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New Study Shows Patchy Resources Drove Evolutionary Changes to Body Size

December 19, 2019

Resource allocation isn’t just a problem for humans preparing a holiday dinner, or squirrels storing up nuts for the winter. It can actually affect the size of an animal or whether it procreates, according to quantitative systems biology Professor Justin Yeakel’s new paper published in the journal Proceedings of the National Academy of Sciences (PNAS).

“Every single thing has to do this, from single-celled life forms all the way up to mammals or dinosaurs,” Yeakel said. “Animals have to decide how they route their energy, whether it's to build more mass on to themselves or invest in reproduction.”

Yeakel authored the study with former postdoctoral researcher Uttam Bhat and Chris Kempes of the Santa Fe Institute. The authors created a new model of how animals budget their energy, which sheds light on how they live and explains evolutionary-scale changes in body size. This study proposes energy budgets have a previously overlooked key mechanism: the variability of resources across a landscape.

To map an animal’s theoretical “life-history,” the authors rely on several key variables: how much energy an animal has, its metabolism, the cost of reproduction, and how much energy is delivered to its offspring. The new model shows that the more spread out and patchy an animal’s food is, the more unstable its population is, with smaller animals more affected and likely to go extinct.

“If you have a lot of uncertainty, you want to have a lot of buffers in your body,” Bhat said.

Typically, this means an increase in body size — bigger bodies have more room for energy storage. But acquiring resources isn’t necessarily easy. In their new model, the authors give resources a spatial dimension. Resources can be spread out over an area uniformly, such as grassy plains, or clustered, such as fruit bushes. Fluctuations in the availability of these resources, such as the distance between fruit bushes becoming larger, can have an enormous impact on animals.

Between 20 million years ago and 5 million years ago, grasslands sprouted up around the world. For mammals that adapted by growing large enough to graze on grasslands, food was ubiquitous; mammals that remained small were unable to partake, and less certain to secure dinner. If you plug in the variables to the author’s model, what you get is what the fossil record shows: an evolutionary pressure for herbivores to grow larger from rodent-sized to bison.

“It's sort of fun, I think, that we build up from single-organism risk all the way to long-term evolutionary trends,” Kempes said. “I think the push to address lots of the major questions in this way is where much of the future of ecology lies.”