Desiccation resistance in tropical insects: causes and mechanisms underlying variability in a Panama ant community

Abstract

Desiccation resistance, the ability of an organism to reduce water loss, is an essential trait in arid habitats. Drought frequency in tropical regions is predicted to increase with climate change, and small ectotherms are often under a strong desiccation risk. We tested hypotheses regarding the underexplored desiccation potential of tropical insects. We measured desiccation resistance in 82 ant species from a Panama rainforest by recording the time ants can survive desiccation stress. Species' desiccation resistance ranged from 0.7 h to 97.9 h. We tested the desiccation adaptation hypothesis, which predicts higher desiccation resistance in habitats with higher vapor pressure deficit (VPD) - the drying power of the air. In a Panama rainforest, canopy microclimates averaged a VPD of 0.43 kPa, compared to a VPD of 0.05 kPa in the understory. Canopy ants averaged desiccation resistances 2.8 times higher than the understory ants. We tested a number of mechanisms to account for desiccation resistance. Smaller insects should desiccate faster given their higher surface area to volume ratio. Desiccation resistance increased with ant mass, and canopy ants averaged 16% heavier than the understory ants. A second way to increase desiccation resistance is to carry more water. Water content was on average 2.5% higher in canopy ants, but total water content was not a good predictor of ant desiccation resistance or critical thermal maximum (CT max), a measure of an ant's thermal tolerance. In canopy ants, desiccation resistance and CT max were inversely related, suggesting a tradeoff, while the two were positively correlated in understory ants. This is the first community level test of desiccation adaptation hypothesis in tropical insects. Tropical forests do contain desiccation-resistant species, and while we cannot predict those simply based on their body size, high levels of desiccation resistance are always associated with the tropical canopy.

Keywords: Body size; CTmax; VPD; canopy; thermal tolerance; water content; water loss.

Figure 1

Worker of Cephalotes atratus, in a Dipteryx panamensis canopy next to the data loggers used for measuring temperature and relative humidity. Cephalotes atratus was the second largest canopy ant we tested.

Figure 2

Log₁₀ of lethal time (h) at which 50% of workers lost their muscle coordination (LT ₅₀), after they have been exposed to air (white) and desiccant (gray). The box and whisker plots are showing median of log₁₀ LT ₅₀, upper and lower quartiles, as well as the maximum values and outliers.

Figure 3

Relationship between species desiccation resistance (LT ₅₀) and log₁₀ body mass (mg) in canopy and understory ants. Both linear models for this relationship differ significantly from a slope of 0 (see text for details): canopy – gray line, understory – black line.

Figure 4

(A) Total water content (%) of canopy and ground nesting ants. (B) Total water loss (%) in canopy and ground nesting ants. The box and whisker plots are showing median of % water content (A) and % total water loss (B), upper and lower quartiles, as well as the maximum values and outliers.

Figure 5

Relationship between desiccation resistance (LT ₅₀) and critical thermal maximum (CT _max) in canopy and understory ants.