Raising laying hens: housing complexity and genetic strain affect startle reflex amplitude and behavioural response to fear-inducing stimuli.

Individual variation in fearfulness can be modified during ontogeny, and high levels of fear can affect animal welfare. We asked whether early-life environmental complexity and genetic strain affect fear behaviour in young laying hens (pullets). Four replicates of brown (B) and white (W) genetic strains (breeds) of layers were each raised in four environmental treatments (housing): conventional cages (Conv) and different rearing aviaries with increasing space and complexity (Low < Mid < High). We used a startle reflex test (weeks 4 and 14) to measure startle amplitude and autonomic response (i.e. comb temperature). A combination of novel arena (NA) and novel object (NO) tests was used (week 14) to assess NA exploration and alertness, latency to approach the centre and initial NO avoidance and investigation. Housing × strain affected startle amplitude (B-Conv, B-High < B-Low, B-Mid; B > W; no housing effect in W) but not autonomic response. Fear behaviour was affected by housing (NA exploration, investigation: Conv < Low, Mid, High; NO avoidance: Conv, High < Low, Mid), strain (NA alertness: B > W, NO avoidance: W > B) and their interaction (NA centre approach: B-Conv < all other groups). We present evidence for strain-specific fear responses depending on early experience.

The development of laying hen locomotion in 3D space is affected by early environmental complexity and genetic strain.

Adult laying hens are increasingly housed in spatially complex systems, e.g., non-cage aviaries, where locomotion between elevated structures can be challenging for these gallinaceous birds. This study assessed the effect of early environmental complexity on spatial skills in two genetic strains. Brown (B) and white (W) feathered birds were raised in: Conventional cages with minimal complexity (Conv) or rearing aviaries with low (Low), intermediate (Mid), or high complexity (High). Birds from each housing treatment were challenged at three different time points in three different, age-appropriate vertical spatial tasks. Whites performed better than brown birds in all tests regardless of rearing environment. In chicks, test performance was predominantly explained by variation between replicates and differences in motivation for test participation. Treatment effects were seen in pubertal birds (pullets), with pullets from aviaries performing better than those from Conv. White High pullets performed better than white Mid or Low, an effect that was not found in browns. Pullets preferred to use a ramp to move downwards, but only when ramps had previously been experienced and when the ramp was not too steep. Overall, early environmental complexity affected spatial skills of laying hen pullets with stronger effects in white than brown feathered birds.

Breed and loading history influence in vivo skeletal strain patterns in pre-pubertal female chickens.

The influence of loading history on in vivo strains within a given specie remains poorly understood, and although in vivo strains have been measured at the hindlimb bones of various species, strains engendered during modes of activity other than locomotion are lacking, particularly in non-human species. For commercial egg-laying chickens specifically, there is an interest in understanding their bones' mechanical behaviour, particularly during youth, to develop early interventions to prevent the high incidence of osteoporosis in this population. We measured in vivo mechanical strains at the tibiotarsus midshaft during steady activities (ground, uphill, downhill locomotion) and non-steady activities (perching, jumping, aerial transition landing) in 48 pre-pubescent female (egg-laying) chickens from two breeds that were reared in three different housing systems, allowing varying amounts and types of physical activity. Mechanical strain patterns differed between breeds, and were dependent on the activity performed. Mechanical strains were also affected by rearing environment: chickens that were restricted from performing dynamic load bearing activity due to caged-housing generally exhibited higher mechanical strain levels during steady, but not non-steady activities, compared to chickens with prior dynamic load-bearing activity experience. Among chickens with prior experience of dynamic load bearing activity, those reared in housing systems that allowed more frequent physical activity did not exhibit lower mechanical strains. In all groups, the tibiotarsus was subjected to a loading environment consisting of a combination of axial compression, bending, and torsion, with torsion being the predominant source of strain. Aerial transition landing produced the highest strain levels with unusual strain patterns compared to other activities, suggesting it may produce the strongest anabolic response. These results exemplify how different breeds within a given specie adapt to maintain different patterns of mechanical strains, and how benefits of physical activity in terms of resistance to strain are activity-type dependent and do not necessarily increase with increased physical activity. These findings directly inform controlled loading experiments aimed at studying the bone mechanoresponse in young female chickens and can also be associated to measures of bone morphology and material properties to understand how these features influence bone mechanical properties in vivo.

Fearfulness in commercial laying hens: a meta-analysis comparing brown and white egg layers.

High fearfulness in commercial laying hens can negatively affect production parameters and animal welfare. Brown and white egg layers differ in several behavioral characteristics, though reported differences in fearfulness are inconsistent. A meta-analysis was conducted to determine whether there are systematic differences in measures of fearfulness between brown and white layers. Twenty-three studies that examined either 1 or both of 2 behavioral tests were included: tonic immobility (TI) (longer duration = higher fearfulness, 16 studies) and novel object (NO) test (lower approach rate = higher fearfulness, 11 studies). The 2 tests were analyzed separately. TI analyses: A generalized linear mixed effect model (GLMM) with a lognormal distribution was fitted to describe the data with experiment nested in study as a random effect. Explanatory (X) variables were considered through backward selection, where potential X-variables included color (brown vs. white layers), decade (1980s, 2000s, 2020s), age (prelay vs. in lay), genetic stock (hybrid vs. grand-/parent stock), and methodology (back vs. side position). NO test analyses: univariable GLMMs with a beta distribution were fitted with approach rate as the Y-variable and color, decade, age, stock, or 2 methodological factors (test duration, single vs. group testing) as X-variables. Models were evaluated by assessing information criteria, residuals/random effect normality, significance of X-variables and model evaluation statistics (mean square prediction error, concordance correlation coefficient). TI duration was best explained by a color-by-decade interaction (P = 0.0006). Whites in the 1980s had longer TI durations (709.43 ± 143.88 s) than browns in the 1980s (282.90 ± 59.70 s), as well as in comparison to browns (208.80 ± 50.82 s) or whites (204.85 ± 49.60 s) in the 2020s. The NO approach rate was best explained by color (P ≤ 0.05 in 3 models), age (P < 0.05 in 3 models), and decade (P = 0.04). Whites had a higher approach rate (0.7 ± 0.07) than browns (0.5 ± 0.11), birds in lay a higher rate (0.8 ± 0.07) than birds prelay (0.4 ± 0.12), and approach rate for papers published in the 2000s (0.8 ± 0.09) was higher than in the 2020s (0.2 ± 0.12). The phylogenetic difference in the 1980s was no longer detectable after enforcing an upper limit on TI durations (10 min), as became common practice in later studies. Our findings suggest that phylogenetic differences in fearfulness and changes over time are test dependent, and this raises important questions and potential consequences for assessing hen welfare in commercial egg production.

Perch Positioning Affects both Laying Hen Locomotion and Forces Experienced at the Keel.

The aim of this study was to assess the effect of perch positioning on laying hens' locomotion and the resulting energy experienced at the keel. Twenty Nick Chick and 20 Brown Nick hens were trained to transition from a platform to a perch in several configurations. Three variables of perch positioning were tested in a 2 × 2 × 2 factorial design: direction (upward vs. downward), angle (flat vs. steep), and distance (50 cm vs. 100 cm). All hens were tested for five jumps of each treatment combination at 27-28 weeks of age. As predicted, we found steep angles and long distances to result in higher peak forces and impulse during take-off, flight, and landing; longer latency to jump; a higher likelihood to perform balancing movements; and a longer latency to peck at the provided food reward. The effect of perch positioning on locomotion and force at the keel during downwards jumps and flight was more pronounced in Brown Nick hens than in Nick Chick hens. Although we cannot state how the observed forces at the keel relate to the risk for keel bone fractures, our results indicated that optimizing perch positioning can reduce accumulated forced at the keel and consequent risk for fracture due to unsuccessful transitions.

Differential predation alters pigmentation in threespine stickleback (Gasterosteus aculeatus).

Animal pigmentation plays a key role in many biological interactions, including courtship and predator avoidance. Sympatric benthic and limnetic ecotypes of threespine stickleback (Gasterosteus aculeatus) exhibit divergent pigment patterns. To test whether differential predation by cutthroat trout contributes to the differences in pigmentation seen between the ecotypes, we used a within-generation selection experiment on F2 benthic-limnetic hybrids. After 10 months of differential selection, we compared the pigmentation of fish under trout predation to control fish not exposed to trout predation. We found that stickleback exhibited more lateral barring in ponds with trout predation. Ponds with trout were also less turbid, and a greater degree of barring was negatively correlated with the magnitude of turbidity across pond replicates. A more benthic diet, a proxy for habitat use, was also correlated with greater lateral barring and green dorsal pigmentation. These patterns suggest that differential exposure to cutthroat trout predation may explain the divergence in body pigmentation between benthic and limnetic ecotypes.