Female investment in reproduction is an energetic expensive component of their life history (Sibly et al., 2012; Healy et al 2019), and within and between egg-laying species, there is substantial variation in reproductive output such as clutch size, clutch numbers, and clutch intervals (Roff, 2002; Shine and Greer, 1991; Pincheira-Donoso and Hunt, 2017; Meiri et al., 2020; Bansal & Thaker, 2021; Fisher et al., 2021; Pincheira-Donoso et al., 2021). Clutch size is the most common reproductive output trait measured and linked to maternal body size, resource availability, and environmental conditions (Lack, 1947; Jetz et al., 2008; Pincheira-Donoso and Hunt, 2017; Meiri et al., 2020; Caracalas et al., 2021). For example, clutch size among egg-laying squamates positively correlates with maternal traits such as body size, age, and growth rate (Tinkle et al., 1970; Pincheira-Donoso and Tregenza, 2011; Scharf et al., 2015; Liang et al., 2021). However, size-dependent clutch size is often constrained and dependent by other reproductive parameters where larger eggs or multiple clutches can result in smaller clutch size (e.g., Shine and Greer, 1991; King, 2000; Shine and Greer, 1991; Werneck et al., 2009; Siliceo and Diaz, 2010; Meiri et al., 2012; Slavenko et al., 2015; Meiri, 2018).
Environmental variation can also impact reproductive investment for ectotherms that depend on the environment to regulate their body temperature (Shine, 2005; Parmesan, 2006; Meiri et al, 2013; Meiri, 2018). For example, ectotherms raised under warmer temperatures often result in smaller body sizes which then leads to lower reproductive output (Fitch, 1970; Gibbons and McCarthy, 1986; Atkinson, 1994; Barneche et al., 2018; Meiri, 2018; Wu et al., 2022). In macroecology studies, variations in climate at large scales are often represented by geographic latitude (Boyer et al., 2010; Griebeler et al., 2010; Laiolo and Obeso 2015). At higher latitude, the climate is generally warmer and consistent throughout the year and becomes colder and more variable at lower latitude. Studies have shown patterns in clutch size and latitude, where clutch size increases at higher latitude among reptile species (e.g., Cody, 1966; Lack, 1947; Tinkle et al., 1970; Fitch, 1985; Iverson et al., 1993; Boyer et al., 2010). For example, lizards at high latitudes lay large clutch sizes primarily by the highly seasonal environment, favouring suitable breeding periods (Cody, 1966; Fitch, 1985). Therefore, the variation in the degree of seasonality may influence clutch size patterns across lizard species (Tinkle et al., 1970; Hao et al., 2006; Meiri et al., 2013; Mesquita et al., 2016; Meiri et al., 2020).
However, there is still substantial variation in clutch size within a similar latitude range (Fitch, 1970; Fitch, 1985; Du et al., 2014; Laiolo and Obeso 2015). It is likely that local scale variation in climates contributes to these clutch size variations. Altitudinal gradients have been incorporated as an alternative proxy to latitudinal patterns in life-history, especially to explain variation within broad wide-ranging lizard species (Griebeler et al., 2010; Roitberg et al., 2013). Unlike the latitudinal gradients in environmental conditions at the macroscale, altitudinal gradients can have greater impact life-history traits at local scales due to synergetic interactions of decreasing air temperature, oxygen levels, and greater variation in weather conditions such as sudden snowstorms and short summers (Fitch, 1985; Morrison and Hero, 2003; Beall, 2014; Hille and Cooper, 2015; Lack et al., 2016). Since life history patterns such as reproduction depend on the environment for ectotherms, the unique climate at high altitude is expected to lead to differentiation of life-history strategies that may impose evolutionary constraints on reproduction (Lack, 1947; Laiolo and Obeso 2015).
Understanding the impact of the environment on reproductive investment has mainly been focused on global patterns and annual averages in environmental conditions (Araújo et al., 2006; Hille and Cooper, 2015; Laiolo and Obeso 2015; Meiri et al., 2020). Local scale studies that examine environmental variation can provide more direct mechanistic explanations of clutch size diversity. This is because maternal investment is directly influenced by resource availability (Vitt and Congdon, 1978; Griebeler et al., 2010; Bansal and Thaker, 2021). In energetic models, greater accumulation of food prior to breeding provides females with more energy resources for reproduction (Forseth et al., 1994; Lika and Kooijman, 2003). At high altitude, food resource is generally scarce and maybe the primary limiting factor for reproductive investment. Additionally, unpredictable weather conditions at high altitudes such as high seasonal temperature and precipitation variability can reduce the window of opportunity to forage (Fitch, 1970; Urban et al., 2014; Burner et al., 2020; Anderson et al., 2022), limiting an individual’s ability to accumulate sufficient energetic resources for egg production (Tinkle et al., 1970; Vitt and Congdon, 1978). Such trends in reproductive investment and environmental variability have been observed at global scales (Andrew et al. 2013; Crozier and Hutchings, 2014; Urban et al., 2014), however, this pattern is not consistent across species (Lyon et al., 2008). Therefore, the need for intraspecific studies at local levels can provide a mechanistic link for within-species variation in clutch size diversity and global patterns in clutch size diversity. If the prediction on clutch diversity at high altitudes holds true at an intraspecific level, populations at high altitudes are expected to lay smaller clutches relative to populations closer to sea level.
In this study, we examined the relationship between clutch size and environmental variables from ten populations of a wide-ranging lizard, Eremias argus from China. E. argus is a small (~70 mm snout‐vent length [SVL]; Zhao et al., 1999), an oviparous lizard that lays several clutches of two to five parchment-shelled eggs per clutch. This species occurs throughout China and other Asian countries across altitudinal gradients between 0–2,900 m above sea level (Zhao et al., 1999), providing an excellent model system to examine the influence of altitude climates on the clutch size within species. We hypothesized that clutch size will: (i) positively correlate with maternal body mass, and (ii) will be lower at higher elevations due to lower temperatures and precipitation and greater climate variability. Understanding the extent to which these hypotheses hold for clutch size diversity within species allows inference of whether the relationship between clutch size with geographic distribution conforms to the global patterns across taxa as seen in Meiri et al. (2020).