The hidden cost of sexually selected traits: the metabolic expense of maintaining a sexually selected weapon

Abstract

Sexually selected weapons are among the most exaggerated traits in nature. Sexual selection theory frequently assumes a high cost of this exaggeration; yet, those costs are rarely measured. We know very little about the energetic resources required to maintain these traits at rest and the difference in energetic costs for the largest individuals relative to the smallest individuals. Knowledge in this area is crucial; resting metabolic rate can account for 30–40% of daily energy expenditure in wild animals. Here, we capitalized on the phenomenon of autotomy to take a unique look at weapon maintenance costs. Using Leptoscelis tricolor (Hemiptera: Coreidae), we measured CO 2 production rates before and after a weapon was shed. Males in this insect species use enlarged hind femora as weapons in male–male combat, and yet can shed them readily, without regeneration, upon entrapment. We found that metabolic rate decreased by an average of 23.5% in males after leg loss and by 7.9% in females. Notably, larger males had less of a drop in metabolic rate per gram of weapon lost. Our findings suggest that sexually selected weapons contribute to a large portion of resting metabolic rate in males, but these costs do not scale in direct proportion to size; larger males can have larger weapons for a reduced metabolic cost. These energetic maintenance costs may be integral to the evolution of the allometries of sexually selected weapons, and yet they remain largely unexplored.

Harder, better, faster, stronger: Weapon size is more sexually dimorphic than weapon biomechanical components in two freshwater anomuran species

“…As already noted in recent papers (Eberhard et al, 2018;McCullough et al, 2016;Palaoro, Peixoto, Benso-Lopes, Boligon, & Santos, 2020;Somjee et al, 2018), more information on how weapons are used, the components that build them, the selective pressures that mould them, and the developmental pathways taken during growth are crucial for understanding animal weapons. Thus, more information F I G U R E 4 Summary of how claw shape is distributed on the first and second principal components analysis.…”

“…First, there are metabolic costs of producing and maintaining metabolic active tissue (Somjee, Woods, Duell, & Miller, 2018). Since resources are limited, allocating them to weapons may trade-off with other fitness-enhancing traits (Zera & Harshman, 2001).…”

Performance, but not size, of hindleg weaponry is sexually dimorphic in the giant mesquite bug (Thasus neocalifornicus)

“…Such increased variation in males is consistent with the hypothesis that squeezing forces and muscle allocation exhibits greater condition dependent in males but not females. Generally, muscle tissue is costly to produce and maintain, and only animals with sufficient resources can invest greatly into greater muscle mass (Somjee, Woods, Duell, & Miller, 2018;Bywater, Seebacher, & Wilson, 2015;Bywater, White, & Wilson, 2014;O'Brien et al, 2019) . Overall, by measuring the variation of sexually dimorphic weapon components, we gain a deeper insight into the functional and selection pressures that have acting on such traits.…”

Variation in movement: multiple locomotor gaits in Neotropical harvestmen

“…The gait types we identified here are only one of many aspects that must be considered to gain a full understanding of the energetics of locomotion in animals. Further work can address this by studying metabolism (for instance, oxygen consumption; Schmitz, 2005;Fleming & Bateman, 2007;Somjee et al, 2018), measuring ground reaction forces of single legs (Jindrich & Full, 1999;Reinhardt & Blickhan, 2014) and modelling energy efficiency across gaits.…”