THE ADAPTIVE SIGNIFICANCE OF MALE POLYMORPHISM IN THE ACARID MITE CALOGLYPHUS-BERLESEI

Two forms of males occur in the acarid mite, Caloglyphus berlesei. One of them uses its thickened third pair of legs to kill other males; th e other, without modified legs, does not attack other males. Previous studies have shown that the form of the male is environmentally determ ined: ''fighter'' males develop only at low population densities and t heir development can be suppressed by substances emanating from dense colonies. In this study, the duration of development, longevity and vi rility of the two male morphs were measured. The only significant diff erence was that fighters had a shorter development time between the tw o last moults. In another experiment, two groups of colonies were main tained under the same conditions but had different numbers of individu als: small colonies contained 2 fighters and 2 non-fighters, whereas l arge colonies contained 30 males of each type. The relative reproducti ve success of both morphs was estimated from numbers of matings. In sm all colonies, the estimated reproductive success (ERS) was significant ly higher for fighters than for non-fighters, mainly because in over 5 0% of these colonies a single fighter male managed to kill all the riv al males and monopolize the females. The opposite was true in large co lonies, in which non-fighters achieved significantly higher ERS, mainl y because the fighters were killed in fights more often than the non-f ighters. This implies that the ratio of costs to benefits obtained by adopting the fighter-male strategy increases with the number of rival males, which may explain conditional male development in C. berlesei.