Negative frequency-dependent selection (NFDS) has been shown to maintain polymorphism in a diverse array of traits. The actionof NFDS has been confirmed through modeling, experimental approaches, and genetic analyses. In this study, we investigated NFDS in the wild using morph-frequency changes spanning a 20-year period from over 30 dimorphic populations of Datura wrightii. In these populations, plants either possess glandular (sticky) or non-glandular (velvety) trichomes, and the ratio of these morphs varies substantially among populations. Our method provided evidence that NFDS, rather than drift or migration, is the primary force maintaining this dimorphism. Most populations that were initially dimorphic remained dimorphic, and the overall mean and variance in morph frequency did not change over time. Furthermore, morph-frequency differences were not related to geographic distances. Together, these results indicate that neither directional selection, drift, or migration played a substantial role in determining morph frequencies. However, as predicted by negative frequency-dependent selection, we found that the rare morph tended to increase in frequency, leading to a negative relationship between the change in the frequency of the sticky morph and its initial frequency. In addition, we found that morph-frequency change over time was significantly correlated with the damage inflicted by two herbivores: Lema daturaphila and Tupiochoris notatus. The latter is a specialist on the sticky morph and damage by this herbivore was greatest when the sticky morph was common. The reverse was true for L. daturaphila, such that damage increased with the frequency of the velvety morph. These findings suggest that these herbivores contribute to balancing selection on the observed trichome dimorphism. K E Y W O R D S : balanced polymorphism, Datura wrightii, glandular trichomes, plant-herbivore interactions. Impact SummaryWe present a long-term observational study of morph frequency changes in naturally dimorphic populations of Datura wrightii. We were able to determine that negative frequency-dependent selection-rather than drift, migration, or directional selection-is the main contributor to the main-tenance of this dimorphism over the past 20 years. We also sampled herbivory across our sample of populations and found evidence suggesting that the damage inflicted by two species of specialist herbivores may underlie this selective regime.Negative frequency-dependent selection (NFDS)-a selective regime in which rare morphs are favored over common 8 3

Many lizards are olfactory foragers and prey upon herbivorous arthropods, yet their responses to common herbivore‐associated plant volatiles remain unknown. As such, their role in mediating plant indirect defenses also remains largely obscured. In this paper, we use a cotton‐swab odor presentation assay to ask whether lizards respond to two arthropod‐associated plant‐derived volatile compounds: 2‐( E )‐hexenal and hexanoic acid. We studied the response of two lizard species, Sceloporus virgatus and Aspidoscelis exsanguis , because they differ substantially in their foraging behavior. We found that the actively foraging A. exsanguis responded strongly to hexanoic acid, whereas the ambush foraging S. virgatus responded to 2‐( E )‐hexenal—an herbivore‐associated plant volatile involved in indirect defense against herbivores. These findings indicate that S. virgatus may contribute to plant indirect defense and that a species' response to specific odorants is linked with foraging mode. Future studies can elucidate how lizards use various compounds to locate prey and how these responses impact plant‐herbivore interactions.

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Cuticular hydrocarbons (CHCs) present on the surface of arthropods are important mediators of both intraspecific and interspecific interactions. They are known to be important chemical cues that help predatory arthropods locate prey, yet less is known about if and how vertebrate insectivores use these nearly ubiquitous chemical cues. We examined behavioral responses of insectivorous lizards (Aspidoscelis exsanguis) to the CHCs of three different species of arthropod prey (crickets, ants, and spiders). We presented cotton swabs with either extracted CHCs or a solvent control to determine if lizards respond to CHCs in the absence of visual cues of prey. We found that CHCs not only increased the frequency of chemosensory behaviors performed by predatory lizards, but that they also elicit feeding‐like behavior from lizards (e.g., biting at the cotton swabs). These results indicate that lizards not only respond to arthropod CHCs, but that they are sufficient for prey‐recognition by our chosen lizard species. This finding lays the ground work for future studies to investigate the precise compounds that are used by foraging lizards, and how the chemical communication systems of arthropods respond to selection exerted by vertebrate predators.