Bones that are separate in small gerbils are fully fused in large ones, but bony structures that are better at dissipating the stresses of jumping are only partially fused.
A bipedal jerboa, one of the rodent species included in a study of unpredictability in animal movements. Image credit: Talia Moore and Kim Cooper
Foot bones that are separate in small, jumping 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 know why.
It appears that once evolution put jerboa bones on the path to fusion, they overcame the optimal amount of fusion, the structure that best dissipated the stresses of jumping and landing, to join completely
Study: Metatarsal fusion resisted bending when gerbils (Dipodidae) transitioned from quadrupedal to bipedal (DOI: 10.1098/rspb.2022.1322)
This finding could inform the design of future robotic legs capable of withstanding the higher forces associated with rapid bursts of agile locomotion.
Jerboas are desert rodents that hop erratically on two legs to avoid predators. In the jerboa family tree, these two legs can look very different: there are species that weigh just three grams to those that weigh 400 grams, with heavier species having very different foot bones, or metatarsals. Lighter gerbils are like most other mammals, including humans: their metatarsal foot bones are separated from each other.
“We wanted to explore why we’re seeing these fused bones only in larger gerbils,” said Carla Nathaly Villacís Núñez, a UM mechanical engineering PhD and first author of the study in Proceedings of the Royal Society B.
“We found that fused bones showed lower stresses than unfused bones, thus strengthening against higher loads,” he 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 overflow.”
To study bone performance between species, the researchers took micro-CT scans of museum specimens and built 3D models of jerboa metatarsals in software, then scaled them to equal sizes and stress tested them while they hit, flexed and bounced off a surface. .
Smaller gerbils have three separate metatarsal bones, which are able to support the small stature of the rodent, even if used for high-impact jumping. More recent and larger species of jerboa have completely fused these three bones into one. Intermediate-weight species have something in between: a metatarsal with internal remnants of bone where it has partially fused, like a bundle of sticks.
Talia Moore
“Our interdisciplinary team applied state-of-the-art engineering techniques to unravel an evolutionary puzzle,” said Talia Moore, UM assistant professor of robotics and lead author of the study.
“Evolution reached an advantageous point of partially fused geometry, but then the evolutionary drive may have continued to fully fuse the metatarsals. Since the fully fused bones are still sufficient to prevent them from breaking, it is likely that no there has been evolutionary pressure to stop fusing.”
The research team notes that similar analyzes could help uncover other ways in which the skeleton changed shape to compensate as species evolved from quadrupedal locomotion, or walking on all fours, to locomotion biped
“While kangaroos, primates and other rodents 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, a graduate student who studies 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 locomotion.”
An additional 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 tracking 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 by a Harvard Chapman Memorial Fellowship, a Collaborative Research Grant from the David Rockefeller Center for Latin American Studies, and the UM Mechanical Engineering Research, Innovation, Service and Entrepreneurship Program.
/ Public communication. This material from the original organization/author(s) may be ad hoc in nature, edited for clarity, style and length. The views and opinions expressed are those of the author(s). See them in full here.