Heat-Induced Hatching of Red-Eyed Treefrog Embryos: Hydration and Clutch Structure Increase Behavioral Thermal Tolerance

Abstract

Climate change is increasing both environmental temperatures and droughts. Many ectotherms respond behaviorally to heat, thereby avoiding damage from extreme temperatures. Within species, thermal tolerance varies with factors such as hydration as well as ontogenetic stage. Many tropical anurans lay terrestrial eggs, relying on environmental moisture for embryonic development. These eggs are vulnerable to dehydration, and embryos of some species can hatch prematurely to escape from drying eggs. Warmer temperatures can accelerate development, and thus hatching, but excess heat can kill embryos. Thus, we hypothesize that embryos may show a behavioral thermal tolerance limit, hatching prematurely to avoid potentially lethal warming. If so, because warming and drying are often associated, we hypothesize this limit, measurable as a voluntary thermal maximum, may depend on hydration. We manipulated hydration of the terrestrial eggs of Agalychnis callidryas, in intact clutches and egg-groups isolated from clutch jelly, then warmed them to assess if embryos hatch early as a behavioral response to high temperatures and whether their thermal tolerance varies with hydration or surrounding structure. We discovered that heating induces hatching; these embryos show a behavioral escape-hatching response that enables them to avoid potentially lethal warming. Hydrated eggs and clutches lost more water and warmed more slowly than dehydrated ones, indicating that hydration buffers embryos from environmental warming via evaporative cooling. Embryos in hydrated clutches tolerated greater warming before hatching and suffered higher mortality, suggesting their behavioral thermal safety margin is small. In contrast lower thermal tolerance protected dry embryos, and those isolated from clutch jelly, from lethal warming. Heat-induced hatching offers a convenient behavioral assay for thermal tolerance of terrestrial anuran embryos and the interactive effects of warming and dehydration at an early life stage. This work expands the set of threats against which embryos use hatching in self-defense, creating new opportunities for comparative studies of thermal tolerance as well as integrative studies of self-defense mechanisms at the egg stage.

Sensitivity of amphibian embryos to timing and magnitude of present and future thermal extremes

“…Although the impact of temperature extremes on ectotherm embryos has been extensively studied, our knowledge is largely biased toward thermal tolerance of embryos from thermally challenging habitats (Angilletta et al, 2013; Guevara‐Molina et al, 2022; Hall & Warner, 2019). In this study, we focused on amphibian embryos from relatively cold aquatic habitats and revealed that thermal extreme events have various nonlethal effects on embryonic life histories and larval behavior (see also Liu et al, 2022).…”

Elevated ammonia cues hatching in red‐eyed treefrogs: A mechanism for escape from drying eggs

“…The eggs are deposited on plants above water (Duellman, 1970) and, without disturbance, hatching occurs after 6–7 days in our Gamboa, Panama, study population (Warkentin, 2005). However, A. callidryas embryos can hatch early and escape in response to multiple natural risks, including attacks by egg‐eating snakes and wasps (Warkentin, 1995, 2000), infection by a fungal pathogen (Warkentin et al, 2001), flooding (Warkentin, 2002), dehydration (Salica et al, 2017), and dangerously high temperatures (Guevara‐Molina et al, 2022). Previous research has documented that physical disturbance cues, sensed by the inner ear and lateral line mechanoreceptors, mediate predator‐induced hatching (Jung et al, 2019, 2020; Warkentin, 2005) and that hypoxia cues flooding‐induced hatching (Warkentin, 2002).…”

“…2-6.8, N = 7 clutches;Cohen et al, 2019). In contrast, drying eggs reached only 4.0 ± 0.34 mm(3.3-4.8, N = 15 clutches) at 2 days, then shrank to 3.7 ± 0.36 mm (2.9-4.4 mm) by 4 days, when induced hatching began(Salica et al, 2017) Guevara-Molina et al (2022). report even greater variation in egg size with (de)hydration, ranging from 2.8 to 7.5 mm diameter, or 11-218 µL volume.…”

“…This variation in volume as eggs gain and lose water must change the concentration of all solutes in the perivitelline fluid; thus embryos may sense and use the concentration of additional solutes for escape-hatching decisions, or perhaps even general osmolality. Moreover, the shrinking egg volume physically constrains embryos' positions and, at lower volumes, even their movements in ovo(Tippet & Warkentin, 2017); these spatial constraints may also contribute to hatching decisions in drying eggs.Dry weather and warm temperatures act synergically to increase water loss by evaporation, increasing the risk of embryo mortality, and even without drying, A. callidryas embryos hatch in response to increasing temperatures(Guevara-Molina et al, 2022). However, embryos in well-hydrated eggs tolerate higher temperatures than do embryos in drying eggs, which hatch at lower temperatures(Guevara- Molina et al, 2022).…”

“…Moreover, the shrinking egg volume physically constrains embryos' positions and, at lower volumes, even their movements in ovo(Tippet & Warkentin, 2017); these spatial constraints may also contribute to hatching decisions in drying eggs.Dry weather and warm temperatures act synergically to increase water loss by evaporation, increasing the risk of embryo mortality, and even without drying, A. callidryas embryos hatch in response to increasing temperatures(Guevara-Molina et al, 2022). However, embryos in well-hydrated eggs tolerate higher temperatures than do embryos in drying eggs, which hatch at lower temperatures(Guevara- Molina et al, 2022). We do not know how these embryos sense either temperature or ammonia, but their clear behavioral responses to these cues would facilitate testing of candidate mechanisms(Saito…”