Which chicken found it harder to cross the road?

By Felipe Brando Cadavid

It has been well established that there is a clear relationship between body size/mass and the metabolic cost of locomotion, generally larger animals are more energy efficient than smaller animals as large animals tend to have longer legs that move with fewer strides which also require fewer muscle contractions.

This generalization seems to hold true for all animals all the way from nimble insects to large megafauna like elephants, between different species the assumption holds but what about within a species? Does it mean that larger members of a species will have a lower metabolic cost?

A study conducted on one our most well-known animal minions, the chicken, has helped us shed some light on the metabolic costs of locomotion within a species and has produced some surprising results.

The study compared chickens of different breeds that were anatomically proportional but disparate in size, the breeds were chosen to avoid bias from gross anatomical proportion differences and, the smaller breed was akin to shrunken down version of the larger breed. The metabolic comparison was made by putting both breeds through their paces in a specially designed treadmill where their respiration could be measured, and gentle encouragement could be given by a helpful volunteer.

To the surprise of the researchers, the assumption did not hold true for species members of differing mass and size, both breeds had similar metabolic costs for locomotion and that size and body mass was not a predictive factor.

The same researchers decided to go further and investigate if gender would influence the findings due the large degree of dimorphism displayed by chickens, to start they looked at juvenile chickens before they displayed sexual maturity and as expected the metabolic and morphological differences were minimal. When they looked at differences in mature chickens they found that paradoxical to the general assumption that the more active, mobile and larger males had a much costlier locomotion than females, despite females being encumbered by their egg mass that can make nearly 15% of their mass.

From their study, it became clear that on average there is no metabolic cost difference within a species but that there is a strong indication that gender and secondary sexual characteristics will impact metabolic locomotion costs.

So, to answer the title's question, all chickens found it just as hard to cross the road unless you are a large and spritely cockerel. But more important are the broader implications that size, body mass and gender can have when studying animal locomotion and selection pressures.

Based on a seminar by Dr Kayleigh Rose