Abstract
Tropical forests experience a relatively stable climate, but are not thermally uniform. The tropical forest canopy is hotter and thermally more variable than the understory. Heat stress in the canopy is expected to increase with global warming, potentially threatening its inhabitants. Here, we assess the impact of heating on the most abundant tropical canopy arthropods—ants. While foragers can escape hot branches, brood and workers inside twig nests might be unable to avoid heat stress. We examined nest choice and absconding behavior—nest evacuation in response to heat stress—of four common twig-nesting ant genera. We found that genera nesting almost exclusively in the canopy occupy smaller cavities compared to Camponotus and Crematogaster that nest across all forest strata. Crematogaster ants absconded at the lowest temperatures in heating experiments with both natural and artificial nests. Cephalotes workers were overall less likely to abscond from their nests. This is the first test of behavioral thermoregulation in tropical forest canopy ants, and it highlights different strategies and sensitivities to heat stress. Behavioral avoidance is the first line of defense against heat stress and will be crucial for small ectotherms facing increasing regional and local temperatures.
Highlights
Tropical forests are biodiversity hotspots characterized by a relatively stable climate (Janzen 1967; Ghalambor et al 2006), but they are not thermally uniform
We focused on four main questions: (1) How do the physical characteristics of twig nests shape their thermal properties and occupancy by focal ant genera?; (2) Are certain genera more likely to abscond in response to heat stress?; (3) Does thermal tolerance predict absconding temperature?; and (4) Does the presence of brood inside the nest promote absconding behavior? We experimentally heated natural and artificial twigs to quantify the behavioral responses of cavity-nesting ants to heat stress
Ectotherms like ants will increasingly depend on behavioral thermoregulation to avoid heat stress, as many are already living at their physiological thermal limits (Sunday et al 2014)
Summary
Tropical forests are biodiversity hotspots characterized by a relatively stable climate (Janzen 1967; Ghalambor et al 2006), but they are not thermally uniform. Surface temperatures of branches in lowland tropical forests can exceed 50 °C (Kaspari et al 2015; Stark et al 2017). These thermal extremes likely are challenging for ectotherms whose metabolism and fitness are governed by local environmental temperatures (Deutsch et al 2008; Angilletta 2009). Changes in microclimate caused by deforestation reduce the abundance of tropical groundnesting ants, selecting for more thermally tolerant genera in disturbed, hotter habitats (Boyle et al 2020). Such changes in abundance and community composition are concerning, because ants are ubiquitous and provide a variety of important ecosystem functions (Folgarait 1998; Philpott and Armbrecht 2006; Clay et al 2013). We do not know how increased temperatures will affect canopy ants, or the importance of behavioral mechanisms used by arboreal ants to avoid thermal stress
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