3.8. Repeatability of male song traits in D. montana and D. littoralis

D. montana and D. littoralis females use male courtship song in their mate choice in the wild preferring males that produce short and dense sound pulses (i.e. short PL and high CN and FRE, Aspi & Hoikkala 1995). In paper II these song characters were found to be repeatable among overwintered males of both species. Male progenies of wild-caught flies reared in the laboratory, and in D. montana also the males collected in the wild before overwintering, exhibited very little variation in the above-mentioned characters. In a fashion different from pulse characters, pulse train characters measured for D. montana song varied also between laboratory-reared males. Our findings suggest that in D. montana and D. littoralis, song characters that play a part in sexual selection in the wild (pulse characters) are more condition dependent than the song characters, which are not direct targets of female choice (pulse train characters). The repeatability of pulse characters also shows that the songs of overwintered males differ from each other sufficiently so that the females may exercise selection during the mating season.

Repeatability provides an indication of the potential utility of the signal as a reliable indicator of competitive ability of the males (Clark & Moore 1995). The high repeatability of pulse characters of the song among wild-caught D. littoralis and D. montana males was due to increased variation between males rather than to decreased variation within individuals (paper II). The fact that some pulse characters changed during the cold treatment suggests that they may carry information on the condition of an overwintered male, which enables the females to choose the males in good condition as a mating partner (Zahavi 1977). Characters of male acoustic signals are repeatable across a wide array of animal species, such as Chorthippus brunneus, a grasshopper (Butlin & Hewit 1986), Hyla versicolor, a tree frog (Gerhardt 1991), Nycticeius humeralis, a bat (Scherrer & Wilkinson 1993) and Grompadorhina portentos a cockroach (Clark & Moore 1995). We do not know, however, how large a proportion the variation between the songs of different males is due to changes in male condition.

The heritabilities of different song characters of D. montana and D. littoralis have been estimated in the laboratory and in the wild (Aspi & Hoikkala 1993, Suvanto et al., in press) and in D. montana also before and after a cold-treatment in laboratory (Suvanto et al., in press). Aspi and Hoikkala (1993) found that in D. montana heritabilities calculated across the environments were nonsignificant (except for PN), while those for PN, IPI and PL in laboratory reared flies were significant. In our study (paper II) the songs of laboratory-reared D. montana males were found to be repeatable in PN and IPI but not in PL. The low repeatability of PL was unexpected, as repeatability should set an upper limit for the heritability of a character (Falconer 1989, Boake 1989).

In the study by Suvanto et al. (in press), most heritability estimates for D. montana song traits were nonsignificant, largely due to high residual variation. During the cold-treatment, however, the additive variation increased significantly in nearly all song traits, so that this increase was most pronounced in traits found to be the targets of sexual selection (PL, CN and FRE). The increase in additive variation was most probably due to the circumstance that the selected song traits became more repeatable following cold-treatment, i.e. to the same phenomenon, which we found in paper II.

One of the central questions in the theories of sexual selection is to what extent the development of male characters depends on the phenotypic condition and overall genotype of the male (Andersson 1986). In cases where Drosophila flies overwinter as adults and mate after overwintering (like in D. montana and in D. littoralis), the male trait could tell the female how well the male has survived the winter (Aspi et al. 1993). If genotype environment-interactions increase phenotypic variation between males in sexually selected song traits prior to the mating season of the flies, male songs may reflect viability and condition of the males at the moment of courtship. This is as suggested in conditional viability variants of the ‘good genes’ model (Zahavi 1977, Iwasa 1991). D. montana females that exercise selection on male song have also been found to gain indirect benefit from their choice in the form of better offspring survival (Hoikkala et al. 1998).