Geographic Variation in the Advertisement Call Structure of Litoria verreauxii (Anura: Hylidae)

Many evolutionary processes have been identified that could lead to signal variability among populations despite stabilizing selection to maintain conspecific recognition. By examining among-population variation in mate recognition signals we may gain insights into the processes behind the evolution of such variation. We documented among-population variation in the advertisement call of Litoria verreauxii in the Australian Snowy Mountains to determine how call structure varied (1) across a broad geographic area, (2) from allopatry to sympatry with northern Litoria ewingii (an undescribed member of the same species complex), and (3) between two subspecies, Litoria verreauxii verreauxii and Litoria verreauxii alpina. Significant variation in all measured call properties was explained by latitude and/or longitude. Most noticeably, a strong east-west trend in introductory note duration and pulse number was detected that coincided with the transition from allopatry to sympatry with northern L. ewingii. The observed variation is discussed within the context of several evolutionary mechanisms including clinal variation and both reproductive and ecological character displacement. We found considerable variation in pulse rate among nearby allopatric populations (based upon current taxonomy). Pulse rate has been implicated in species recognition and reinforcement in L. verreauxii. Some of the among-population variation in pulse rate may reflect the presence of cryptic taxa. However, these results suggest that factors other than interspecific interactions with closely related species can lead to considerable among-population divergence in pulse rate. Finally, there was little difference in call structure between the two subspecies.

Fig. 1.

The study populations of Litoria verreauxii with groupings used in this study and their patric relations with northern Litoria ewingii. Open circle = populations of L. verreauxii in sympatry with northern L. ewingii, solid circle = Litoria verreauxii verreauxii, open square = montane populations, and solid square = Litoria verreauxii alpina. Refer to Table 1 for an explanation of population abbreviations

Fig. 2.

Wave form of the advertisement call of Litoria verreauxii showing the introductory and repeated notes and magnification of an individual note. The measurements used in this study are shown

Fig. 3.

Relationships between longitude and (A) introductory note pulse number (PN_IN) and (B) introductory note duration (D_IN). Solid line = line of best fit and dashed lines = 95% confident limits. Values on the Y-axis are residuals after correction for call-site temperature, weight, and latitude

Fig. 4.

Plots showing variation among populations in advertisement call components of Litoria verreauxii. All properties have been standardized and corrected for call-site temperature and weight. Populations are ordered in increasing longitude from left to right. Solid square = mean, box = ± 1 standard deviation, lines = range. Refer to Table 1 for an explanation of population abbreviations and see text for call property abbreviations. Dashed vertical line demarks sympatry (left) and allopatry (right) with northern Litoria ewingii.

Fig. 5.

Graph showing the relationship between raw repeated note pulse rate (PR_RN) and call-site temperature at Yaouk (open circle) and the Upper Cotter (solid circle)

Fig. 6.

Variation along a canonical discriminant axis (CDA 1) of advertisement call properties of Litoria verreauxii expressed as rotated principal components. CDA 1 was loaded most strongly by C 1 (correlation = −0.77) and then by C 3 and C 2 (correlation = −0.35 and 0.18, respectively). Refer to text for a description of the relationships between the raw call properties and the components. The graph was based upon an analysis with eight groups: Litoria verreauxii verreauxii (L.v.v.: Pre, Kio, Bun, Gun, Mul, Num, Blo1 and Blo2), Litoria verreauxii alpina (L. v. a.: Bul, Ogi, TMD, Thr, Dig, Hap and Cha), the two Tumbarumba populations (Tum), Yaouk (Yao), Lake Jindabyne (Jyn) Lake Eucumbene (Euc), Tantangara Dam (Tan), and Upper Cotter (Cot). Solid square = mean, box = ± 1 standard deviation, lines = range

Contributor Notes

Applied Ecology Research Group, University of Canberra, Australia Capital Territory, 2601, Australia. (MJS)smithmicha@missouri.edu; and (WO)osborne@scides.canberra.edu.au Send reprint requests to WO.

Accepted: 04 Aug 2003