The evolution of allometric outliers

Fig. 1 The allometric relationship between the basal metabolic rate (BMR) and body mass (w) in marsupial mammals; with an estimated exponent of 0.75 ± 0.01. Data from McNab (2008), with the honey possum (left red) and the southern hairy-nosed wombat (right red) removed as outliers from the regression.

The number of generations through which a given lineage is selected is an essential factor that affects the current evolutionary stage of the species. This is maybe best illustrated by species that evolve a small or large body mass at an early stage relative to the other species in a clade. These species will evolve over a larger or smaller number of generations that the main clade and, thus, they will have a tendency to evolve a higher or lower metabolic rate.

This evolution may explain shrews (Soricidae) with strongly increased metabolism (Platt, 1974), and bowhead whales (Balaena mysticetus) with smaller metabolism than expected from mass alone (George, 2009). Another example is Fig. 1, that shows the high metabolism of the small honey possum (Tarsipes rostratus), and the low metabolism of the large southern hairy-nosed wombat (Lasiorhinus latifrons), relative to the metabolic allometry for marsupial mammals.

These outliers from typical Kleiber scaling do not reflect adaptations in a traditional sense. Instead, they reflect differences in natural selection time; where each lineage has its own timescale that is defined by the number of generations it evolves.

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References

• George, J.C. 2009. Growth, morphology and energetic of bowhead whales (εm Balaena mysticetus). (Thesis, Ph.D.). University of Alaska, Fairbanks.
• Platt, W.J. 1974. Metabolic rates of short-tailed shrews. Physiological Zoology 42:75--90.