Disentangling sex allocation in a viviparous reptile with temperature‐dependent sex determination: a multifactorial approach

Abstract

Females are predicted to alter sex allocation when ecological, physiological and behavioural variables have different consequences on the fitness of male and female offspring. Traditionally, tests of sex allocation have examined single causative factors, often ignoring possible interactions between multiple factors. Here, we used a multifactorial approach to examine sex allocation in the viviparous skink, Niveoscincus ocellatus. We integrated a 16-year observational field study with a manipulative laboratory experiment to explore whether the effects of the maternal thermal environment interact with the resources available to females for reproduction to affect sex allocation decisions. We found strong effects of temperature on sex allocation in the field, with females born in warm conditions and males in cold conditions; however, this was not replicated in the laboratory. In contrast, we found no effect of female resource availability on sex allocation, either independently, or in interaction with temperature. These results corresponded with an overall lack of an effect of resource availability on any of the life history traits that we predicted would mediate the benefits of differential sex allocation in this system, suggesting that selection for sex allocation in response to resource availability may be relatively weak. Combined, these results suggest that temperature may be the predominant factor driving sex allocation in this system.

Degrees of change: between and within population variation in thermal reaction norms of phenology in a viviparous lizard

“…It is unlikely that other environmental factors contributed to this effect, as previous research has demonstrated that such factors, including maternal condition at ovulation, and food availability and quality during gestation, do not affect parturition date in this species. (Cadby et al 2011, Gruber et al 2018.…”

“…To explore population‐mean–level response of parturition dates to temperature, we first fit a linear mixed model (LMM) with mean cohort parturition date as a response variable. We have previously shown that factors other than temperature, including environmental variables (humidity, cloud cover, and wind speed; Cadby et al 2010), food quality during gestation (Cadby et al 2011), females’ condition at ovulation and food availability during gestation and their interactions with temperature (Gruber et al 2018) do not affect birth dates in N. ocellatus . We, therefore, included the site‐specific mean maximum daytime temperature during the gestation period (October 1–December 31 and October 15–January 15 for the lowland and highland sites, respectively (Pen et al 2010).…”

“…Both sites are surrounded by either unsuitable habitat or road and there is, consequently, little immigration or emigration in either population (Cadby et al 2010, Uller et al 2011, Cunningham et al 2018). Each year the majority (90–95%) of pregnant females were captured late in the gestation period but prior to birth (mid‐December and mid‐January for the lowlands and highlands, respectively, because of differences in the timing of birth; Cadby et al 2010, Uller et al 2011, Gruber et al 2018). Females were identified using existing toe clips, and their capture location was recorded (±5 m).…”

Sex reversal explains some, but not all, climate-mediated sex ratio variation within a viviparous reptile

“…By contrast, in a low-elevation population, sex ratios correlate with temperature [22]; sex ratios are female-biased in warm seasons/developmental conditions and male-biased in cool seasons/developmental conditions [26,27]. The populationspecific sex ratio response to temperature observed in C. ocellatus in the wild has been replicated by manipulating maternal thermal opportunity in the laboratory [22,[26][27][28][29][30]. Low-elevation females subjected to cooler short basking treatments (e.g.…”

“…10 h) produce female-biased cohorts [29,30]. By contrast, highelevation females subjected to the same thermal regimes produce sex parity cohorts [22,[26][27][28][29][30]. An adaptive explanation for these patterns is that the production of males or females is favoured at different temperatures at low elevation because the concomitant variation in birth date has sex-specific fitness consequences.…”