Maximum animal mass over 3.5 billion years of evolution – at the metabolic limit
An upper bound on mass specific metabolism generates body mass evolution that is unbend and log-linear
Given constant selection for an increase in mass specific metabolism, we may expect macro evolution at the largest scale to reach some biochemical limit where further increase in mass specific metabolism is difficult, if not impossible, to obtain.
Evolution at this limit will proceed with a mass that increases from primary selection on resource handling and/or increased resource availability. The increase in mass will generate an allometric downscaling of mass specific metabolism away from the upper limit. And if the primary increase occurs relatively slow, this will allow for a selection increase that will return mass specific metabolism to the upper limit. This form of evolution follows the yellow curves in Fig. 1, and it is log-linear in physical time with a dw/dt-exponent of one in both 2D and 3D, and a rββ/rα-ratio of 1/2 in 2D, and 1/4 in 3D.
These trajectories balance the upward bending from time contraction due to metabolic acceleration against the downward bending from the time dilation of mass-rescaling, with no overall bend in physical time.
In a classical figure, Bonner (1965) showed a continued increase in the maximum length of animals over 3.5 billion years of evolution. I recalculated length to mass (Witting, 2008, 2020) and estimated the rate of change as a function of mass, and found an almost log-linear trajectory with a dw/dt-exponent of 1.07 (left figure). While the increase wasn’t uniform (Payne et al., 2009), it illustrates a macro evolution that is bound by an upper limit on metabolism. This is confirmed also by a mass specific metabolism that is invariant of mass across taxonomic groups from bacteria to mammals (Makarieva et al., 2008; Kiørboe and Hirst, 2014).
The natural selection of metabolism explains curvature in fossil body mass evolution
The natural selection of metabolism explains curvature in allometric scaling
Inevitable evolution: back to
- Bonner, J.T. 1965. Size and cycle. Princeton University Press, Princeton.
- rboe and Hirst, 2014Kiorboe:Hirst:2014Kiørboe, T., and A.G. Hirst 2014. Shifts in mass scaling of respiration, feeding, and growth rates across life-form transitions in marine pelagic organisms. The American Naturalist 183:E118--E130.
- Makarieva, A.M., V.G. Gorshkov, B.Li, S.L. Chown, P.B. Reich and V.M. Gavrilov 2008. Mean mass-specific metabolic rates are strikingly similar across life's major domains: Evidence for life's metabolic optimum. Proceedings of the National Academy of Sciences 105:16994--16999.
- Witting, L. 2008. Inevitable evolution: back to The Origin and beyond the 20th Century paradigm of contingent evolution by historical natural selection. Biological Reviews 83:259--294, https://doi.org/10.1111/j.1469--185X.2008.00043.x.
- Witting, L. 2020. The natural selection of metabolism explains curvature in fossil body mass evolution. Evolutionary Ecology 47:56--75, https://dx.doi.org/10.1007/s11692--020--09493--y.