Seasonal changes in weapon size in both sexes – but in different seasons

Blog by Whitney Heuring and Melissa Hughes. Read the full paper here.

Weaponry and ornamentation often differ between males and females; males can have brightly colored plumage, long horns, or thick manes compared to their female counterparts. Whether females lack the trait entirely or express it in reduced from (not-quite-so-bright plumage, smaller horns), such sexual dimorphisms are widespread across animals, and we’re rarely surprised to learn that the male is brighter or has more fearsome weapons.

Superficially, snapping shrimp are similarly unsurprising. These small marine crustaceans, also known as pistol shrimp, have an enormous claw that ranges from approximately one-third to over one-half the animal’s body size. When the claw is rapidly closed, a jet of water is released and a cavitation bubble is formed. The implosion of the bubble produces the snap for which these shrimp are named, a sound that makes them one of the loudest animals in the sea [1]. A snap directly against the body of another shrimp can cut through the opponent’s carapace or snip off limbs; claws are deadly weapons. In many species, including the one we study (Alpheus angulosus), males have larger claws than females. More unusual, however, is the sexual dimorphism in how the claw is used: females snap more than males in competitions with conspecifics [2], and are more likely to kill their opponents [3]. And even stranger: while in other animals, larger weaponry in males is often favored by sexual selection (males using their large weapons to compete with other males for access to females), in snapping shrimp, we have yet to find any evidence that larger claws benefit males in a reproductive context.

In our study, we explored seasonal variation in weapon size as a means to investigate possible differences in weapon function in males and females. If weapons are primarily beneficial for competitive interactions over reproductive resources (i.e., mates), then claws would be predicted to show seasonal variation in size, being larger during the reproductive season. Conversely, if claws are primarily advantageous to defend nonreproductive resources (i.e., food, shelter), then claw size would not be predicted to show seasonal variation.

What we found, however, was an unexpected pattern of seasonal variation in which male and female claw sizes mirror one another: male claw sizes are largest when female claw sizes are smallest, and vice versa. For males, the ratio of the claw size to body size is higher in the reproductive season than in the nonreproductive season; the regression of claw size on body size is steeper in the reproductive season. For females, on the other hand, claw/body ratios are highest and the slope of the claw x body regression is steeper in the nonreproductive season as compared to the reproductive season. As a result, sexual dimorphism in claw size is maximized during the reproductive season, due to changes in both sexes. To the best of our knowledge, this the first evidence of seasonal variation in sexual dimorphism due to males and females having opposing seasonal patterns of weapon size, suggesting that different mechanisms are operating in each sex.

Shrimp graph
Seasonal variation in claw/body ratio. Mean ± SE claw/body ratio across collections within months for males (blue, above) and females (red, below); open circles are reproductive season (n = 40), and closed circles are nonreproductive season (n = 14)

In males, increased claw size during the reproductive season suggests that male snapping shrimp, like males of many other animals, benefit from larger weaponry due to advantages in reproductive competitions, although how exactly the claw provides this advantage remains unclear. Larger snapping claws in snapping shrimp do not increase the likelihood of winning competitive interactions [4], or the likelihood of being paired with a reproductive female [2]. It is possible that larger claws are beneficial primarily as signals; the role of claws in female mate choice has yet to be explored.

For females, larger claws during the nonreproductive season are intriguing and unexpected. There may be a trade-off between investment in weapon size vs. investment in reproduction, with females investing more in egg production or body size (increasing the number of eggs that can be brooded) during the reproductive season; such a trade-off would explain the reduction in claw/body size during the reproductive season. But why invest in claw growth during the nonreproductive season? Female fecundity depends on body size; continued investment in body size rather than claw size during the nonreproductive season, then, would maximize fecundity in the following reproductive season. The benefit to females for increased claw size during the nonreproductive season is unknown. Females snap more in competitive interactions [2] and are more likely to kill their opponents than males [3]; in females (unlike in males), larger claws predict greater success in competitive interactions [4]. Together with the results of this study, these results suggest that aggressive behavior – and the claw as a weapon – play a critical role in female fitness.

Sexual dimorphisms in weaponry are common, and not surprisingly we often focus on the sex with larger weapons, typically the male. Our results demonstrate that studying sexual dimorphisms from the perspective of only one sex risks missing half the story.

 

References:

[1] Versluis, M., B. Schmitz, A. von der Heydt, D. Lohse. 2000. How snapping shrimp snap: Through cavitating bubbles. Science 289:2114-2117.

[2] Hughes, M., T. Williamson, K. Hollowell, and R. Vickery. 2014. Sex and weapons: contrasting sexual dimorphisms in weaponry and aggression in snapping shrimp. Ethology 120:982–994.

[3] Knowlton N, B. D. Keller. 1982. Symmetric fights as a measure of escalation potential in a symbiotic, territorial snapping shrimp. Behavioral Ecology and Sociobiology 10:289–292.

[4] Hughes, M.  1996.  Size assessment via a visual signal in snapping shrimp. Behavioral Ecology & Sociobiology 38:51-57.

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