Differences among domestic chicken breeds in tonic immobility responses as a measure of fearfulness

Abstract

Background: One priority for animal welfare is for animals to experience less fear, especially during human contact. For domestic animals, breeds that are less fearful may provide genetic resources to develop strains with improved welfare due to lower susceptibility to fear. Genetic predispositions inherited in these breeds might reflect the large diversity of chicken breeds. The goal of the present study was to systematically test a diverse group of chicken breeds to search for breeds that experience less fear.

Methods: Nineteen chicken breeds from commercial hybrid lines, native layer-type, meat-type and dual-purpose breeds, ornamental breeds as well as bantam breeds were tested in a standardized tonic immobility (TI) test. Chickens were manually restrained on their back, and the time to first head movement and first leg movement, the duration of TI, as well as the number of attempts needed to induce TI were measured.

Results: The TI response differed among chicken breeds (p ≤ 0.001) for naïve, mature hens. The median number of attempts required to induce TI ranged from 1 to 2 and did not differ significantly among breeds. Median durations were much more variable, with Lohmann Brown showing shortest durations (6 s, 12 s, 58 s for time to first head movement, first leg movement and total duration of TI, respectively). In contrast, medians reached the maximum of 600 s for all three measures in German Creepers. Repeated tests on the same individuals did not affect attempts needed to induce TI nor TI durations. Breeds clustered into two main groups, with layer-type native breeds and ornamental breeds having longer TI durations, and bantam, dual-purpose and meat-type native breeds having shorter TI durations.

Conclusions: Our findings provide evidence for substantial variation of fearfulness among breeds. This variation could be linked to the intended use during the breed's specific history. Knowledge and quantitative measurement of these behavioural responses provide the opportunity to improve welfare through selection and future breeding.

Keywords: Animal genetic resources; Animal welfare; Chicken; Fear; Poultry; Tonic immobility.

Figure 1. Duration of Tonic Immobility response of chicken breeds.

Total duration of the tonic immobility (TI) response of 19 breeds of domestic chickens (part A; mature, experimentally naïve hens). Breeds are sorted by median duration. Medians [s] are indicated by a vertical line, 25th and 75th percentiles form the bar, 10th and 90th percentiles are indicated by whiskers, and outliers are shown as points. Grey shading reflects the categorization of the breeds based on the intended use and breeding history.

Figure 2. Breed categories based on TI responses.

Total duration of the tonic immobility (TI) response of all individuals in each of the six breed categories (Part A; mature, experimentally naïve hens). Breed categories are sorted by median duration. Medians [s] based on all individuals in a given breed category are indicated by a vertical line, 25th and 75th percentiles form the bar, 10th and 90th percentiles are indicated by whiskers and outliers are shown as points.