Why are very large herbivores absent from Australia? A new theory of micronutrients
We propose a Megacatalyst Theory, based on the pivotal role of the micronutrients iodine (I), cobalt (Co) and selenium (Se), in answer to the body size anomaly of herbivores on different continents, and the previously unexplained absence of megaherbivores in certain environments.Location
It is anomalous that megaherbivores are absent from Australia while present in even dry and nutrient-poor parts of southern Africa, and that they have been exterminated from the Americas, but not south-east Asia.Methods
We hypothesize that I, Co and Se are micronutrients in quantity, but megacatalysts in effect, determining maximum body size and pace of life, hence whether energy is used by animals or fire. The Megacatalyst Theory suggests that the greater the reproductive rate and brain size relative to body size, the greater the probable demand for I, Co and Se.Results
Balanced supply of I, Co and Se, within narrow tolerances, is elusive because of disparate cycles: I gravitates towards the sea, whereas Co and Se are concentrated in ultramafics and organic shales, respectively. Sufficiency of these micronutrients, at less than toxic concentrations, is vital for rapid metabolism and growth, particularly of the nervous system. Iodine controls thermogenesis, Co controls the gut fermentation supplying herbivores, and Se controls biochemical damage where both processes occur rapidly. The supply of Co allows vegetation to be metabolized instead of combusted, by promoting digestion of fibre by gut microbes. Herbivores demand I, Co and Se in greater concentrations than palatable plants necessarily contain, as an increasing proportion of energy is fermented from fibre with increasing body size. Economy of scale is limited by loss of I in urine (partly compensated by thyroid size), Co in faeces (partly compensated by gut compartments), and Se both ways.Main conclusions
The larger the herbivore species, the more it may depend on supplementation in order to survive predation by humans. As body mass increases, Co becomes deficient before I, because it is essential for rumination, and cannot be absorbed by the skin. Moderate uplift of a fairly flat landscape sustainably supplies I from mineralized springs, and Co from rocks (and Se from both), avoiding the excess of I in the sea and the excess of Co on high mountains. Iodine and Se leached to groundwater under dry climates are inaccessible to herbivores on a continent as flat as Australia, where even kangaroos have limited fecundity and intelligence compared to southern African ruminants of similar body mass. Where springs and associated earth-licks were available in the late Pleistocene, megaherbivores could evolve to survive the era of domestication.