A note on simulating the 'observer effect' using constant photoperiod on nursery pigs

Often, researchers using video cameras to record animal behavior must resor t to illuminating the environment to obtain data during the night hours. Th is unnatural lighting during the night hours may change the behavior of exp erimental animals. Therefore, this study was designed to determine if the b ehavior of pigs is altered by constant illumination. Nursery pigs (24 +/- 0 .95 days of age, 5.58 +/- 0.18 kg) were randomly assigned to one of two tre atments. Treatment pigs were exposed to a constant photoperiod (24:0, light :dark cycle) while control pigs were exposed to a 12:12 light:dark cycle (l ights on at 0600 h and off at 1830 h). The pigs for each treatment (n = 30) were randomly assigned to one of six pens, blocked by sex, so that each pe n contained five pigs (two females and three males). Pigs were acclimated t o the photoperiods for 1 week, and during this time, no observations were m ade. After this period, the activity of both treatments was recorded by vid eo cameras for 6 days. Upon completion of this week of observation, the pho toperiod for all pigs were reverted to the 12:12 light:dark illumination sc hedule for 1 week, after which observations resumed for an additional 6 day s (days 14 to 19). The 24:0 pigs were more active than the 12:12 pigs (P < 0.001) between 1830 and 0630. This increase in night time activity resulted in a trend for 24:0 pigs to be more active during the entire day (P < 0.08 ). The 24:0 pigs were most active during the first 3 h and last 3 h of obse rvation (P < 0.001) from 1830 to 0630. At no point during the study did we find that pigs exposed to constant illumination were less active than contr ol pigs. Therefore, we conclude that constant illumination caused an altera tion in the behavior of pigs during video recording effectively simulating the 'observer effect'. Altering the activity budgets of experimental animal s could have profound effects on experimental results as alterations in act ivity may lead to alterations in eating behavior, agonistic encounters, and levels of social stress. (C) 1999 Elsevier Science B.V. All rights reserve d.