Overview
Ants are abundant in desiccating environments despite their high surface area to volume ratios and exposure to harsh conditions outside the nest. Red harvester ant (Pogonomyrmex barbatus) colonies must spend water to obtain water: colonies lose water as workers forage outside the nest, and gain water metabolically through seeds collected in foraging trips. Here we present field experiments showing that hydrated P. barbatus foragers made more foraging trips than unhydrated nestmates. The positive effect of hydration on foraging activity is stronger as the risk of desiccation increases. Desiccation tests showed that foragers of colonies that reduce foraging in dry conditions are more sensitive to water loss, losing water and motor coordination more rapidly in desiccating conditions, than foragers of colonies that do not reduce foraging in dry conditions. Desiccation tolerance is also associated with colony reproductive success. Surprisingly, foragers that are more sensitive to water loss are from colonies more likely to produce offspring colonies. This could be because the foragers of these colonies conserve water with a more cautious response to desiccation risk. An ant’s hydration status may influence its response to the olfactory interactions that regulate its decision to leave the nest to forage. Thus variation among ant colonies in worker physiology and response to ambient conditions may contribute to ecologically significant differences among colonies in collective behavior.
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Methods and contributions
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Findings and contributions
- Pooled across five colonies, hydrated ants made 1.18-fold more foraging trips than unhydrated nestmates (paired Wilcoxon p = 0.0006).
- The hydration effect on foraging grew with desiccation risk (VPD), with a quadratic model fitting better than linear, supporting a non-linear effect.
- Foragers from colonies that reduce foraging in dry conditions lost water faster and reached morbidity and death sooner, without differing in total or critical water content.
- Surprisingly, colonies with offspring colonies had foragers with lower desiccation tolerance, which the authors suggest may reflect more cautious, water-conserving foraging.
- The authors infer that an ant's threshold response to olfactory interactions that trigger foraging is influenced by its current hydration level.
Methods
- Field hydration manipulation of marked P. barbatus foragers — Foragers from five colonies were cold-anesthetized, paint-marked by treatment, given 0.2 μL water or none, returned to the nest, and their next-day foraging trips counted.
- Vapor Pressure Deficit regression of hydrated/unhydrated trip ratio — Field temperature and humidity (iButton) were combined into VPD and regressed (linear vs quadratic, AIC/ANOVA) against the ratio of hydrated to unhydrated trips.
- Laboratory desiccation assays on 74 foragers from 24 colonies — Ants were held over Drierite at about 40 °C and weighed every 20 minutes, recording time to morbidity and death, blinded, to derive water loss rates and water contents.
- Colony grouping by offspring success and foraging regulation — Colonies were classified by prior parentage-analysis offspring status and by whether they reduce foraging in dry conditions, then compared with Wilcoxon and Chi-square tests.
- PCA of five water-loss physiology measures — Principal Component Analysis (factoextra in R) on water contents, times to morbidity and death, and area-independent water loss rate.
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