
1) Genes do not play an exclusive role in regulating behavior: biologists have long realized that behavior is influenced by genes, the environment, and interactions between the two.
2) Genes never act alone. They must operate in an environment where they code for proteins that participate in many systems in an organism, with these systems in turn influencing the expression of genes. Consequently, biologists must take a broad approach in assessing the impact of any gene.
3) The research group of the author uses the Western honey bee, Apis mellifera. Honey bees pass through different life stages as they age, and their behavioral responses to environmental and social stimuli change in predictable ways. Although worker bees go through a consistent path of behavioral development, this path is not rigidly determined. Bees can accelerate, retard, or even reverse their behavioral development in response to changing environmental and colony conditions.
4) Experimental evidence indicates that juvenile hormone, one of the most important hormones influencing insect development, helps time the pace of behavioral maturation in honey bees. The rate of endocrine-mediated behavioral development is influenced by inhibitory social interactions. Older bees inhibit the behavioral development of younger bees: the rate of behavioral development is negatively correlated with the proportion of older bees in a colony. Inhibitory social interactions that influence the rate of behavioral development involve chemical communication between colony members.
5) Evidence from the laboratory of the author in 1993 indicated the so-called mushroom bodies in the bee brain are involved in the behavioral changes occurring during maturation, the volume of the bodies increasing, and the volume increase associated with an increase in synapses with neurons from brain regions devoted to sensory input. The author suggests this was the first report of brain plasticity in an invertebrate.
6) The author suggests that, in general, two-way interactions between the nervous system and the genome contribute fundamentally to the control of social behavior. Information about social conditions that is acquired by the nervous system is likely to induce changes in genomic function that in turn produce adaptive modifications of the structure and function of the nervous system.
7) The author proposes a new research initiative called "sociogenomics", defined as a "wide-ranging approach to identify genes that influence social behavior, determining the influence of these genes on underlying neural and endocrine mechanisms, and exploring the effects of the environment -- particularly the social environment -- on gene action."
American Scientist http://www.americanscientist.org
ScienceWeek http://www.scienceweek.com
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