The pupillary light reflex and menace response in neonatal calves: the role of environmental isolation on development of the menace response.

The aim of this study was to investigate the development of the pupillary light reflex in neonatal calves and the effect on the development of the menace response of keeping them in individual stalls. Seventy-three Holstein calves were divided into test (n=44) and control (n=29) groups and the animals were kept in individual stalls for the first 32+/-4 and 73+/-5 days post partum, respectively. They were then moved to group pens. All calves in both groups were observed daily from the first day after birth until the pupillary light reflex and menace response had become well established. All 73 calves had a positive pupillary light reflex on the first day post partum. None of the animals developed a menace response during the time they were housed in individual stalls. Over 14 days (test group) and 12 days (control group) after the calves had been moved to the group pens they became positive to the menace response at ages 41+/-4 and 81+/-5 days, respectively. The Mann-Whitney test revealed a significant difference (P<0.001) in the ages of the calves with positive menace responses between groups. Group rearing may produce more visual threats to the eyes and the calves learn the menace response by closing the lids in response to sudden alarming movements.