Plenty of Room at the Bottom?
Tiny animals solve problems of housing and maintaining oversized brains, shedding new light on nervous-system evolution
A basic fact of life is that the size of an animal’s brain depends to some extent on its body size. A long history of studies of vertebrate animals has demonstrated that the relationship between brain and body mass follows a power-law function. Smaller individuals have relatively larger brains for their body sizes. This scaling relationship was popularized as Haller’s Rule by German evolutionary biologist Bernhard Rensch in 1948, in honor of Albrecht von Haller, who first noticed the relationship nearly 250 years ago. Little has been known, however, about relative brain size for invertebrates such as insects, spiders and nematodes, even though they are among Earth’s more diverse and abundant animal groups. But a recent wave of studies of invertebrates confirms that Haller’s Rule applies to them as well, and that it extends to much smaller body sizes than previously thought.
These tiny animals have been able to substantially shift their allometric lines—that is, the relationship between their brain size and their overall body size—from those of vertebrates and other invertebrates. Animals that follow a given allometric line belong to the same grade and changes from one grade to another are known as grade shifts. The result is that different taxonomic groups have different, variant, versions of Haller’s Rule.
The mechanisms that are responsible for grade shifts are only beginning to be understood. But this combination of generality and variability in Haller’s Rule appears to call into question some basic assumptions regarding the uniformity of how the central nervous system functions among animals. It also reveals a number of overlooked design challenges faced by tiny organisms. Because neural tissue is metabolically expensive, minute animals must pay relatively higher metabolic costs to power their proportionally larger brains, and they thus face different ecological challenges. There is reason to expect that tiny animals might cut corners wherever possible, for example by adopting lifestyles that are behaviorally less demanding. Yet available evidence indicates that at least some small-bodied animals express the same kinds of behavior as their large-bodied relatives.
Biologists have tended to ignore the lower limits of body size and the physiological processes that are associated with evolutionary decreases in brain mass. Instead they have focused on evolutionary increases in brain size, and its possible links to intelligence and other mental processes. And almost all of the current data have come from adults. But problems associated with the demands of a relatively large nervous system in a small animal are not limited to taxa with miniaturized adults. Many species have extremely small immature stages that are free-living, and whose growth and survival depends on their behavioral capabilities.
The new data on invertebrate brain allometry have several important implications. They challenge vertebrate-based hypotheses that were proposed to explain Haller’s Rule that invoked factors such as surface-volume relations, longevity and metabolic rates. They also challenge the idea, again derived from studies of vertebrates, that animals with relatively and absolutely larger brains have more sophisticated behavioral abilities and mental capabilities. For these reasons, the time is ripe for exploring the ways that central nervous systems are organized at very small scales, within constraints that differ from those of large-bodied organisms.