Thermal sensitivity of lizard embryos indicates a mismatch between oxygen supply and demand at near-lethal temperatures

Abstract

<jats:title>ABSTRACT</jats:title><jats:p>Aspects of global change (e.g. urbanization, climate change) result in novel, stressful thermal environments that threaten biodiversity. Though much research quantifies the thermal sensitivity of adult organisms, effects of global change on developing offspring (e.g. embryos) are also important. Oviparous, non-avian reptiles have received considerable attention because eggs are left to develop under prevailing environmental conditions, making them vulnerable to increases in ambient temperature. Though many studies assess embryo thermal tolerance and physiology in response to long-term (i.e. chronic), constant incubation temperatures, fewer assess responses to acute exposures which are more ecologically relevant for many species. We subjected eggs of the brown anole lizard (<jats:italic>Anolis sagrei</jats:italic>) to heat shocks, thermal ramps, and extreme diurnal fluctuations to determine the lethal temperature of embryos, measure the thermal sensitivity of embryo heart rate and metabolism, and quantify the effects of sub-lethal but stressful temperatures on embryo development and hatchling phenotypes and survival. Most embryos died at heat shocks of 45 or 46 °C, which is ∼12 °C warmer than the highest constant temperatures suitable for development. Heart rate and O<jats:sub>2</jats:sub> consumption increased with temperature; however, as embryos approached the lethal temperature, heart rate and CO<jats:sub>2</jats:sub> production continued rising while O<jats:sub>2</jats:sub> consumption plateaued. These data indicate a mismatch between oxygen supply and demand at high temperatures. Exposure to extreme, diurnal temperature fluctuations depressed embryo developmental rates and heart rates, and resulted in hatchlings with smaller body size, reduced growth rates, and lower survival in the laboratory. Thus, even brief exposure to extreme temperatures can have important effects on embryo development, and our study highlights the role of both immediate and cumulative effects of high temperatures on egg survival. Such effects must be considered to predict how populations will respond to global change.</jats:p>

The thermal ecology and physiology of reptiles and amphibians: A user's guide

“…Importantly, the difference in critical temperatures obtained via chronic versus acute exposure to thermal stress can be dramatic. For example, Hall and Warner (2020) found that the lethal temperature differed by 12°C for squamate embryos depending on whether acute or chronic methods were used. Understanding both chronic and acute thermal tolerance is likely more important for embryos than for adults due to the inability of embryos to behaviorally thermoregulate in most circumstances.…”

“…A similar method can be used to assess cold tolerance by cooling eggs (see Levy et al, 2015). Second, the monitor has been used to assess embryo survival after exposure to brief thermal extremes (e.g., thermal fluctuations, Smith et al, 2015; heat shocks, Hall & Warner, 2020). There are some important considerations when using the Buddy.…”

“…When designing future experiments, care should be taken to expose animals to naturalistic thermal variation. Constant temperatures may be appropriate for testing acute effects of temperature during the activity period, but more chronic experiments need to utilize variable temperature regimes to provide biological rather than pharmacological measurements (Carter et al, 2018; Fabrício‐Neto, Gavira, & Andrade, 2019; Georges et al, 2005; Hall & Warner, 2020). Historically, most experiments used constant temperatures due to technological constraints (While et al, 2018).…”

“…For example, many genes in reptiles and amphibians are as yet undescribed, and the physiological mechanisms underlying thermal performance, tolerance, and behavior are still largely unknown (Campbell‐Staton et al, 2017, 2020; Garcia‐Porta et al, 2019). For example, the mechanism(s) setting absolute thermal tolerances in reptiles and amphibians is hotly debated, with some data supporting failure of subcellular mechanisms (e.g., classical and marginal stability hypotheses, Hochachka & Somero, 2002), some supporting failure of organ systems such as the cardiovascular and respiratory system (oxygen and capacity limited thermal tolerance hypothesis; Hall & Warner, 2020; Pörtner, 2001; Smith et al, 2015), and some data suggesting a combination of these mechanisms (e.g., hierarchical mechanisms of thermal limitation, for a full review see Gangloff & Telemeco, 2018). Thus, much work is needed to provide a more mechanistic understanding of why reptiles and amphibians display the thermal preferences, performances, and tolerances observed.…”

Heat tolerance of reptile embryos: Current knowledge, methodological considerations, and future directions

“…However, we propose that when researchers measure CT max using acute exposures to temperature, they are measuring a different phenotype than assessing heat tolerance with constant temperatures. Although each assay assesses embryo survival, the mechanisms resulting in death may differ: Acute heat tolerance may result from cardiac arrest or oxygen limitation (Angilletta et al, 2013; Hall & Warner, 2020; Smith et al, 2015), while chronic heat stress results in morphological abnormalities (Sanger et al, 2018), or depletion of energy stores (e.g., Sun, Li, Gao, Ma, & Du, 2015; Yang, Niu, & Sun, 2002).…”