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Plenty of Room at the Bottom?

Tiny animals solve problems of housing and maintaining oversized brains, shedding new light on nervous-system evolution

William G. Eberhard, William T. Wcislo

Haller’s Rule, Grades, Behavior

2012-05EberhardF8.jpgClick to Enlarge ImageThe common supposition derived from vertebrates—that animals in lower grades are necessarily inferior or less capable in their behavior—is not well supported by facts. Among adult orb-weaving spiders that vary in body mass by 400,000 times, including those near the lower size limit for spiders, there is no evidence of inferior performance with respect to the precision of executing certain web construction behaviors or the ability to adjust web designs to local conditions or to build alternative web designs. Qualitative observations also suggest that size limitation of behavior fails to explain some other grade differences, such as that between honey bees and salamanders. Miniature salamanders and honey bees have approximately the same body size, but the latter fall on an allometric line lower than that of the salamander. A honey bee is nevertheless capable of such feats as navigating using landmarks and polarized sunlight that is adjusted for time of day; learning complex patterns involving different sensory modalities or general concepts such as different or similar and above or below; and using language to tell nest mates where to find food. It would be difficult to defend the argument that a honey bee is behaviorally inferior to a miniature salamander.

Even the neuroanatomically simple nematodes behave in ways that are not fundamentally different from many animals with many more neurons. For example, C. elegans senses and responds to various stimuli, including physical contact with environmental objects, and perceives various chemicals, oxygen concentration, osmolarity, pH, temperature, light and pheromones. These various inputs are used to coordinate motor output, and to evaluate conditions such as the density and sex of conspecifics. Motor outputs include different movements for swimming and for crawling on a surface, for turning or reversing those movements or for altering them after fixed time periods. These animals orient and move toward or away from point stimuli; forage for food, engulf it, perform rhythmic swallowing movements and defecate; seek mates, copulate and lay eggs. In addition, nematodes can learn to modify a variety of motor behaviors on the basis of their experience.

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