|
ScienceWeek
EVOLUTIONARY BIOLOGY: ON ANTS
The following points are made by Philip S. Ward (Current Biology 2006 16:R152):
1) Ants are one of evolution's great success stories. Arising in the mid-Cretaceous about 120 million years ago, they now comprise a diverse assemblage of approximately 20,000 species and have colonized most of the world's terrestrial biomes. They impose a strong ecological footprint in many communities in their varied roles as scavengers, predators, granivores, and herbivores. In some tropical forests the biomass of ants exceeds that of terrestrial vertebrates by a factor of four, and their soil-turning activities dwarf those of earthworms. There is a word for "ant" in most languages, reflecting their ubiquity and distinctiveness to humans. The ecological dominance and conspicuous social behavior of ants have long engaged the attention of natural historians. In terms of their species diversity, relative abundance, ecological impact and social habits, ants emerge as one of the most prominent groups of arthropods.
2) There is impressive breadth in the ecological characteristics of ants and in the range of environments to which they have adapted. From deserts to tropical rainforests, from grasslands to mangrove swamps, most terrestrial habitats are tenanted by ants, and usually in moderate to high densities. Ants are entirely absent only from polar regions and poorly insolated high altitude locations (for example, tropical cloud forests above 2400 meters). Nests are situated in a wide variety of sites from high in the forest canopy to deep underground. While most ant species are rather generalized scavengers, others have become specialized predators, seed-harvesters, and fungus-growers. Many species avidly tend honeydew-producing hemipterans, imbibing liquids processed through the gut of these plant-feeding insects, and thereby acting as indirect herbivores (as well as mutualists with their hosts).
3) A legion of other arthropods has become intimately associated with ant colonies, exploiting the rich concentration of resources therein. Relationships range from parasitic to mutualistic. One of the more striking symbioses is the coevolved association between attine ants and certain basidiomycete fungi which they culture in their nests. This relationship is now known to involve additional participants, including a mutualistic bacterium that helps the ants suppress unwanted parasitic fungi. Mutualistic interactions between ants and vascular plants are also common, with many plants supplying rewards to ants in the form of food and/or shelter in return for protection against herbivores.
4) Nearly all ant species are eusocial, that is, they live in perennial colonies with overlapping generations, cooperative care of the brood, and -- crucially -- reproductive division of labor, such that most colony members belong to a non-reproductive (worker) caste. The only exceptions are a few socially parasitic species which have secondarily lost the worker caste and rely on labor supplied by workers of their host ants. It is reasonable to assume that the most recent common ancestor of ants was eusocial, and that subsequent evolution has involved elaborations of that trait. This is in contrast to bees and wasps in which worker-based societies have evolved several times from solitary ancestors.[1-5]
References (abridged):
1. B. Bolton, Synopsis and classification of Formicidae, Mem. Am. Entomol. Inst. 71 (2003), pp. 1 370
2. A.F.G. Bourke and N.R. Franks, Social evolution in ants, Princeton University Press, Princeton (1995)
3. D.W. Davidson, S.C. Cook, R.R. Snelling and T.H. Chua, Explaining the abundance of ants in lowland tropical rainforest canopies, Science 300 (2003), pp. 969 972
4. D. Grimaldi and D. Agosti, A formicine in New Jersey Cretaceous amber (Hymenoptera: Formicidae) and early evolution of the ants, Proc. Natl. Acad. Sci. USA 97 (2000), pp. 13678 13683
5. B. Hölldobler and E.O. Wilson, The ants, Harvard University Press, Cambridge, Massachusetts (1990)
Current Biology http://www.current-biology.com
--------------------------------
Related Material:
ANIMAL BEHAVIOR: ON ANT NAVIGATION
The following points are made by Francis Ratnieks (Nature 2005 436:465):
1) There are probably 20,000 ant species and they do not all use the same navigational methods or have equal navigational abilities. Many can reorient on trails by using external cues, including landmarks and the position of the Sun. Leafcutter ants are even thought to use the Earth's magnetic field. But recent research has shown that one common ant, the pharaoh's ant, Monomorium pharaonis, has a sense of geometry, and other species probably do as well.
2) A pharaoh's ant colony forms a foraging-trail network leading from the nest entrance into the surrounding environment. These trails form Y-shaped branches with an internal angle of approximately 60 degrees as they lead away from the entrance. Ants walking the wrong way along a trail are unable to reorient at a trail bifurcation if the angle is 120 degrees. But if the angle is less, then they can. Angles less than 120 degrees give the "Y" bifurcation a nest-environment polarity, whereas at 120 degrees there is only symmetry. The ability to reorient is maximized at the natural bifurcation angle of 60 degrees.
3) Natural selection has made insect societies good at solving a problem that is simple to state but hard to solve -- to send foragers to where the food is. Because social insects have been solving this complex dynamic problem for millions of years, they have probably evolved some simple and elegant solutions. We should care about these solutions because human life depends more and more on engineering systems that must solve similar problems to function efficiently -- electronic messaging, grid computing, transmitting electricity and traffic regulation to name a few. One obvious lesson we might learn is how to make our systems more reliable and robust. If there is one thing that natural selection should be good at, it is eliminating solutions that are not robust. The colony or organism that "crashes" will soon be a dead one.
4) When a physicist plays ants at their own game -- trying to understand a problem fundamental to colony survival that ants have been working on, by means of natural selection, for millions of years -- ants can come out ahead. It is no disgrace to be outsmarted by ants. But are we smart enough to learn from them?[1-3]
References:
1. Feynman, R. P. Don't You Have Time To Think? (ed. Feynman, M.) (Allen Lane, 2005).
2. Feynman, R. P. Surely you're joking, Mr. Feynman! (Norton, New York, 1985).
3. Jackson, D. E., Holcombe, M., Ratnieks, F. L. W. Nature 432, 907-909; 2004.
Nature http://www.nature.com/nature
--------------------------------
Related Material:
GENETIC CONTROL OF SOCIAL ORGANIZATION IN ANTS
Notes by ScienceWeek:
Biological evolution is marked by a number of major transitions, one of which is the evolution of complex social behavior. Animal social life can take a variety of forms, each distinguished by features such as group size and the reproductive roles of group members. One focus in evolutionary biology is to identify the causes of social behavior and its conspicuous variation, and to determine the extent to which social organization is under genetic control. Such information is useful for reconstructing pathways of animal social evolution. Current views on insect social evolution stress the importance of ecological and behavioral environments in molding what are largely plastic social behaviors.
The following points are made by K.G. Ross and L. Keller (Proc. Natl. Acad. Sci. 1998 95:14232):
1) The authors report evidence that major variation in the social organization of fire ant colonies is under simple genetic control, providing a demonstration of an apparent strong genetic component to complex social behavior.
2) The authors report that a single genomic element (the gene [Gp-9]) is responsible for the existence of two distinct forms of social organization in the fire ant *Solenopsis invicta. This genetic factor apparently influences the reproductive *phenotypes and behavioral strategies of ant queens and determines whether workers tolerate a single fertile queen or multiple queens per colony. The authors suggest "these findings reveal how a single genetic factor can have major effects on complex social behavior and influence the nature of social organization."
--------------------------------
Notes by ScienceWeek:
Solenopsis invicta: The fire ant S. invicta is an introduced pest species in the southern US, the species existing in two distinct social forms. The "monogyne" form features colonies with a single fertile (egg-laying) queen, whereas the "polygyne" form features colonies with multiple fertile queens. The two social forms differ in other major aspects of their reproductive biology.
phenotypes: The term "phenotype" refers to the total appearance of an organism as determined by the interaction during development between its genetic constitution (genotype) and the environment.
Proc. Nat. Acad. Sci. http://www.pnas.org
ScienceWeek http://scienceweek.com
|