No evidence of male-biased sexual selection in a snake with conventional Darwinian sex roles

Abstract

Decades of research on sexual selection have demonstrated that ‘conventional’ Darwinian sex roles are common in species with anisogamous gametes, with those species often exhibiting male-biased sexual selection. Yet, mating system characteristics such as long-term sperm storage and polyandry have the capacity to disrupt this pattern. Here, these ideas were explored by quantifying sexual selection metrics for the western diamond-backed rattlesnake ( Crotalus atrox ). A significant standardized sexual selection gradient was not found for males ( β SS = 0.588, p = 0.199) or females ( β SS = 0.151, p = 0.664), and opportunities for sexual selection ( I s ) and selection ( I ) did not differ between males ( I s = 0.069, I = 0.360) and females ( I s = 0.284, I = 0.424; both p > 0.05). Furthermore, the sexes did not differ in the maximum intensity of precopulatory sexual selection (males: s′ max = 0.155, females: s′ max = 0.080; p > 0.05). Finally, there was no evidence that male snout–vent length, a trait associated with mating advantage, is a target of sexual selection ( p > 0.05). These results suggest a lack of male-biased sexual selection in this population. Mating system characteristics that could erode male-biased sexual selection, despite the presence of conventional Darwinian sex roles, are discussed.

Exceptional long-term sperm storage by a female vertebrate

“…atrox , while the species exhibits characteristics that could promote male-biased sexual selection (e.g., male-male combat for access to females, male-biased sex ratio, and male-biased sexual size dimorphism), Levine et al [ 30 ] found no such pattern. In the population studied by Levine et al [ 30 ], high instances of multiple paternity were previously reported [ 33 ]. Furthermore, of 12 females for which multiple paternity litters were identified, 10 were radio-tracked over the mating season with both courtship and mating behaviors reported.…”

“…In snakes, while there is limited evidence that sperm stored over multiple reproductive seasons can compete with spermatozoa from recent inseminations [ 1 , 12 , 24 , 32 , 55 ], general models show that overlapping ejaculates resulting in multiple paternity can erode the strength of post-copulatory sexual selection in males [ 1 , 25 , 54 ]. In studies investigating post-mating sexual selection, a primary aim is to measure the opportunity for sexual selection [ 25 , 54 ], or selection intensity acting directly on mate numbers using Bateman gradients or traits such as body size [ 28 – 30 , 33 , 54 , 56 , 57 ]. These types of evolutionary analyses would clearly profit from a broader understanding of LTSS in populations given the eroding effect it may have on sexual selection [ 24 , 30 ].…”

“…In studies investigating post-mating sexual selection, a primary aim is to measure the opportunity for sexual selection [ 25 , 54 ], or selection intensity acting directly on mate numbers using Bateman gradients or traits such as body size [ 28 – 30 , 33 , 54 , 56 , 57 ]. These types of evolutionary analyses would clearly profit from a broader understanding of LTSS in populations given the eroding effect it may have on sexual selection [ 24 , 30 ].…”

“…The capacity for, and duration of, LTSS has important implications for mating systems and their analysis [ 1 , 2 , 25 – 27 ]. This is particularly important when considering male reproductive success and the opportunity for sexual selection [ 25 , 28 – 30 ]. Accordingly, polygyny-polyandry and multiple paternity have the potential to be strongly influenced by LTSS [ 22 , 24 , 31 – 33 ], and its impact on post-copulatory sexual selection, including sperm competition [ 23 , 34 , 35 ] and cryptic female choice [ 36 , 37 ], should be considered when interpreting results.…”

Fission–fusion dynamics in the social networks of a North American pitviper

“…At capture, Global Positioning System (GPS) coordinates were obtained, and subjects were measured (snout-vent length, tail length, head dimensions to the nearest millimeter; body mass to the nearest 1.0 g) and sex confirmed (via probing) while under light anesthesia (isoflurane). Individuals were photographed, implanted with a unique passive integrated transponder (PIT) tag (AVID, Inc.), and their proximal rattle segments were colored via (Amarello et al, 2010;Greene et al, 2002;Levine et al, 2020Levine et al, , 2021Repp, 1998;Schuett, Clark, et al, 2016;. Shed cycle refers to skin shedding (ecdysis).…”