Cryptic male dimorphism and fighting in a fig wasp

In some nonpollinating fig wasps, male competition for mates often results in serious injury or death. We studied the factors associated with fighting behaviour in an undescribed Philotrypesis fig wasp species. We quantified morphological traits in 440 males and revealed the presence of two discrete, but cryptic, male morphs, termed 'aggressive' (A) and 'passive' (P). For a given head size, morph A had larger mandibles. However, the body size distributions of the two morphs overlapped considerably, such that morph designation required calculation of the ratio of mandible (weapon) size to head size. Importantly, this means that standard analyses for dimorphism do not identify the two morphs correctly. We also sampled 62 pairs of males engaged in escalated fights and compared their size matching with that of randomly chosen pairs of males from the same patch. There was a significantly greater discrepancy in mandible size in fighting pairs than in randomly chosen pairs of males. Furthermore, our cryptic morph designation revealed the process underlying this pattern, as follows. Although only 22% of males were morph A, 45% of these males were involved in fights. In contrast, 78% of males were morph P, but only 16% of them were involved in fights. The different population frequencies and fighting tendencies of the two morphs combine to ensure that most fights occur between 'heteromorphic' pairs and this, in turn, generates the significant jaw size discrepancy recorded between fighting males.