Assessing the benefits of aggregation: thermal biology and water relations of anomalous Emperor Moth caterpillars

1. Three hypotheses have been proposed to account for the maintenance of aggregation behaviour in caterpillars. While the benefits of aggregation for defence and for the elevation of growth rates have been investigated in some species, both of these hypotheses have been criticized, and in many cases the alternative hypothesis regarding the benefits of physiological regulation has not been investigated. 2. In this paper the field thermal biology and water relations of solitary caterpillars (instars IV-V) and aggregations of caterpillars (instars I-III) are examined in Imbrasia belina (Westwood) (Lepidoptera: Saturniidae), a species where an instar-related change in aggregation behaviour is obligatory (instars I-III always aggregate, instars IV-V are always solitary), and where the benefits of defence and of overwhelming inducible host plant defences are unlikely to be significant. 3. Aggregations of instars II-III maintain body temperatures equivalent to those of larger solitary individuals of instars IV and V, but significantly higher than those of operative models of single caterpillars of instars II-III. In addition, behavioural thermoregulation is not used to elevate body temperatures, although hanging behaviour takes place when upper lethal temperature limits are approached. Nonetheless, solar radiation has a marked influence on the temperature of aggregations because of the accumulation of higher heat loads by the larger masses of large aggregations. 4. Instars IV-V can survive higher temperatures than instars I-III, and this may be because of their accumulation of very high heat loads in the field. Instars I and LII can avoid these high heat loads by partial dispersal from the aggregation. In contrast, instars I-III have significantly lower critical thermal minima than the later instars, which may allow larvae to regain aggregations if they are separated from them. 5. Water loss rates of real aggregations, of a given size, within a particular instar are lower than the sum of the rates of the same number of individual caterpillars of the same instar. In addition, survival of water loss tends to be higher in individuals from aggregations than in single individuals. 6. Using this evidence in conjunction with studies that demonstrate significant predation in all instars, aggregation in the early instars irrespective of whether outbreaks take place or not, and the absence of inducible defences in the major host (Colophospermum mopane) of I. belina, it is concluded that physiological regulation, of temperature and water balance, is the most likely explanation for the maintenance of aggregation behaviour in this species.