Fear and stress measurements in pure and hybrid red-legged partridges

The purposes of this study were to examine the duration of induced tonic immobility (a reliable fear indicator considered a passive anti-predator behavioural trait) and two stress indicators (heterophil to lymphocyte ratio, and fluctuating asymmetry of leg length, middle-toe length, and wing length) in females and males of the red-legged partridge, and to analyze the differences between pure and hybrid populations. A total of 132 birds (66 pure and 66 hybrid partridges; 48 females and 84 males) at 52 weeks of age were used. There was a significant difference between pure and hybrid birds in tonic immobility duration (P < 0.005) and heterophil to lymphocyte ratio (P < 0.02), the duration being longer and the ratio being higher in the pure group of birds. There was a significant difference between pure and hybrid birds in the relative fluctuating asymmetry of middle toe length (P < 0.002) and the combined fluctuating asymmetry value of the three traits (P < 0.03). There were no significant differences between sexes in any indicator of fear or stress. Thus, pure red-legged partridges were more fearful, stressed, and asymmetrical than hybrid partridges, having increased tonic immobility duration, heterophil to lymphocyte ratio, and relative fluctuating asymmetry. Results indicate that levels of fear and stress, e.g. tonic immobility duration, heterophil to lymphocyte ratio, and fluctuating asymmetry, are higher in the pure partridge, and that fear and stress indicators have been modified in hybrid partridges leading to more passive animals.