Size‐dependent predation risk partly explains the sex‐related marking polymorphism in the sexually size‐dimorphic pygmy grasshopperTetrix japonica

Abstract

The sexually size-dimorphic grasshopper Tetrix japonica exhibits variation in body-color markings on the pronotum even within a single local population. Such markings have been suggested to reduce the visual detectability of grasshoppers. However, some grasshoppers have no markings. In the present study, we examined the effect of the sex-related difference in body size and the spotted markings on the degree of camouflage. We hypothesized that: (i) large individuals (females) are potentially more readily detectable than small individuals; (ii) large individuals (females) with spotted markings would realize a moderate degree of the camouflage effect, whereas large individuals (females) without spotted markings would be quite poorly camouflaged; (iii) small individuals (males) would be sufficiently less detectable, with or without markings; and (iv) large individuals (females) would tend to have spotted markings in the wild. To test this hypothesis, we conducted a morph-frequency field survey and laboratory experiments on the body-size dependency of the spotted markings. The field survey confirmed that all females exhibited spotted markings and that the majority of males were non-spotted morphs. Next, to determine whether body size and the spotted markings affected crypsis, we conducted detection task experiments using humans as dummy predators by manipulating the body size, presence/absence of spotted markings, or both, of printed grasshoppers. The absence of spotted markings increased the detection risk in large and small grasshoppers, particularly in large-sized females. These results suggest that female-biased selective predation could have eliminated non-spotted female morphs because they were too conspicuous.

A comparative perspective on longevity: the effect of body size dominates over ecology in moths

“…The insects appear to be a promising candidate as a target taxon, due to the immense species richness and diversity of the class. Intriguingly, in several insect groups, there may be a positive rather than a negative correlation between extrinsic mortality risk and body size (Remmel et al ., ; Penney et al ., ; Tufto et al ., ; Tsurui et al ., ; Smith et al ., ). If this is the case, predictions stemming from the physiological and ecological explanations of ageing would oppose each other (positive vs. negative size‐longevity correlation, respectively) and could provide information about the relative importance of these two mechanisms in nature.…”

Body size influences differently the detectabilities of colour morphs of cryptic prey

“…Yet, whether size and colour morph interactively influence detection, leading to correlational selection, has not previously been systematically investigated in this species. Tsurui, Honma & Nishida () explored how the presence of a certain type of colour morph (spotted) influenced detection of males and females in the sexually size dimorphic T. japonica . They report that spotless individuals were detected more easily than spotted individuals, and that large individuals were more readily detected than small individuals, although no effect of the interaction between colour morph and body size.…”

On the role of sex differences for evolution in heterogeneous and changing fitness landscapes: insights from pygmy grasshoppers

“…Work on PGHs shows that the protective value of alternative colour patterns differ between males and females according to behaviours and movement patterns [61,63], body size [42] and visual characteristics of the habitat [44,64]. That predation may impose opposing selection on colour pattern in males and females has also been reported in Tetrix japonica [65,66], and in lizards [110] and snakes [28,113]. …”

“…Their small size and locally high population densities render grasshoppers susceptible to visual vertebrate predators such as birds [ 59 – 61 ] and lizards [ 62 ]. Studies using different approaches indicate that colour morph shifts are driven at least in part by differential predation and selection for camouflage that varies according to sex [ 42 , 44 , 61 – 64 ], as outlined in §4, also in other Tetrix species [ 65 , 66 ].…”