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

Abstract

Sexual selection drives the evolution of some of the most exaggerated traits in nature.Studies on sexual selection often focus on the size of these traits relative to body size, but few focus on energetic maintenance costs of the tissues that compose them, and the ways in which these costs vary with body size. The relationships between energy use and body size have consequences that may allow large individuals to invest disproportionally more in sexually selected structures, or lead to the reduced per-gram maintenance cost of enlarged structures. Although sexually selected traits can incur energetic maintenance costs, these costs are not universally high; they are dependent on the relative mass and metabolic activity of tissues associated with them. Energetic costs of maintenance may play a pervasive yet little-explored role in shaping the relative scaling of sexually selected traits across diverse taxa. Also see the video abstract here: https://youtu.be/JyuoQIeA33Q

Exaggerated sexually selected weapons maintained with disproportionately low metabolic costs in a single species with extreme size variation

“…Our study reveals that the reduction in the relative proportion of metabolically active tissue in giraffe weevil weapons may allow them to overcome constraints of maintaining positive allometry in a weapon despite a remarkably large range in adult body size. Other traits that scale with positive allometry such as bird ornamental feathers, beetle and bovid horns, elephant tusks and cervid antlers all require specific metabolically active muscle and neural investment (Somjee, 2021). These supportive nerve and muscular tissues can be large (Larramendi et al., 2020) and may contribute significantly to daily energy budgets.…”

“…Alternatively, honest signalling may be maintained without costs being intrinsic to the weapon itself, in this case selection may act to minimize the cost of the structure while maintaining its advertising potential or functional use (Biernaskie et al., 2014; Eberhard et al., 2018; O'Brien et al., 2018; Weaver et al., 2017). If weapons themselves are not inherently costly, we would predict selection to decouple the size of the traits from its relative maintenance costs, such that large individuals pay lower energetic costs per gram to maintain disproportionally larger sexually selected structures (Somjee, 2021; Somjee et al., 2018).…”

Sexual selection leads to positive allometry but not sexual dimorphism in the expression of horn shape in the blue wildebeest, Connochaetes taurinus.

“…This is possible, but is not universal among bovids because the females of many bovid taxa are hornless (26). The probable cost of growing and maintaining such large and apparently costly traits suggests that their presence likely serves some adaptive function (10), given the regularity with which they are lost in females of related taxa, and it is possible that female horns are maintained by a combination of factors in C. taurinus.…”

“…This is revealed as a slope of greater than 1 when trait size is regressed against body size (8). This phenomenon is widely observed across the animal kingdom and is thought to arise by being an 'honest signal', because growing and maintaining proportionally larger traits is more e cient for larger individuals (9,10). Studies of secondary sexual traits in extant taxa tend to focus on the competing sex, which may impede their application to extinct taxa, where sex is often unknowable (8,11,12).…”

Sexual selection leads to positive allometry but not sexual dimorphism in the expression of horn shape in the blue wildebeest, Connochaetes taurinus

“…This is revealed as a slope of greater than 1 when trait size is regressed against body size (O’Brien et al, 2018). This phenomenon is widely observed across the animal kingdom and is thought to arise by being an ‘honest signal’, because growing and maintaining proportionally larger traits is more efficient for larger individuals (Rodríguez and Eberhard, 2019; Somjee, 2021). Studies of secondary sexual traits in extant taxa tend to focus on the competing sex, which may impede their application to extinct taxa, where sex is often unknowable (Mallon, 2017; O’Brien et al, 2018; Cooper et al, 2019).…”