Sex-specific fitness returns are too weak to select for non-random patterns of sex allocation in a viviparous snake

Abstract

When environmental conditions exert sex-specific selection on offspring, mothers should benefit from biasing their sex allocation towards the sex with the highest fitness in a given environment. Yet, studies show mixed support for such adaptive strategies in vertebrates, which may be due to mechanistic constraints and/or weak selection on facultative sex allocation. In an attempt to disentangle these alternatives, we quantified sex-specific fitness returns and sex allocation (sex ratio and sex-specific mass at birth) according to maternal factors (body size, age, birth date, and litter size), habitat, and year in a viviparous snake with genotypic sex determination. We used data on 106 litters from 19 years of field survey in two nearby habitats occupied by the meadow viper Vipera ursinii ursinii in south-eastern France. Maternal reproductive investment and habitat quality had no differential effects on the growth and survival of sons and daughters. Sex ratio at birth was balanced despite a slight female-biased mortality before birth. No sexual mass dimorphism between offspring was evident. Sex allocation was almost random apart for a trend towards more male-biased litters as females grew older, which could be explained by an inbreeding avoidance strategy. Thus, a weak selection for facultative sex allocation seems sufficient to explain the almost equal sex allocation in the meadow viper.

Intermittent breeding and the dynamics of resource allocation to reproduction, growth and survival

“…In habitat B, females gain less body mass during non‐breeding years, lose more body mass during breeding years, are smaller on average, and produce clutches with a smaller slope of the major axis regression of clutch mass to litter mass. As offspring mass at birth does not differ between habitats (Baron, Tully & Le Galliard ), the lower slope in habitat B either means a lower relative egg hydration, or a lower investment in egg membranes and uterine fluids. Thus, the xeric habitat B is less productive than the mesophilic habitat A, and we would predict that females from habitat B should decrease their threshold for breeding (Madsen & Shine ).…”

“…Data on survival and growth during the juvenile stage have been reported elsewhere (Baron et al . ; Baron, Tully & Le Galliard ).…”

“…On average 11 days (±7 SD) after parturition, mothers were released with their offspring at the maternal capture location. Data on survival and growth during the juvenile stage have been reported elsewhere (Baron et al 2010a;Baron, Tully & Le Galliard 2010b).…”

Micro‐geographic shift between negligible and actuarial senescence in a wild snake

“…The field site was divided into two contiguous habitats including a north‐facing and mesophilic hillside protected from human disturbance (habitat A), and a south‐facing and xerophilic hillside more exposed to human disturbance (habitat B, Supporting Information 1, Figure S1). Previous studies have shown that the meadow viper has a slow life history with continuous growth, delayed sexual maturation at the age of 3–6 years, biennial capital breeding at adulthood, and increased reproductive output with body size (Baron, Galliard, Ferrière, & Tully, 2013; Baron, Le Galliard, Tully, & Ferrière, 2010; Baron, Tully, & Le Galliard, 2010). Vipers living in the more xerophilic and disturbed habitat grow slower but invest similarly into reproduction than vipers from the mesophilic and less disturbed habitat (Baron, Tully, et al, 2010).…”

“…(2004) that species that grow indefinitely should escape senescence more frequently than those that are definite in their growth form, we predict that meadow vipers should exhibit actuarial negligible or negative senescence (prediction 2). As for other snakes, the meadow viper has indeed an indeterminate adult growth (Baron, Le Galliard, et al, 2010; Baron, Tully, et al, 2010) and its fecundity increases significantly with female body size (Baron, 1997). Third, given the reported pattern of slower body growth but similar reproductive effort for females from the sub‐optimal habitat B, we further predicted that there could be micro‐geographic variation in the pace and/or the shape of mortality (prediction 3).…”