“Why have multiple plastic responses? Interactions between color change and heat avoidance behavior in Battus philenor larvae”

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Matthew E. Nielsen and Daniel R. Papaj

Color change and heat avoidance behavior complement each other, helping caterpillars stay cool on different timescales

Red pipevine swallowtail (Battus philenor) caterpillar seeking a refuge off of its host, on a morning-glory (Ipomea ternifolia).
(Credit: Matthew Nielsen)

Pipevine swallowtail caterpillars cool themselves off in the summer by changing color when they molt from a light-absorbing black to a brighter, cooler red, but they can also cool off by leaving their short host plant to seek a refuge further above the hot ground. However, do these two responses to high temperatures work well together, or is one simply better? As part of his dissertation research, Matthew Nielsen along with his advisor Daniel Papaj at the University of Arizona addressed this question using experiments with both live caterpillars and painted models of them at the Santa Rita Experimental Range in southern Arizona. They found that not only can behavior cool caterpillars much more than color change, but once caterpillars leave their host for a refuge, color hardly affects their temperature or survival at all.

Black pipevine swallowtail (Battus philenor) caterpillar on its host plant in Arizona, Watson’s pipevine (Aristolochia watsonii).
(Credit: Daniel Papaj)

Nevertheless, color change reduces the amount of time caterpillars need to spend on refuges, which gives them more time on their host plant to eat. Thus, color change and refuge-seeking each work best on different timescales; refuge-seeking provides a strong, rapid response to daily temperature variation while color change provides a less costly response to temperature changes that persist for multiple days. Interactions between different responses to temperature have received little attention, but similar interactions to these likely occur between behavior and other temperature responses in many other animals. This research improves our understanding of how animals can combine changes in multiple traits when responding to temperature change in their environment, including climate change, and provides a new framework which can be applied to the study of responses to many different environmental changes. Read the Article

Area of the Santa Rita Experimental Range where this study was conducted. Flags mark the locations of experimental enclosures for the experiment with live caterpillars. (Credit: Matthew Nielsen)