Competition and Thermoregulatory Behavior of the Namib Desert Tenebrionid Bettle Genus Cardiosis

Tenebrinonid beetles of the genus Cardiosis are confined to sand dunes and other sandby substrates of the Namib Desert in South West Africa and Angola. In the northern Namib the breadth of environments occupied by each species is considerably greater in the southern Namib. This may be due to competition resulting in ecological divergence, since there is sympatry in this genus only in the southern Namib. All species of the genus Cardiosis, including the sympatric C. fairmairei and C. hamiltonuli are diurnal. The daily activity rhythm of these species responds to ambient thermal conditions, and fluctuations in ambient conditions result in differing patterns of activity. Substrate and ambient temperatures are the important microclimatic variables influencing the actual timing of activity. This usually results in a concentration of activity in the least humid hours of the day. On cool days or when winds cool the sand surface, activity may extend through the middle hours of the day. On hot or calm days activity becomes bimodal, with activity limited to intervals when body temperatures can be maintained at levels well above 30 degrees C and substantially below lethal limits. The interval of morning activity is less pronounced than the afternoon activity period and on especially hot days morning activity may be omitted altogether. Unconfined populations of Cardiosis extend their activity over considerably greater intervals than confined populations exposed to identical macroclimatic conditions. This is due to the ability of the unconfined population to utilized a broad spectrum of environments, thus locating favorable thermal conditions as they develop in the course of a daily cycle.