Let's mate here and now – seasonal constraints increase mating efficiency

Abstract

1. Latitudinal climatic conditions shape the length of the mating season and could thus influence reproductive traits. Knowledge of how animals behave along latitudinal clines will increase understanding of the impact of climate on sexual selection and might help in the prediction of whether peripheral populations will spread or shrink in response to changes in climate.2. This study investigated variation in the mating efficiency of a temperate insect, the emerald damselfly Lestes sponsa, under semi-natural field conditions along a latitudinal gradient covering three regions of the species' distribution: south, central and north.3. A comparison was done of the proportion of copulating males, the proportion of males that formed tandems but did not copulate (unsuccessful males), and the proportion of males that did not attempt to form a tandem (passive males) in these three regions.4. It was found that the proportion of copulations was significantly higher at northern latitudes than in the southern and central regions. Southern latitudes had a higher proportion of successful copulations compared with central latitudes. The northern region had a significantly lower frequency of passive males. The southern region had an intermediate proportion of passive males, and the central region had the highest proportion. The proportion of unsuccessful males did not differ between regions. The population density across sites did not affect these results.5. The study shows that damselflies inhabiting northern populations mate more intensively than individuals from southern and central populations. This suggests that more restrictive environmental conditions during a brief mating season select for higher mating efficiency.

Body and wing size, but not wing shape, vary along a large-scale latitudinal gradient in a damselfly

“…Sexual selection is predicted to be stronger at northern edge populations since mating seasons are shorter and there are less favorable reproductive conditions 10 , 12 – 14 . The same prediction is expected for southern edge populations, where mating seasons are also shorter due to high temperatures that can lead to overheating 13 . However, dispersal and reproductive traits can be subject to conflicting selection pressures at edge populations 15 , possibly limiting the range expansion process 7 .…”

“…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 pairs 38 . 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 seasons 13 , 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 populations 13 .…”

Two experimental designs generate contrasting patterns of behavioral differentiation along a latitudinal gradient in Lestes sponsa—Common‐garden not so common after all?

“…Interestingly, the trend of central damselflies expressing the lowest, and southern populations showing intermediate trait values has also been found in mating behavior of adult L. sponsa studied in seminatural conditions (Golab et al., 2019).…”

“…Also, heat stress can impact damselfly behavior in southern populations. It has been proposed that high air temperature (exceeding 31 degrees) reduces L. sponsa mating activity (Golab et al., 2019). If this is the case, it would explain why individuals from the southern populations are more time constrained than individuals from the center, yet less so than those from the north (cf.…”

“…For example, insects that are plastic in their generation‐time (voltinism) commonly take an extra season to complete a generation and show slower growth and development further north. This strategy results in larger size at emergence (Arnett & Gotelli, 2003; Johansson, 2003), which is often associated with increased reproductive success (Honěk, 1993; Sokolovska, Rowe, & Johansson, 2000) and performance in lower temperatures (Nielsen & Papaj, 2015; Stevenson, 1985, but see Golab, Johansson, & Sniegula, 2019). However, the opposite pattern can also occur if species with fixed voltinism are seasonally constrained, and compensate for this by increasing development and growth rate at northern latitudes.…”