fru⁴/fru^4-40 males exhibit very little courtship behaviour towards females and show a small but significant increase in courtship of other males compared to wild-type. Unlike wild-type males, these mutant males form male mating chains.

The male-female courtship index and fertility of fru^GAL4/fru⁴ males is significantly reduced compared to wild-type. These males court single wild-type males and form chains with males of their own genotype.

25% of fru^0-1/fru⁴ males are fertile. Male-female courtship as measured by courtship index (CI) or wing extension index (WEI) is almost completely abolished in fru⁴/fru^w9, fru⁴/Df(3R)fru^w24, fru⁴/fru^w27 or fru⁴/fru^4-40 males. fru⁴/fru^4-40 males show significant male-male courtship as measured by CI. fru⁴/fru^w9, fru⁴/fru^w12, fru⁴/Df(3R)fru^w24, fru⁴/fru^w27 or fru⁴/fru^4-40 males show substantial chaining. fru⁴/Df(3R)fru^w24 and fru⁴/fru^4-40 males do not produce a courtship song even though they show a small amount of wing extension.

Expression of 5-HT in serotonergic-abdominal ganglion neurons in adult males is only weakly detectable.

fru¹/fru⁴ males mated individually to a single virgin female show vigorous courtship behaviour, comparable to that of wild-type males. High levels of abdominal bending are seen in the males that show courtship behaviour. However, the proportion of males that mate is reduced compared to wild type; 47% of the transheterozygous males court but do not mate. The fertility of the males (as assayed by the ability of the mated females to produce progeny) is reduced compared to wild-type males. The transheterozygous males show longer than normal mating-initiation latencies compared to heterozygous controls. Mating duration is also longer than normal and shows a far more scattered distribution than that of wild-type males.

The number of progeny (number of resulting pupae) obtained from a fertile mating between a fru¹/fru⁴ male and a wild-type female are not significantly different to wild type.

An appreciable fraction of matings by fru¹/fru⁴ males lead to subnormal quantities of sperm being transferred to the female.

The number of varicosities of the sAbg neurons which are associated with the male reproductive organs is variable in fru⁴ males (often no varicosities are present).

Homozygous males show high levels of head-to-head interactions compared to wild-type males. Most of these would-be aggressive actions involve bringing their heads together but not escalating the interactions into the rising-up and boxing motions that are displayed by wild-type males. The level of head-to-head interactions shows a temporal dependency; when males are grouped together on the day they eclose they do not show significantly higher than normal head-to-head interactions, but the frequency of head-to-head interactions shows a marked increase beginning on day 2 and peaking on days 4-5. Mutant males aged individually for 5-7 days and then grouped together show low levels of head-to-head interactions 1 day after being grouped together, but show an increase in the frequency of head-to-head interactions by days 4-5. These flies (which were aged individually) show lower levels of head-to-head interactions compared to mutant males that have been aged for essentially the same number of days, but in the presence of other males since eclosion, indicating a social component in the phenotype. Mutant males show some chaining behaviour. Chaining also show temporal dependence, with the frequency of chaining showing a marked increase beginning on day 2 and peaking on days 4-5.

Males are less stimulated to court females than fru¹ or fru^0-1 and display less chaining behaviour than fru¹ males. All males that exhibit any courtship do exhibit tapping behaviour (tapping of the female with the forelegs). No courtship song pulse is generated, even though they perform rare and unsustained wing extensions. The anomalous wing usage exhibited is not due to a general thoracic etiology, such as a defect in neuromuscular morphology or physiology. Homozygous and transheterozygous males with fru³ exhibit complete behavioural sterility, failure to attempt copulation. Males, when presented with both sexes simultaneously, will perform vigorous and indiscriminate courtship directed at either sex. Chaining behaviour exhibited by males is displayed mostly on the food surface. Courtship song is not required for, nor is particularly correlated with, chain formation. The behaviour of fru³/fru⁴ males is very similar to that of either homozygote. Males lack the Muscle of Lawrence (MOL). Subnormal levels of courtship are not related to any drop in activity or viability. Mutant females are courted by wild-type males at normal levels.

Males court indiscriminately, fail to copulate and have muscle of Lawrence defects. Males show very little wing extension and the wing displays generate no song pulse signals (courtship specific as flight is normal). fru⁴/fru^w12 males barely court at all but show normal locomotor activity. Mutant combination shows some male-male chaining. Early and late steps of courtship are disrupted.

In fru⁴ homozygotes the number of adult muscle nuclei per fibre is greater than that for other fru^- mutants but there are fewer nuclei in muscle of Lawrence (MOL)-position fibres than medial fibres or wild-type male MOL fibres. The number of myoblasts in unaffected.

Males court males and females but fail to mate. Male specific abdominal muscle is reduced.