In Harmony with the Earth Rotation: Is 1 kg Equal to 1 Year in Mammals?

Abstract

In an interesting paper based on a comprehensive data set of 557 mammal species (Damuth 1993), a striking regularity was revealed: mammals with body masses about 1 kg have higher ecological population densities (within their dietary groups and habitat types) than expected for their body sizes on the grounds of the energetic equivalence rule. In other words, they control a greater part of the energy flow in local ecosystems, compared to both larger and smaller mammal species (Damuth 1993). The well-known relationship between body size and life-history traits could explain this phenomenon. If we consider the regression line of age at first reproduction (including both age at maturation and gestation time) on body mass based on the most complete data set of 547 mammal species (Wootton 1987), we find that a body mass of 1 kg corresponds exactly to 1 year (Wootton 1987: Fig. 1). Thus, life-cycles of such species seem to be best synchronized with an annual phenological cycle, which might have ecological advantages. The large deviations of many smaller species along the mass axis at the point of 1-year age, i.e., the retardation of their maturation (Wootton 1987: Fig. 1), suggests the great advantage of this age value. These species tend to reach this value though their masses are not optimal for it from the standpoint of a typical allometric relationship between age and mass in mammals. Such a deviation of smaller species may involve a disadvantage of wasting of time (increase of age at first reproduction not conditioned by body mass). However, reproduction more frequent than once a year (i.e., at two different seasons, one of which may be unfavorable for it) seems to incur a greater cost. On the side of larger masses, there is a corresponding 'notch' in the cloud of species points and increased density of the points at an age of 1 year. Although a similar rationale might apply to any integer-year age of first reproduction, for two-, three-, orhigher-year ages, the effect should be weaker since a piece of time (a fraction of year), on which the first reproduction is retarded or accelerated so to reach an integer age value, is smaller relative to the total age value at first reproduction (two, three or more years). In the cloud of species points in Fig. 1 of Wootton's work, a similar protrusion (increased deviations) on the smaller masses side, and a corresponding notch on the larger masses side can be seen in the two-year region also, but it is less pronounced, and evidently has a lesser effect on the regularity revealed in Damuth's work (and nothing similar can be discerned in the three-year region). It would be interesting to check whether the connection noted here (maximum energy flow in species with a body size that corresponds to age at first reproduction of about 1 year on a typical regression line) occurs in other groups of animals. It would help to reveal the significance of synchrony between the life cycle of an animal and the phenological cycle of its environment (a resonance-like effect?).