Adrian Bejan | Running, from Design in Nature

In this video, Adrian Bejan extends his analysis of animal locomotion to running, showing how flow architecture and resistance shape the evolution of movement on land. He introduces the factor R, a measure of environmental resistance, which varies between air, water, and land, influencing how animals move and how much effort they expend. Bejan connects these insights to frequency of undulation, size effects, and the food chain, explaining how physics predicts not only speed but also the natural hierarchy of who catches whom.

• Bejan defines R with three memorable values: around 10 for flying in air, 1 for swimming in water, and an intermediate value for running on land, dependent on terrain quality.

• He uses examples like the hippo on muddy ground and the ostrich or impala on flat land, showing how surface conditions change R and, therefore, running speed.

• The frequency of undulation, leg stride, wing flapping, and tail movement decreases with animal size, predicting that bigger animals should move their limbs less frequently but still run faster than smaller ones.

• Bejan illustrates how this prediction explains why elephants and hippos can outrun humans despite appearing sluggish, making them dangerous to overconfident observers in the wild.

• He extends the food chain prediction: runners catch fish, birds catch runners, following the order of R values and showing how movement evolved toward easier access, from slow and burdensome to fast and economical.

• Bejan concludes by linking this to the evolution of terrestrial locomotion, showing that life spread toward environments that offered greater access and lower effort per distance traveled.

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Umit Gunes, Ph.D.
Assoc. Prof. | Yildiz Technical University
Editor | International Communications in Heat and Mass Transfer
Guest Editor | Philosophical Transactions of the Royal Society A
Guest Editor | BioSystems
Web | umitgunes.com