How evolution overshot the optimum bone construction in hopping rodents

Bones that are separate in small jerboas are fully fused in large jerboas, but the bony structures that best support the stresses of jumping are only partially fused

A two-legged jerboa, one of the rodent species included in a study of the unpredictability of animal movements. Photo credits: Talia Moore and Kim Cooper

Foot bones, which are separate in small hopping rodents, have fused in their larger relatives, and a team of researchers from the University of Michigan and the University of California, San Diego wanted to know why.

It seems that once evolution put jerboa bones on the path to fusion, they exceeded the optimal amount of fusion—the structure that best dissipates stress during jumping and landing—to become fully fused.

Study: Metatarsal fusion resisted flexing when jerboas (Dipodidae) transitioned from quadruped to bipedal (DOI: 10.1098/rspb.2022.1322)

This finding could inform the design of future robotic legs that can withstand the higher forces associated with rapid bursts of agile locomotion.

Jerbois are desert rodents that erratically hop on two legs to avoid predators. Across the jerboa family tree, these two legs can look very different: there are species that weigh as little as three grams and those that weigh 400 grams, with heavier species having very different foot bones, or metatarsals. Lighter jerboas are like most other mammals, including humans: their metatarsal bones are separated.

“We wanted to investigate why we only see these fused bones in larger jerboas,” said Carla Nathaly Villacís Núñez, UM PhD student in mechanical engineering and first author of the study in Proceedings of the Royal Society B.

“We found that the fused bones exhibited lower stresses than unfused bones, causing them to strengthen against higher loads,” she said. “But we also found that the partially fused bones are subjected to even lower stresses than the fully fused bones. One hypothesis is that fully fused jerboas overshoot evolutionarily.”

To examine the bone performance of different species, the researchers took micro-CT scans of museum specimens and created 3D models of the jerboa’s metatarsals in software, then scaled them to equal size and tested them under load when they hit a surface , bent and jumped .

The smaller jerboas have three separate metatarsals that are able to support the rodent’s small stature even when used for highly effective jumps. The newer, larger jerboa species have completely fused these three bones into one. The medium-weight species have something in between: a metatarsal with internal bony remnants where it’s partially fused together, like a bundle of sticks.

Talia Moore

“Our interdisciplinary team applied cutting-edge engineering techniques to solve an evolutionary puzzle,” said Talia Moore, assistant professor of robotics at UM and senior author of the study.

“Evolution hit an advantageous point of partially fused geometry, but then evolutionary dynamics may have fully fused the metatarsals. Since the fully fused bones are still enough not to fracture, there probably wasn’t any evolutionary pressure to stop the fusion.”

The research team notes that similar analyzes could help uncover other ways in which the skeleton changed shape to compensate for the species’ evolution from quadruped or quadruped to bipedal locomotion.

“While kangaroos, primates, and other rodents approached bipedalism, the dynamics of their locomotion and the anatomical changes associated with this shift are quite different in each case,” said Andrew Ray, a graduate student studying materials science and engineering in Moore’s lab.

“Through similar analysis, we were able to simulate how the foot bones of extinct human ancestors might have been subjected to stress during walking, running, or other locomotion.”

Another author is Kimberly Cooper, a professor of developmental biology at the University of California, San Diego, who formulated the idea for the project with Moore during a separate study tracing the evolution and development of metatarsal fusion in jerboas. Cooper’s expertise was key to understanding the evolutionary implications of the findings.

The research was supported in part by a Harvard Chapman Memorial Fellowship, a David Rockefeller Center for Latin America Studies Collaborative Research Grant, and the UM Mechanical Engineering Research, Innovation, Service and Entrepreneurship Program.