The volitional illuminance preferences of 9 closely related, captive-born cactus mice were determined by allowing them a choice between 10 illuminance levels ranging from darkness to 1.1 lux. The levels could be altered bidirectionally to brighter or dimmer values, one step at a time in predetermined sequence, by instrumental means. The mice were confined in exercise-wheel enclosures and tested and retested at ages of 4 to 35 months. For all 9 animals, the time spent running in the wheel had marked, minor, incipient, or presumptive peaks at level 5 (0.0014 lux) or level 6 (0.0025 lux) ; these peaks probably reveal the optimum illuminance range for the animals' vision. But the major peak in the active time of most individuals was in darkness (or extremely dim light). There was generally a marked tendency for the animals to spend a greater percentage of inactive time than active time in darkness. Five animals retested after a lapse of at least 5 months showed changes in illuminance preferences, there being a tendency for the minor preference for dim light either to increase with age or to appear first long after maturity. In addition, one 5-month-old mouse showed a remarkable series of changes in illuminance preferences over a period of several weeks. The illuminance regimes to which small nocturnal mammals are exposed are analysed in terms of a paradigm involving two sets of factors-illuminance preferences and factors unrelated to these preferences. It is proposed that, although both sets of factors determine illuminance exposures in the wild, only factors in the illuminance preference category determine illuminance exposures (selectivity) in captivity. The major preference of cactus mice for darkness and the lahility of their preferences are analysed in terms of this paradigm.
J. Lee Kavanau and Roy M. Havenhill
Roy M. Havenhill, Judith Ramos and J. Lee Kavanau
Thirteen individuals belonging to the carnivore species Bassariscus astittus, Genetta genetta, Mustela frenata, Mustela rixosa, Tayra barbara, and Urocyon cinereoargenteus were studied in activity-wheel enclosures in which they could control the intensity of ambient illumination in 10 steps by instrumental means. The light levels available ranged from darkness to a brightest value of 150, 300, 500, or 950 lux. The acivity indices determined at each level were: time spent running (active time) and not running (inactive time), running speed, visit lengths during activity, and number of visits. Twelve of the animals preferred the brightest or second-brightest light available. The thirteenth, a genet, had not adapted to the experimental situation after 47 days of study and spent most of the time in dim light or darkness. Active time, lengths of visits, and number of visits gave essentially the same indications of light preferences during activity, with the first two indices giving the greatest resolution. Running speed showed no noteworthy correlation with light level. Plots of the ratios of both active time and visit lengths at the brightest level to those at the second brightest appear to fall into species-specific groupings that do not correlate with body size or species affinity. There were mild to notable shifts in light level preferences from activity to inactivity. In a general way the directions of the shifts are consistent with observations of the behaviour of the animals under other conditions, i.e., light-sleeping sunbathers tended to prefer brighter light during inactivity, while sound-sleeping animals that sought protective cover during the day tended to prefer dimmer light. One of the longtailed weasels consistently adjusted the lights to a dim level or darkness when disturbed daily at feeding time. During undisturbed activity, this animal preferred the brightest light available on five different programs in which switch functions, level sequences, and illuminance values were altered, in some case to markedly different arrangements. As a working hypothesis we suggest that the preference of our carnivores for light in the range of 10 to 950 lux depends on one or more aspects of their vision - such as color, contrast, pattern, and intensity discriminations - being much improved in this range compared to that in dimmer light. On the other hand, all but one of the same animals was found to be primarily or strictly nocturnal in an independent study of activity phasing. Accordingly, it appears possible that one or more of the visual discriminations mentioned above is most sought after when the animals can alternate between conditions in which he discrimination is alternately good and poor. The animals often continued to press the levers controlling the light level after the corresponding extreme level had been attained. The relative numbers of these "false steps" on the on levers and off levers suggest that, of two means available, the animals tended to seek additional environmental modification using the means (the on levers) that yielded the preferred conditions. In addition, the animals that showed the most marked preferences for bright light also were the ones with the lowest rates of "false ons", suggesting that less motivation existed to seek change from highly preferred conditions than from less preferred ones.