Environmental complexity positively impacts affective states of broiler chickens

Abstract

Affective state can bias an animal's judgement. Animals in positive affective states can interpret ambiguous cues more positively ("optimistically") than animals in negative affective states. Thus, judgement bias tests can determine an animal's affective state through their responses to ambiguous cues. We tested the effects of environmental complexity and stocking density on affective states of broiler chickens through a multimodal judgement bias test. Broilers were trained to approach reinforced locations signaled by one color and not to approach unreinforced locations signaled by a different color. Trained birds were tested for latencies to approach three ambiguous cues of intermediate color and location. Broilers discriminated between cues, with shorter latencies to approach ambiguous cues closest to the reinforced cue than cues closest to the unreinforced cue, validating the use of the test in this context. Broilers housed in high-complexity pens approached ambiguous cues faster than birds in low-complexity pens-an optimistic judgement bias, suggesting the former were in a more positive affective state. Broilers from high-density pens tended to approach all cues faster than birds from low-density pens, possibly because resource competition in their home pen increased food motivation. Overall, our study suggests that environmental complexity improves broilers' affective states, implying animal welfare benefits of environmental enrichment.

Figure 1

Least squares mean estimates (± SEM) for latency to approach (s) all five cues (positive [P], near positive [NP], middle [MID], near neutral [NN], and neutral [N]) in the judgement bias test for 4 test rounds (n = 9). Means with different superscripts (^a–d) differ at p < 0.001.

Figure 2

Mean (± SEM) latency to approach (s) all five cues (positive, P; near positive NP; middle, MID; near neutral, NN; and neutral, N) for birds from both high- and low-complexity pens in the judgement bias test for 4 rounds (n = 9). Means with different superscripts (^a–d) differ at p < 0.01.

Figure 3

Percentage of birds with each gait score (ranging from 0, walks with no obvious impairments, to 2, unable to walk) in each treatment group. Percentage of birds (n = 72) in high-complexity, low-complexity, high stocking density (42.08 kg/m² at day 50), and low stocking density (23.83 kg/m² at day 50) pens receiving a gait score of either 0, 1, or 2.