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Despite 100 million years of evolutionary divergence, people, platypuses, and kangaroos all allocate the same proportional amount of energy to reproduction and offspring development, according to a study appearing this week in Proceedings of the Royal Society B: Biological Sciences.

“This is more evidence that fundamental metabolic constraints underlie the development of wildly different mammals on very different branches of the tree of life,” says SFI Postdoctoral Fellow Marcus Hamilton, the paper’s lead author.

The researchers compared data from three lineages of mammals: placentals (wombed mammals), marsupials (pouched mammals), and monotremes (egg-laying mammals). Historically these groups diversified rapidly on different land masses as the supercontinent Pangaea fragmented and drifted apart -- and each evolved conspicuously distinct reproductive and lifespan energy tradeoff strategies.

Marsupials, for example, are born sooner at a smaller size and are nurtured in the pouch. Placentals are born later and larger and are nourished longer by lactation until they become independent.

The researchers find that despite their very different life histories and reproductive physiologies, all three groups of mammals produce new body mass at an equivalent and predictable rate.

Their study supports the belief among a growing number of scientists that many properties of living things, such as life spans, growth rates, and reproductive capacities, change in predictable ways with organisms’ body sizes -- a phenomenon known as allometric scaling.

Such scaling theories, based in part on work by SFI External Professor Jim Brown, SFI Distinguished Professor Geoffrey West, and SFI External Professor Brian Enquist, point to constraints imposed by a mammal’s metabolic processes. Essentially, the bigger the organism, the more slowly its transport networks (such as blood vessels) deliver resources to cells. Based on the size of the organism, resources can be delivered to cells efficiently but only so efficiently.

“Metabolic processes underlie the fundamental characteristics of mammalian life, and our findings add support to the notion that metabolism is to ecology as genetics is to evolution,” Hamilton says. “In essence, placentals, marsupials, and monotremes are simply evolutionarily equivalent ways of being a mammal.”

The paper’s co-authors are Ana Davidson of the Universidad Nacional Autonoma de Mexico, Richard Sibly at the University of Reading, and Jim Brown, with SFI and the University of New Mexico.