Different direct effects of light intensity on the entrained activity rhythm in neotropical bats (Chiroptera,Phyllostomidae)

In the two neotropical Chiroptera species, Artibeus lituratus and Phyllostomus discolor, the influence of illumination intensity during the D-time of an LD 12:12 h on the locomotory activity rhythm was investigated by offering specific rectangular illumination schedules. In LD 12:12 (10¹:10^-4lux), A. lituratus and P. discolor exhibit a stable unimodal basic activity pattern. By increasing the light intensity during D-time to 10^-1 lux the activity pattern, especially in A. lituratus, can be modified in almost every way. This indicates a strong direct inhibitory effect of higher light intensities (> 10^-4 lux). Hereby light effects via the circadian system have practically no importance. The response of A. lituratus to changes of the D-light intensity is more pronounced than that of P. discolor. It is suggested that the different direct effects of light intensity on the activity pattern of the two bat species reflect specific ecological adaptations.