Introduction

The distribution ranges of species change over space and time and the processes that lead to such dynamic ranges have received major interest in biogeographical studies1,2,3. The study of the edges of distribution ranges is of particular interest, since it can inform on the resilience of species and their populations to environmental variation4,5. Edge populations experience the extremes of species ecological gradients, are usually smaller and more fragmented, and can be subject to different selection regimes than the core populations4,6. Increased dispersal ability and larger investment in reproduction at edge populations are predicted by theory and largely supported by empirical studies7,8. Higher dispersal ability at edge populations can occur as a result of local natural selection and spatial sorting of migrating phenotypes8,9,10,11. Spatial sorting is an accumulation of phenotypes that facilitate higher rates of dispersal at expanding range edges, combined with assortative mating of those phenotypes at the edge front9. Sexual selection is predicted to be stronger at northern edge populations since mating seasons are shorter and there are less favorable reproductive conditions10,12,13,14. The same prediction is expected for southern edge populations, where mating seasons are also shorter due to high temperatures that can lead to overheating13. However, dispersal and reproductive traits can be subject to conflicting selection pressures at edge populations15, possibly limiting the range expansion process7. Moreover, intralocus sexual conflict can vary along a species distribution range, potentially leading to smaller sex differences in traits under relaxed selection at edge populations16,17. Exploring variation of morphological traits across large latitudinal ranges and the signatures of selection and spatial dynamics at edge populations is important to understand the evolutionary and ecological fate of species facing environmental change18,19.

In flying species, flight is a fundamental trait that impacts many of the aspects related to natural and sexual selection of the species20,21,22,23. Flight-related traits (i.e., morphological and physiological traits that impact flight performance) are predicted to change following environmental gradients, and differences at edge populations related to increased sexual selection and dispersal ability have been previously reported11,24,25,26,27. However, studies incorporating the whole distribution range of the species and both the south and north margins are rare28,29. Damselflies and dragonflies are an interesting group to ask questions on flight morphology along latitudinal ranges because they are experiencing rapid changes in their distribution ranges30,31. Moreover, their flight-related traits are not only impacted by selection at the adult stage32, but also by selection at the larval stage33,34.

In this study, we used males and females of the damselfly Lestes sponsa (Hansemann, 1823) (Fig. 1A) to investigate the variation of morphological traits associated with flight across its entire latitudinal range. This damselfly is very well suited for investigating these questions. First, this species spans a large distribution range in Europe, from populations in northern Spain and Greece at the south margin, to populations in northern Sweden at the north margin35. Second, flight-related traits such as body size and wing morphology are subject to both natural (e.g., survival) and sexual selection (e.g., mating success) in this species32. Indeed, these selective pressures can reinforce or oppose each other when it comes to flight32: survival and mating success both favored intermediate body sizes; survival selection favored long and slender forewings and short and broad hindwings, whereas mating success favored short and broad forewings and narrow-based hindwings. Finally, L. sponsa is a strictly univoltine insect, i.e., one generation per year, that overwinters in the egg stage36. Thus, the fixed voltinism does not bias selection pressures on flight-related traits across latitudes37. At edge populations, individuals that disperse are predicted to be more abundant and hence dispersal-related flight traits should differ between core and edge population26. Males of L. sponsa show a mating system known as scramble competition, where males do not defend territories but actively search for females, can engage aggressively towards other males, and in the absence of available females will try to disrupt mating pairs38. Males of this species mate more intensively and invest more in armaments related to sexual selection (i.e., the male grasping apparatus that attaches to the female prothorax) at northern edge populations than in core or southern edge populations, presumably due to constraints imposed by short mating seasons13,39. Thus, we also expected stronger effects of sexual selection in traits related to flight in males at the northern edge populations compared the central core and southern edge populations13.

Figure 1
figure 1

(A) Image of a male Lestes sponsa (credit: Bogusław Daraż). (B) Locations of the study populations (black dots) across the distribution range of L. sponsa (grey shade) in Europe (modified from Dijkstra and Schröter35). Map created using CorelDraw Graphics Suite 2021 (https://www.coreldraw.com/).

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We studied four main flight-related morphological traits: body size, wing size, wing ratio (the proportion of wing size by body size) and wing shape. Regarding body size, previous studies in Lestes damselflies have shown that sexual selection (measured as mating sucess32,38) may favor intermediate male sizes, while larger sizes are generally expected to confer higher dispersal ability40. Under strong sexual selection, females would be predicted to show larger body sizes, since female body size is generally correlated with fecundity in insects41. Wing size is generally highly correlated with body size, but larger wing lengths and larger wing ratios can be related to dispersal ability42,43,44,45. Wing ratio also informs on relative investment on wing structures for a certain body size, which could be related to the aerodynamic needs of flight for each sex. Wing shape can also reflect both sexual selection pressures and dispersal ability. In males of L. sponsa, short and broad forewings (and to a lesser extent narrow-based hindwings), were favored by sexual selection, suggesting a role of flight maneuverability in mating success32. Moreover, wing shapes that are energetically more efficient can reflect dispersal ability, i.e., long and narrow wings46,47. In summary, we predicted latitudinal differences in these traits that would be the signature of both natural and sexual selection. More specifically, the flight-related traits should better reflect dispersal ability at the edge populations26, and therefore we predicted, for both sexes, larger body and wing size, larger wing ratio, and longer and narrower wing shapes at the northern and southern edges40,42,43,44,45,46,47. Alternatively, the signature of sexual selection could be stronger than dispersal selection for males at the northern and/or southern edge populations13,39. In that scenario, at edge populations males would show intermediate body sizes and short and broad forewings, favoring scrambling behavior32,38.

Results

A total of 331 females and 337 males were collected across 17 populations spanning the entire distribution range of L. sponsa in Europe, from southern France to northern Sweden (Fig. 1B). The latitudinal variation of body size, wing size, wing ratio and wing shape were studied.

Body size

The general linear model used to study the variation of body size (adjusted R2 = 0.47) showed that body size was larger in females than males (F1,581 = 80.09, partial R2 = 0.12, p < 0.001; mean ± SD, mm: females: 4.75 ± 0.17; males: 4.66 ± 0.17). Body size showed a slightly U-shaped pattern in relationship to latitude (linear term: F1,581 = 94.86, partial R2 = 0.14, p < 0.001; quadratic term: F1,581 = 88.64, partial R2 = 0.13, p < 0.001), with the highest values in the southernmost latitude, a steady decrease in the center, and a small increase again at the northernmost latitudes, especially for females (Fig. 2A). The relationship between body size and latitude did not differ between the sexes, since the interaction term sex × latitude was not significant (removed from the final model).

Figure 2
figure 2

(A) Latitudinal variation of body size in Lestes sponsa (measured as head width, mm) for females (black) and males (gray). (B) Latitudinal variation of wing centroid size (derived from landmark coordinates in pixels) for females and males, including fore- and hindwings. (C) Latitudinal variation of wing ratio (ratio between wing centroid size and body size) for fore- and hindwings of females and males. (female forewings: black circles and solid line; female hindwings: black squares and dashed line; male forewings: gray circles and solid line; male hindwings: gray squares and dashed line; the shade around the regression lines shows the 95% CI).

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Wing size

The effects of sex and latitude on fore- and hindwing size were first explored. The general linear models showed that wing size significantly differed between the sexes (Table 1A). Female wings were larger than male wings, and forewings were larger than hindwings in both sexes (mean ± SD, mm: female forewings: 156.77 ± 1.31; female hindwings: 155.18 ± 1.30; male forewings: 155.03 ± 1.28; male hindwings: 153.46 ± 1.28). Wing size also showed a significant U-shaped pattern in relation to latitude (Table 1A, Fig. 2B). Latitude explained 27.6% of the forewing size variation and 27.4% of the hindwing size variation (Table 1A). The shape of the quadratic curve of latitude slightly differed between the sexes (Fig. 2B), although this difference was not significant.

Table 1 A. Results of the General Linear Models inspecting the effects of latitude and sex on the variation of fore- and hindwing size in Lestes sponsa. B. Results of the General Linear Models inspecting the effects of latitude, body size (measured as head width) and sex on the variation of fore- and hindwing size in Lestes sponsa.
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The same general linear models were run accounting for the allometric component of wing size by incorporating body size into the models. Wing size showed a strong allometric component for both fore- and hindwings (Table 1B). The slopes of the regressions of wing size on body size were significantly larger than 1 both for fore- (slope ± 95% CI 1.36 ± 0.01) and hindwings (slope ± 95% CI 1.35 ± 0.09). When accounting for allometry, the effect sizes of latitude and its quadratic component accounted for less than 2% of the total wing size variation (Table 1B).

Wing ratio

The two general linear models used to study the variation of fore- and hindwing ratio showed that this trait differed between the sexes (Table 2). Wing ratio was larger in males than in females for both wings (mean ± SD: male forewing: 1.22 ± 0.03; male hindwing: 1.21 ± 0.03; female forewing: 1.21 ± 0.03; female hindwing: 1.21 ± 0.03). Moreover, fore- and hindwing ratio showed a significant positive quadratic relationship with latitude (Table 2). Latitude and its quadratic component accounted for almost 20% of the total variation of wing ratio. Females showed lower values of wing ratio than males at the north edge populations (Fig. 2C), although this difference was not significantly different.

Table 2 Results of the General Linear Models inspecting the effects of latitude and sex on the variation of fore- and hindwing ratio in Lestes sponsa.
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Wing shape

Wing shape was studied using geometric morphometrics (13 landmarks and 1 semi-landmark, see Supplementary Fig. S1). To visualize wing shape variation in the entire dataset of fore- and hindwings, a PCA on the wing shape variables was performed, and the variation of wing shape along the first two PCs (variance explained: PC1: 47.71%; PC2: 24.50%) was depicted as thin-plate spline deformation grids (Fig. 3). The plot of PC2 on PC1 showed large overlap between the sexes, but clear separation between fore- and hindwings (Fig. 3). Forewings were longer, slenderer and showed a shorter petiole; hindwings were shorter, broader and had a more elongated petiole (Figs. 3, 4).

Figure 3
figure 3

Wing shape variation of Lestes sponsa across all the populations studied. The thin-plate splines grids are exaggerated × 3 times for ease of visualization and show the extremes of variation for each corner of the plot. (female forewings: black circles; female hindwings: black squares; male forewings: gray circles; male hindwings: gray squares).

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Figure 4
figure 4

Thin-plate splines depicting the wing shape differences between males and females and fore- and hindwings of Lestes sponsa. The grids are exaggerated × 3 times for ease of visualization.

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Two Procrustes ANOVAs on the fore- and hindwing shape variables showed that the effects of sex and latitude accounted for less than 4% of the total wing shape variation (Table 3A). The differences in wing shape between the sexes were indeed subtle: female wings showed an overall slightly broader appearance and a slightly shorter petiole than male wings (Fig. 4, Supplementary Fig. S2). The deformation grids regarding wing shape variation along latitude are not shown since the wing shape variation was almost imperceptible.

Table 3 (A) Results for the Procrustes ANOVAs investigating the effects of latitude and sex on fore- and hindwing shape of Lestes sponsa. (B) Results for the Procrustes ANOVAs investigating the effects of latitude, size (measured as wing centroid size) and sex on fore- and hindwing shape of Lestes sponsa.
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When the allometry of wing shape was accounted for by incorporating wing centroid size in the Procrustes ANOVAs (Table 3B), neither sex, latitude nor wing centroid size explained more than 5.5% of the total wing shape variation. Wing shape variation in relation to wing centroid size was subtle, with small wings showing a more pedunculated shape, broader tip, longer petiole and narrow wing base, and large wings showing a thicker shape with shorter petiole and broad base (Fig. 5).

Figure 5
figure 5

Allometric variation of wing shape of Lestes sponsa. Wing centroid size was derived from landmark coordinates in pixels. Fore- and hindwings of males and females are coded differently (female forewings: black circles; female hindwings: black squares; male forewings: gray circles; male hindwings: gray squares), but note that the regression slopes did not differ between these groups. Deformation grids depicting the extremes of allometric wing shape variation are included. The grids are exaggerated × 10 times for ease of visualization.

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Discussion

We predicted that edge populations would differ in flight-related traits compared to core populations and that they should show trait values favoring dispersal ability and reproductive investment7,8,13. We found some support for these predictions. Our study highlights two main findings. We found that traits such as body size, wing size and wing ratio showed variation across the entire latitudinal range (contributing to 17–27% of the total variation of each trait), but wing shape showed very little variation across the same span (latitude contributed to less than 4% of the total variation of wing shape). We also observed differences between the sexes regarding the latitudinal variation of body size, wing size and wing ratio. We discuss below how the observed patterns for body and wing size agree with our predictions regarding selection favoring dispersal and fecundity in females, but not with selection on male sexual behaviors. We also discuss why wing shape might show very limited latitudinal variation.

Body size showed a U-shaped pattern in relationship with latitude. This relationship was especially clear for females, although the differences between the sexes were not statistically significant. In females, we predicted larger body sizes at the north and south edge populations to facilitate dispersal ability40 and increase their investment in reproduction via fecundity41. Our results agree with this prediction, but we also note that body size is much larger in the south than in the north edge populations. A plausible explanation is that body size in hemimetabolous insects such as damselflies is determined in part by the length of the season favorable for larval growth28,37,48: obligatory univoltine species such as L. sponsa usually tend to show a decrease in body size with latitude37, since the growth season decreases northwards. For males, we predicted that intermediate body sizes would be favored at edge populations (especially at the north edge13) due to their scrambling reproductive behavior32,38, but larger body sizes would be favored due to selection for dispersal ability. We found no support for this prediction since the results showed a similar body size at the north edge compared to the core populations, which could reflect conflicting selection pressures between sexual (scrambling behavior) and natural selection (dispersal). Similar to females, body size in males was the largest at the southern populations, probably due to the longer available growth season towards the south37,49. The observed differences in body size between the sexes along the latitudinal gradient, might be explained by latitudinal variation of sex-specific selection on body size. A recent study on the damselfly Ischnura elegans (Vander Linden, 1820) has found a decrease in sexual size dimorphism at north edge populations, as a result of an increase in male body size16. Our results show, however, a likely increase in female body size at north edge populations, resulting in an increase in sexual size dimorphism. One reason for these differences might be related to sex biases in dispersal: Ischnura damselflies can show male-biased dispersal16, while Lestes damselflies have been suggested to show female-biased dispersal50. Moreover, since females invest more in fecundity than males, females are probably under stronger fecundity selection at edge populations.

Our results for wing size in relation to latitude were similar to body size, a U-shaped pattern. Larger wing sizes can be related to better dispersal ability42,43, and our results suggest (similarly to our results for body size) that dispersal ability might be under stronger selection for females than males50, since the pattern was more accentuated in females. This latitudinal effect decreased from an effect size of 27% to less than 2% when allometric effects of body size were introduced in the model. Thus, our results suggest that selection on wing size might be acting through selection on body size.

Wing ratio showed a positive quadratic relationship with latitude, with significant differences between the sexes: wing ratio is lager in males than in females, and this difference is more pronounced at the north edge populations. Wing ratio has been also proposed as a proxy for better dispersal ability43, and we therefore predicted larger wing ratios at the north and south edge populations. However, our results showed a different pattern, with the smallest values of wing ratio at the south edge, an increase at core populations, and a different response between the sexes at the north edge populations. For females, the wing ratio values somewhat decreased compared to core populations, while for males, the values increased. Thus, wing ratio showed an inverse pattern to body size in relation to latitude. Part of the variation in wing ratio could be explained by the allometric relationship between wing size and body size. The allometric slopes between wing size and body size were larger than one for both fore- and hind wings, suggesting that either large-bodied damselflies invest more in larger wings or that smaller-bodied damselflies invest less in larger wings. It is likely that different latitudinal selection on body and wing size, and the particular aerodynamic needs of damselfly flight, are imposing the observed pattern in wing ratio.

Contrary to our expectations, wing shape showed very little variation in relation to latitude. We predicted a different wing shape at the edge populations compared to the core populations, favoring dispersal ability and reproduction32,46,47. However, latitudinal variation explained less than 5% of the total shape variation, regardless of considering or not the allometric component of wing shape. This is very interesting because flight traits such as wing shape are understood as the result of adaptive evolution driven by natural and sexual selection in many extant flying animal groups20,51,52. We suggest that the aerodynamic needs of each pair of wings for a given size of the individual might be under strong selection, probably differing between front- and hind wings53,54,55.

Our results showed that within the entire latitudinal range of a species, certain components of flight morphology, body size and wing size, might be under different selection regimes between core and edge populations, and that selection for dispersal and fecundity in females is probably stronger than sexual selection in males. The patterns observed at edge populations for body size and wing size might be a signature of a process of spatial sorting9. However, population genetics studies would be necessary to support this hypothesis.

Methods

Ethics declaration

The damselfly L. sponsa is considered a species of least concern by the IUCN red list of threatened species56. Damselfly collection in southern France was permitted by The Camargue Regional Nature Park, France. No other sampling permits were required for field collection.

Field sampling and specimen preparation

Males and females of L. sponsa were sampled in 17 populations ranging from southern France to northern Sweden, covering the entire south-north distribution range of this species (Fig. 1B; see samples sizes in Table 4). Damselflies were collected during the peak of the flying season at each latitude (see sampling dates in Table 4). The altitude of the sampled populations ranged from 0 to 252 m a.s.l. (Table 4). These same damselflies were used in earlier cross-latitude comparison of adult size37. Animals were captured using a butterfly net and sacrificed in 70% ethanol. Head width was measured using a digital caliper to the nearest 0.01 mm. Head width is a commonplace measure of body size in damselflies and dragonflies, correlated with overall body size57,58,59. The measures of head width were log-transformed for use in further analyses. Fore- and hindwings were carefully removed and scanned in a flatbed scanner at 600 pixels/inch. The images of the wings were subsequently used for studying wing morphology. Individuals with torn or broken wings were discarded from the dataset.

Table 4 Sample size (N) of males and females of Lestes sponsa studied at each population including country of origin, sampling date, latitude and longitude, and altitude (head: head width, FW: forewings, HW: hindwings). The sample size for measurements of head width is variable depending on the corresponding measurements for fore- and hindwings of the same individual (N head FW-N head HW). No females were captured in the populations of Vistula Oxbow 1 Oświęcim (Poland) and Norrköping (Sweden).
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Wing morphology

To study wing size and shape, 13 landmarks and 1 semi-landmark along the wing outline and the arculus were digitized in tpsDig260 (Supplementary Fig. S1). The landmark configurations were used to calculate the centroid size of each wing, i.e., the square root of the sum of squared distances between each landmark of the configuration and the centroid of the configuration. Centroid size was log-transformed for further analyses. Landmark configurations for all the wings were subject to Generalized Procrustes Analysis (GPA), in order to remove the effects of position, rotation and isometric size by minimizing the total sums-of-squared deviations of the landmark sets from all specimens to the average landmark configuration61. The semi-landmarks were permitted to slide along their tangent directions to minimize the Procrustes distance between each wing and the average configuration62. The GPA was first run for fore- and hindwings together (for visualization purposes), and then separately for fore- and hindwings. The aligned Procrustes coordinates resulting from the GPAs were projected into a linear tangent space to obtain the Procrustes shape variables. The sets of Procrustes shape variables for each separate wing (one for fore- and one for hindwings) were subsequently used as the variables representing wing shape variation. All the geometric morphometric analyses were performed in the R package geomorph v 3.3.263.

Variation of flight-related traits

Fore- and hindwing shapes can be subject to different flight constraints, so the statistical analyses were run separately for fore- and hindwings. The variation across latitudes of body size, wing size, wing ratio (the ratio between wing size and body size, calculated for fore- and hindwings) and wing shape was explored in separate models. Latitude was log-transformed before being entered in the models to improve normality and use similar scaling as for the other continuous variables. Altitudinal variation did not correlate with latitudinal variation (Pearson’s r = − 0.035). Altitude did not explain variation of body size, wing size or wing ratio (all cases, R2 < 0.003, p > 0.103), and was therefore excluded from the corresponding models. Altitude showed very little effects on wing shape variation (forewing shape: R2 = 0.017, p = 0.001; hindwing shape: R2 = 0.014, p = 0.001) so it was excluded from the final models.

For body size, fore- and hindwing size and fore- and hindwing ratio, separate general linear models were run, with sex as a fixed factor and latitude as a covariate. The interaction term was not significant and therefore removed from the final models. Visual inspection of the relationship between these variables and latitude suggested a possible quadratic relationship. Therefore, the models including the quadratic term of latitude were also run. The linear and the quadratic models were compared inspecting the ∆AIC using the R package MuMIn64. The models that included the quadratic term were selected over the models with only the linear term (body size: ∆AIC = 81.03; forewing size: ∆AIC = 95.25; hindwing size: ∆AIC = 94.73; forewing ratio: ∆AIC = 54.73; hindwing ratio: ∆AIC = 8.17). Finally, wing size can be affected by body size allometry. Therefore, another set of four general linear models including body size (measured as head width) as a covariate was also run. The non-significant interactions were removed one by one following a hierarchical approach, starting from the full model with all possible interactions, leading to no interactions present in the final model.

Wing shape variation was first visualized using Principal Component Analysis (PCA). The matrix of shape variables resulting from the full GPA (including fore- and hindwings together) was subject to a PCA, and the scores for PC1 and PC2 were used to plot wing shape variation. The variation of wing shape along each PC was depicted using thin-plate spline deformation grids. Wing shape variation for each wing was then analyzed using a Procrustes ANOVA on the shape variables resulting from the separate GPAs. These analyses were run in the R package geomorph63. The significance of the amount of shape variation attributable to each effect was evaluated using a residual randomization permutation procedure65,66. Latitude and sex and their interaction were included in the model. The interaction term was not significant and thus removed from the model. The fitting significantly improved by incorporating the quadratic term of latitude (p = 0.001), so this term was included in the final model. To account for the potential effects of allometry on wing shape, two more Procrustes ANOVAs were run incorporating also wing centroid size as a covariate.