The effects of selective breeding on the architectural properties of the pelvic limb in broiler chickens: a comparative study across modern and ancestral populations

Abstract

Intensive artificial selection has led to the production of the modern broiler chicken, which over the last few decades has undergone a dramatic increase in growth rate and noticeable changes in body conformation. Unfortunately, this has been associated with musculoskeletal abnormalities which have altered the walking ability of these birds, raising obvious welfare concerns, as well as causing economic losses. Here we present a comparative study of ancestral and derived muscle anatomy in chickens to begin to tease apart how evolutionary alterations of muscle form in chickens have influenced their locomotor function and perhaps contributed to lameness. We measured the muscle architectural properties of the right pelvic limb in 50 birds, including the Giant Junglefowl, a commercial strain broiler and four pureline commercial broiler breeder lines (from which the broiler populations are derived) to identify which features of the broiler's architectural design have diverged the most from the ancestral condition. We report a decline in pelvic limb muscle mass in the commercial line birds that may compromise their locomotor abilities because they carry a larger body mass. This greater demand on the pelvic limb muscles has mostly led to changes in support at the hip joint, revealing significantly larger abductors and additionally much larger medial rotators in the broiler population. Differences were seen within the commercial line bird populations, which are likely attributed to different selection pressures and may reflect differences in the walking ability of these birds. In addition, Junglefowl seem to have both greater force-generating capabilities and longer, presumably faster contracting muscles, indicative of superior musculoskeletal/locomotor function. We have provided baseline data for generating hypotheses to investigate in greater depth the specific biomechanical constraints that compromise the modern broiler's walking ability and propose that these factors should be considered in the selection for musculoskeletal health in the chickens of the future. Our new anatomical data for a wide range of domestic and wild-type chickens is useful in a comparative context and for deeper functional analysis including computer modelling/simulation of limb mechanics.

Fig. 1

A typical modern broiler chicken breeding programme, represented as a pyramid where each level represents a generation. The great-grandparent line/purelines on the top of the production line are where desired traits are selected across four lines. Within the pedigree segment are the specific male and female lines, with the males typically selected for heritable growth and production traits and the female lines selected for early growth and conformation (Anthony, 1998). The commercial broiler (fifth generation) is derived from the cross of a male and female parent line.

Fig. 2

The PCSA against fascicle length for the pelvic limb muscles of the purelines A, B, C and D. Values (means) are normalized for comparison. The colour of the labelled data points for each muscle matches the colour of that particular muscle in the anatomical diagram above. The uncoloured points represent the remaining muscles of the pelvic limb.

Fig. 3

The PCSA against fascicle length for the pelvic limb muscles of the broiler population. Values (means) are normalized for comparison. The colour of the labelled data points for each muscle matches the colour of that particular muscle in the anatomical diagram above. The uncoloured points represent the remaining muscles of the pelvic limb. The data points for the pureline populations are shaded in grey.

The PCSA against fascicle length for the pelvic limb muscles of the juvenile and adult Junglefowl population. Values (means) are normalized for comparison. The colour of the labelled data points for each muscle matches the colour of that particular muscle in the anatomical diagram above. The uncoloured points represent the remaining muscles of the pelvic limb. The data points for the broiler population are shaded in grey.

Fig. 5

Functional distribution of muscle mass within the pelvic limb for all commercial line birds and Junglefowl populations. *Significant difference at the 0.05 level to the broiler population. ^{ˇ ”}Significant difference at the 0.05 level within the Junglefowl populations and pureline populations, respectively. Data are median ± interquartile range.