Male and female gibbons (Hylobates) produce sexually dimorphic song bouts which appear to serve to guard mates and defend territories respectively. On the basis of differences in both the familiarity and the costs of conflict between sender and receiver, males but not females are hypothesized to use high-energy assessment signals to advertise resource-holding potential. This hypothesis is tested by examining the evidence for differential energy constraints in the production of male and female song bouts across 21 gibbon populations. The results indicate that song performance is reduced when the availability of high-energy foods is reduced and that this effect is greatest in males. Male song bout frequency also declines in populations where the energetic costs of thermoregulation are likely to be greater (i.e. in high-latitude populations). Females do not show this pattern. Females appear to perform the minimum number of songs per bout required for signal transmission and may perform bouts less often when more songs are required. In contrast, males sing for longer periods the more frequently bouts are performed. Overall, these results support the hypothesis that the song bouts of males but not females act as assessment signals. Observational and experimental studies which would provide a more powerful test of this hypothesis are briefly outlined.