Costs of weaponry: Unarmed males sire more offspring than armed males in a male‐dimorphic mite

Abstract

Morphological structures used as weapons in male–male competition are not only costly to develop but are also probably costly to maintain during adulthood. Therefore, having weapons could reduce the energy available for other fitness‐enhancing actions, such as post‐copulatory investment. We tested the hypothesis that armed males make lower post‐copulatory investments than unarmed males, and that this difference will be most pronounced under food‐limited conditions. We performed two experiments using the male‐dimorphic bulb mite Rhizoglyphus robini, in which males are either armed “fighters” or unarmed “scramblers.” Firstly, we tested whether fighters and scramblers differed in their reproductive output after being starved or fed for 1 or 2 weeks. Secondly, we measured the reproductive output of scramblers and fighters (starved or fed) after one, two or three consecutive matings. Scramblers sired more offspring than fighters after 1 week, but scramblers and fighters only sired a few offspring after 2 weeks. Scramblers also sired more offspring than fighters at the first mating, and males rarely sired offspring after consecutive matings. Contrary to our hypothesis, the fecundity of starved and fed males did not differ. The higher reproductive output of scramblers suggests that, regardless of nutritional state, scramblers make larger post‐copulatory investments than fighters. Alternatively, (cryptic) female choice generally favours scramblers. Why the morphs differed in their reproductive output is unclear. Neither morph performed well relatively late in life or after multiple matings. It remains to be investigated to what extent the apparent scrambler advantage contributes to the maintenance and evolution of male morph expression.

Size-structured mating niche differentiation in the male-polyphenic mite Rhizoglyphus robini

“…Our results align well with evidence from horned beetles that larger males are generally more successful fighters than smaller males (Emlen 1997;Hongo 2007;McCullough and Simmons 2016;Mitchem et al 2019), so that small males resort to sneaking behaviour to gain access to females, adaptively expressing a different, weaponless morphology (Moczek and Emlen 2000). We currently do not have evidence that R. robini scramblers are sneakers, but they have been shown to obtain higher fertilization success from a single copulation than fighters (Van den Beuken et al 2019). Furthermore, scramblers move quicker than fighters and are better able to navigate spatially complex environments (Tomkins et al 2011), which could benefit a hypothetical sneaking tactic.…”

“…In contrast, small and poorly-fed males tend to develop an unmodified third leg pair, which is morphologically similar to the other legs (the 'scrambler' phenotype) (Radwan 1995;Smallegange 2011). Fighter males are more aggressive than scrambler males (Radwan et al 2000), and can monopolize females by killing rivals (Radwan and Klimas 2001), while scramblers have a mobility advantage (Tomkins et al 2011) and can obtain higher fertilization success from a single copulation ( Van den Beuken et al 2019). Male morph expression depends on body size immediately prior to metamorphosis, with larger males becoming fighters and smaller males scramblers (Radwan 1995;Smallegange 2011), although adult fighter males are on average not larger than adult scrambler males (Smallegange et al 2012); fighters shrink during metamorphosis while scramblers do not (Radwan et al 2002).…”

The trade‐off between investment in weapons and fertility is mediated through spermatogenesis in the leaf‐footed cactus bug Narnia femorata

“…The trade-off between weapons critical for precopulatory mating success and traits critical for postcopulatory fertilization success has now been demonstrated in a number of species. Males that lack weapons tend to have increased fertilization success (Joseph et al, 2018;Somjee et al, 2018;Van den Beuken et al, 2019). Males that invest in precopulatory traits may not be able to fully invest in postcopulatory traits (Parker et al, 2013;Parker & Pizzari, 2010;Simmons et al, 2017).…”

Positive allometry of sexually selected traits: Do metabolic maintenance costs play an important role?

“…[100] In the male dimorphic bulb mite, Rhizoglyphus robini, fighter-males with exaggerated leg weapons consistently sire less offspring compared to scrambler males without these weapons regardless of different nutrient intakes during adulthood. [101] While a number of studies have revealed allocation trade-offs with the investment of weapons relative to other traits during development, [95,102] far fewer examine the ways in which energy may trade-off among traits in adults. Future studies that relate energy budgets to the behavior and reproductive outputs for individuals of different size, or sexually selected trait size have the potential to uncover yet unidentified energetic constraints that may be shaping exaggerated sexually selected traits.…”