Continental variation in wing pigmentation in Calopteryx damselflies is related to the presence of heterospecifics

Abstract

Wing pigmentation in Calopteryx damselflies, caused by the deposition of melanin, is energetically expensive to produce and enhances predation risk. However, patterns of melanisation are used in species identification, greater pigmentation is an accurate signal of male immune function in at least some species, and there may be a role for pigment in thermoregulation. This study tested two potential hypotheses to explain the presence of, and variation in, this pigmentation based on these three potential benefits using 907 male specimens of Calopteryx maculata collected from 49 sites (34 discrete populations) across the geographical range of the species in North America: (i) pigmentation varies with the presence of the closely related species, Calopteryx aequabilis, and (ii) pigment increases at higher latitudes as would be expected if it enhances thermoregulatory capacity. No gradual latitudinal pattern was observed, as might be expected if pigmentation was involved in thermoregulation. However, strong variation was observed between populations that were sympatric or allopatric with C. aequabilis. This variation was characterised by dark wings through allopatry in the south of the range and then a step change to much lighter wings at the southern border of sympatry. Pigmentation then increased further north into the sympatric zone, finally returning to allopatry levels at the northern range margin. These patterns are qualitatively similar to variation in pigmentation in C. aequabilis, meaning that the data are consistent with what would be expected from convergent character displacement. Overall, the results corroborate recent research that has suggested sexual selection as a primary driver behind the evolution of wing pigmentation in this group.

Keywords: Allopatry; Dragonfly; Immune function; Melanin; North America; Odonata; Species recognition; Sympatry; Thermoregulation; Wing.

Figure 1. Map of sites with transparency of wings.

Geographical distribution of Calopteryx maculata (light grey) and Calopteryx aequabilis (hashed region). Points mark the location of sampling sites for C. maculata and the size of the point is proportional to the grayscale value of the wing pigmentation intensity (larger symbol = lighter wings) for areas of sympatry (filled symbols) and allopatry (open symbols). See Fig. 3 for illustrations of wing pigmentation variation.

Figure 2. Variation in wing pigmentation in Calopteryx maculata.

Latitudinal patterns of pigmentation in C. maculata males where populations are allopatric (open circles) and sympatric (closed circles) with C. aequabilis. Error bars are 1SE. The y-axis is arranged to show increasing levels of pigment from bottom to top, but note that the measurement was grayscale where higher values correspond to lighter colours. Displayed on the right are examples of wings with pigmentation intensities corresponding to their locations on the y-axis. See Table S1 for sample sizes and means, and Table S2 for raw data.

Figure 3. Comparison of wing pigmentation variation between Calopteryx maculata and C. aequabilis.

Comparison of variation wing pigmentation between (A) C. maculata males (from the present study), (B) C. aequabilis females (from Waage, 1979), (C) C. maculata males (from Waage, 1979), and (D) C. maculata females (from Waage, 1979). All y-axes represent qualitatively the same trait: pigmentation of the wing, with low values (top of y-axes) corresponding to darker wings and higher values (bottom of y-axes) to greater transparency. However, the measures vary in the way in which they were collected: for C. aequabilis males it is the proportion of the wing length that is unpigmented, for C. aequabilis and C. maculata females it is the transparency of the dark area of the wing (using a densitometer), and for C. maculata males it is the mean grayscale value of a wing scan (see methods above). All error bars are standard errors. Note in all cases that pigmentation is highest at the southern margin of the zone of sympatry with subsequent declines further north.