Analysis of mating calls of males recorded near Skukuza versus those calls gathered near Shingwedzi (separated by approximately 270 km) showed significant differences in basic call structure and thus supported our hypothesis. We found that the call structure of males around specific fruiting sycamore fig trees were consistent with single-species presence for males at a very localized scale. However, there is also range overlap between the species, and because the male mating calls travel at least 0.5 km from the calling perch near where females are feeding on figs (Wickler and Seibt 1976, personal observations), there is potential for females of each species in the area to approach calling males of the other species if there is not a discernible difference in call structure.
From our data, we conclude that the male mating calls we recorded from each area depicted E. wahlbergi from Skukuza and E. crypturus from Shingwedzi. As mentioned, the mating call of male epauletted bats appears to serve both the function of attracting females to their perch because male call rates increase as females approach, but secondly, male calls appear to act as a deterrent to other conspecific males attempting to call from perches too close to other calling males (Wickler and Seibt 1976; Adams and Snode 2013). Although one could argue that because we could not definitively identify individuals to species from which calls were recorded, we could not make this determination. However, several factors help in corroborating our interpretation of results: a) there are unequivocal distinctions in the type of male mating calls recorded in different areas of KNP; b) the calls are consistently different in the two regions of KNP, with exceedingly small degrees of variation in each of the call parameters; c) our capture data, as well as data from other researchers (Bonaccorso et al. 2014), indicate that E. wahlbergi dominates the area around Skukuza, whereas E. crypturus dominates the area near Shingwedzi; d) capture data from other studies in KNP (Fenton et al. 1985), and other areas in Africa, show spatial separation between epauletted fruit bat species on both local (Wickler and Seibt 1976) and, in some cases, regional scales (Kingdon 1974); and e) there are no records of mixed-species feeding groups of epauletted fruit bats at the same fig trees, and radio-telemetry data indicate that female E. wahlbergi from a particular colony moved to the same feeding area of ripe fig trees nightly (Fenton et al. 1985).
An alternative hypothesis to ours would be that males of either E. wahlbergi or E. crypturus alter their call structure in different areas of KNP, and thus, we were recording the same species in both areas using calls that were geographically distinctive within species. Although we could not refute this hypothesis with our study, from a theoretical perspective, this seems unlikely, and we can think of no benefits of such mating behavior. In addition, the extreme lack of variation in call structure in the variables measured for this study suggests the possibility of stabilizing selection via mate choice that would not likely promote such strong regional differences in intraspecific mating calls. In fact, just the opposite would be expected (Ptacek 2000).
It should be noted that the call structure of male E. wahlbergi we recorded in KNP was substantially different from those recorded by Wickler and Seibt (1976) in Kenya. Our recordings contained only a fundamental and single second harmonic, whereas they found a fundamental with three harmonics (however, this could simply be the distance away from a perch where recording was taken). In addition, the fundamental frequency reported by Wickler and Seibt (1976) equated to 1.7 kHz, whereas our recordings show the fundamental at a much higher frequency of about 7 kHz. Although there are no published descriptions of E. crypturus calls available for comparison, it is also curious to note that E. crypturus calls in KNP correspond in frequency to the second harmonic of E. wahlbergi calls. Tonal differences in the calls of each species may be the product of differences in the slope of the calls. Although both species produce relatively flat calls, the total slope of the calls emitted in the Shingwedzi area was twice that of the calls we recorded in the Skukuza area on average. Alterations in call structure attributes presented herein represent a relatively simple measure by which males of co-occurring species of epauletted bats may use harmonic displacement to avoid call overlap with another similar species. The question of if species-specific differences in mating calls are learned or have inherent genetic components remains unanswered. However, our study raises the possibility that differences in mating calls between species of male epauletted fruit bats may serve to avoid hybridization between the two species of epauletted fruit bats.