How evolution overtook optimal bone structure in hopping rodents

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Bones that are separated in small jerboas are fully fused in large ones, but the bony structures that are best at dissipating the stresses of jumping are only partially fused.

A bipedal jerboa, one of the rodent species included in a study of the unpredictability of animal movements. Image credit: Talia Moore and Kim Cooper

Foot bones that are separated in small, hopping rodents are fused together in their larger cousins, and a team of researchers from the University of Michigan and the University of California, San Diego wanted to find out why.

It seems that once evolution put jerboa bones on a path to fusion, they passed the optimal amount of fusion – the structure that best dissipates the stresses of jumping and landing – to become completely linked.

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

Jerboas are desert rodents that jump erratically on two legs to avoid predators. In the family tree of the jerboa, these two legs can be very different: there are species that weigh only three grams and those that weigh 400 grams, with the heavier species sporting very different foot bones or metatarsals. Lighter jerboas are like most other mammals, including humans: their foot metatarsal bones are separated from each other.

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

“We found that the fused bones exhibited lower stresses than the unfused bones, thus strengthening against higher loads,” she said. “But we also found that partially fused bones had even lower stresses than fully fused bones. One hypothesis is that fully fused jerboas have evolutionary overshoot.

To study bone performance across species, the researchers micro-scanned museum specimens and built 3D models of jerboa metatarsals in software, then scaled them to equal sizes and stress tested them as they kicked, flexed and jumped off a surface. .

The smallest jerboas have three separate metatarsal bones, which are able to support the rodent’s small size even if used for high impact jumps. Later and larger species of jerboa have completely fused these three bones into one. Mid-weight species have something in between: a metatarsal with remnants of interior bone where it has partially fused, like a bundle of sticks.

Talia Moore
Talia Moore

“Our interdisciplinary team applied state-of-the-art engineering techniques to unravel an evolving puzzle,” said Talia Moore, Assistant professor of robotics at UM and lead author of the study.

“Evolution reached an advantageous point of partially fused geometry, but evolutionary momentum may have continued to fully fuse the metatarsals. Because fully fused bones are still sufficient to prevent breakage, there was probably no no evolutionary pressure to stop fusion.

The research team notes that similar analyzes could help uncover other ways the skeleton has changed shape to compensate for the evolution of species from quadrupedal locomotion, or walking on four feet, to bipedal locomotion.

“While kangaroos, primates and other rodents have converged on bipedalism, the dynamics of their locomotion and the anatomical changes associated with this change are quite different in each case,” said Andrew Ray, an undergraduate student studying the materials science and engineering in Moore’s lab.

“Through a similar analysis, we could simulate how the foot bones of extinct human ancestors might have experienced stress during walking, running, or other means of locomotion.”

Another author is Kimberly Cooper, 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 the jerboas. Cooper’s expertise was essential in understanding the evolutionary implications of the findings.

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

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