Interpopulation variation in sexual dichromatism in the Neotropical grasshopperSphenarium purpurascens(Orthoptera: Pyrgomorphidae)

Abstract

Cryptic coloration is an adaptative defensive mechanism against predators. Colour patterns appear cryptic through general background coloration matching or disruptive coloration. Disruptive coloration might evolve in visually heterogeneous microhabitats, whereas background matching could be favoured in chromatically homogeneous microhabitats. In this study, we used digital photography to explore the potential use of disruptive coloration and background matching in males and females of the Neotropical grasshopper Sphenarium purpurascens in different habitats. We found chromatic differences in three habitats and sexual dichromatism that might be explained by local adaptation. Although females and males were sexually dichromatic, interpopulation differences were found in the magnitude of the sexual dichromatism. In a highly contrasting environment, both males and females seemed to follow a disruptive strategy, whereas in a heterogeneous environments males and females followed different colour cryptic strategies, in which males were more disruptive than females, and females exhibited high background matching with fewer disruptive elements. Selective predation in different microhabitats and differences in mobility between the sexes might explain the colour pattern divergence between females and males.

Testing background matching and disruptive colouration in a sexually dichromatic grasshopper: a computer detection experiment

“…On a colour monitor, images of the grasshopper individuals were displayed randomly on a heterogeneous chromatic background. Given the different visual properties of males and females, and because in other Sphenarium spp., males are usually found in heterogeneous and contrasting chromatic environments, whereas females exhibit high background matching (Ramírez‐Delgado & Cueva del Castillo, 2020; Cueva del Castillo et al, 2021), we expected to find differences between predators' time searching for striped and unstriped individuals. Moreover, we expected that searching time for various disruptive individuals (potentially the striped males, of which we distinguished two patterns, see below) would be similar.…”

“…This is especially important in organisms with high mobility (Stevens et al, 2006). Moreover, if females and males use distinct microhabitats owing to their different sexual roles, and/or display differences in mobility, natural selection can favour a divergence in cryptic colour patterns between females and males (Forsman & Appelqvist, 1999; Medina et al, 2016; Ramírez‐Delgado & Cueva del Castillo, 2020; Cueva del Castillo et al, 2021).…”

“…However, the females tend to be cryptic to decrease the risk of being predated, whereas males are conspicuous due to sexual selection pressures (Badyaev & Hill, 2003; Medina et al, 2017). In invertebrates, the study of the evolution of sexual dimorphism in colour patterns has focused on insects, mainly orthopterans and butterflies (Forsman & Appelqvist, 1999; Ramírez‐Delgado & Cueva del Castillo, 2020; van der Bijl et al, 2020; Cueva del Castillo et al, 2021), but also spiders (Li et al, 2008). In orthopterans, evidence suggests that males and females could have different ecological niches and mobility that may favour divergence in colouration.…”

“…Following reproduction, oviposition, as well as the highest adult mortality, occur during winter, from mid‐December to mid‐February (Sanabria‐Urbán & Cueva del Castillo, 2020). Adults exhibit large variation in chromatic patterns (Figure 1), and like other species of the genus, are sexually dichromatic (Ramírez‐Delgado & Cueva del Castillo, 2020; Cueva del Castillo et al, 2021). Males usually have more complex and contrasting patterns than females, with stripes that cross both longitudinally and transversely and some bright spots on the first two segments of the abdomen in their dorsal area.…”

“…In pygmy grasshoppers, background matching and disruptive colouration have been related to the habitation of different environments and grasshopper size (Tsurui et al, 2010, 2013). In contrast, Sphenarium purpurascens Charpentier and Sphenarium planum Bruner & L. show different patterns that could be associated with both cryptic strategies (Ramírez‐Delgado & Cueva del Castillo, 2020; Cueva del Castillo et al, 2021). In these two species, males and females are sexually dichromatic.…”