Evolution made jerboa legs optimal for jumping, but then went further

(ORDO NEWS) — Analyzing the evolution of the hind legs of various jerboas, the scientists concluded that even after the bones of their feet had fused enough to form an optimal structure for jumping, they continued to grow together until they merged into a single whole.

The results of the study will be relevant for creating robot legs and studying the evolution of bipedalism in other species, including humans.

The bones of the foot, which are divided in small jerboas, are almost completely fused in their larger counterparts. Scientists from Michigan and California Universities (USA) decided to find out how this happened.

Their study showed that the bones of large jerboas fused much more than would be required by an optimal design that would best dissipate the loads from jumps and landings.

The discovery, described in the journal Proceedings of the Royal Society, will help develop robotic legs that can withstand high loads and make quick and agile jumping movements.

Jerboas are desert rodents that hop beautifully on their two hind legs. However, the legs of tiny species weighing only a few grams and those weighing up to half a kilogram look different.

The legs of small jerboas differ little from those of most other mammals: their foot bones are separated from each other. At the same time, large species may have different degrees of fusion of the foot.

To study the legs of different species, the authors took micro-CT scans of museum specimens and created 3D models of their feet, which were then scaled to the same size and subjected to various tests.

An analysis of the phylogenetic tree of jerboas showed that the bones of these animals were originally separated.

The first jerboas were small, their legs could support their weight even when jumping with high impact. In more evolutionarily later, large species of jerboas, these bones completely “merge”. In those species that are located in the middle of the evolutionary tree, partial fusion is observed.

Fused bones are subjected to less jumping stress than non-fused bones, thereby increasing resistance to stress. However, partially fused bones have the best characteristics – it is this design that can be called optimal.

According to scientists, evolution has reached the optimal point of partial fusion, but continued until the bones of the metatarsus are completely fused. Because fully fused bones were still strong enough to remain effective at jumping, jerboas experienced no evolutionary pressure to stop their bones from fusing.

Such an analysis may help to identify other ways of changing the shape of the skeleton, which the animals underwent during the transition from walking on four legs to upright posture. The authors of the study also plan to model the stresses faced by the foot bones of extinct human ancestors.

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