The impact of behavioural thermoregulation on reproductive rates in a grasshopper
Oviposition rates of the European grasshopper Stenobothrus lineatus (Panzer) are clearly related to temperatures above a threshold of 23.5°C. Considering this temperature dependency, oviposition rates estimated for an experimental population on base of ambient field temperatures yielded much lower values in comparison to those actually determined in the field. These findings indicated that the thermoregulation of the females influences their reproductive rates in a decisive way. Field investigations showed that the body temperatures of S. lineatus females were much higher then the ambient air conditions raising intercepts of more than 20 K. Applying an empirical and a biophysical thermal model, the influence of passively acquired heat from environmental temperatures could be separated from active behavioural thermoregulation. The oviposition rates in the field can be estimated by simulating the body temperatures of the females on the base of weather data (temperature and sunshine periods). On the population level, the impact of behavioural thermoregulation, first by visiting sunflecks within the habitat and second by posturing towards the sun, could be quantified. In the Central European population studied between 1992 and 1995, the two modes of behavioural thermoregulation explained on average about 53% (basking in sunflecks) and 4% (posturing) of both the mean individual fertility and the population net reproductive rates.