It takes two: Seasonal variation in sexually dimorphic weaponry results from divergent changes in males and females

Abstract

Sexually dimorphic weaponry often results from intrasexual selection, and weapon size can vary seasonally when costs of bearing the weapon exceed the benefits outside of the reproductive season. Weapons can also be favored in competition over nonreproductive resources such as food or shelter, and if such nonreproductive competition occurs year‐round, weapons may be less likely to vary seasonally. In snapping shrimp ( Alpheus angulosus ), both sexes have an enlarged snapping claw (a potentially deadly weapon), and males of many species have larger claws than females, although females are more aggressive. This contrasting sexual dimorphism (larger weaponry in males, higher aggression in females) raises the question of whether weaponry and aggression are favored by the same mechanisms in males and females. We used field data to determine whether either sex shows seasonal variation in claw size such as described above. We found sexual dimorphism increased during the reproductive season due to opposing changes in both male and female claw size. Males had larger claws during the reproductive season than during the nonreproductive season, a pattern consistent with sexual selection. Females, however, had larger claws during the nonreproductive season than during the reproductive season—a previously unknown pattern of variation in weapon size. The observed changes in female weapon size suggest a trade‐off between claw growth and reproduction in the reproductive season, with investment in claw growth primarily in the nonreproductive season. Sexually dimorphic weaponry in snapping shrimp, then, varies seasonally due to sex differences in seasonal patterns of investment in claw growth, suggesting claws may be advantageous for both sexes but in different contexts. Thus, understanding sexual dimorphisms through the lens of one sex yields an incomplete understanding of the factors favoring their evolution.

Tradeoffs and Benefits Explain Scaling, Sex Differences, and Seasonal Oscillations in the Remarkable Weapons of Snapping Shrimp (Alpheus spp.)

“…Concurrently, males had significantly lower abdomen residuals during the breeding season, whereas females exhibited a slight but nonsignificant decrease in abdomen residuals. Similar trends have been reported in A. angulosus, although in that species, females significantly reduce proportional abdomen sizes during the breeding season (Heuring & Hughes, 2019). We speculate that males shift investment from their abdomens into weapons during the breeding season because it increases their likelihood of being paired.…”

“…In addition to scaling relationships, costly traits can also differ depending on sex and season. For example, some secondary sexual traits are expressed in both sexes but at greater magnitudes in males than females (Heuring & Hughes, 2019;Nolazco et al, 2022). Moreover, costly traits might be expressed more intensely during the breeding season compared to the nonbreeding season, such as the annual shedding and regeneration of deer antlers (Brockes et al, 2004;Clements et al, 2010;Price et al, 2005).…”

“…Yet, snapping shrimp don't grow weapons to arbitrary sizes. Instead, they vary along three axes: 1) larger individuals have larger weapons, 2) at any given body size, males have larger weapons than females, and 3) the sex difference amplifies during the summer breeding season (Heuring & Hughes, 2019). Therefore, costs and benefits of weapon size can be examined across these three axes: body size, sex, and breeding season.…”

Large and exaggerated sexually selected weapons comprise high proportions of metabolically inexpensive exoskeleton

“…This is likely because female snapping shrimp reduce relative claw sizes during the breeding season as they invest more in egg production and brood care (e.g. A. angulosus , [39]). In my dataset, I found ovigerous A. heterochaelis females, but I did not find ovigerous A. estuariensis females, which could explain why A. heterochaelis female weapons scaled with negative allometry but A. estuariensis female weapons did not.…”

Snapping shrimp have helmets that protect their brains by dampening shock waves

“…These indicators should reliably distinguish females from males because we conducted this experiment in the middle of the A. heterochaelis reproductive season. 39 Outside of these indicators, A. heterochaelis do not have obvious sexual dimorphisms: males and females appear to have similar ranges of both body and claw size. The sexes included in each treatment group were as follows: UE 13 f/17 m; AE 15 f/15 m; UC 15 f/15 m; AC 15 f/15 m. We measured the size of each individual from the tip of its rostrum to the end of its telson.…”