What is the Function of the Color of the Iris?
Ok, I took liberties with the wording of the title in order to preserve the ambiguity, but only a little bit.
A recent paper by Hansen et al. in Proceedings of the Royal Society – B investigates one putative function of the pattern of white triangles visible on the petals of Lapeirousia oreogena (see image), a member of the genus iris. One proposal is that these markings function in much the same way as landing lights on a runway – or signposts, to use the authors’ favored analogy. Below the petals of this flower is a long tube, with nectar in a little pool at the bottom. Insects have specialized structures – a long proboscis – designed to reach the nectar, extracting it from the flower. When the insect is positioned properly to do this, it simultaneously – and not coincidentally – is positioned such that it rubs up against the anther and stigma, picking up and/or depositing pollen. In this way, the flower uses the insect for fertilization, providing nectar as the incentive.
The insect must be positioned just so. Hansen et al proposed that the white markings in the iris they chose were “nectar guides,” aiding the pollinating insect so that it could insert its appendage properly into the iris’ long tube.
To explore this possibility, Hansen et al. conducted an experiment in a natural environment — literally a field study – in a wildlife preserve in South Africa.
Their manipulation was clever but simple. They used a black marker to cover none, some, or all of the white triangles on the flowers. To control to ensure that it was the lack of markings – rather than being written on with a marker – they also applied ink to the control flowers, but over the already-black portions rather than the white triangles.
Lapeirousia oreogena (Photo: Tim Waters)
One dependent variable was the behavior of the flies. They monitored their test subjects for two and a half hours during the day, counting how many fly approaches there were, also counting decisions insects made when faced with flowers with different numbers of intact markings. In addition, the authors applied fluorescent powder to the flowers’ anthers, and then surveyed the area after dark with an ultraviolet lamp, which allowed them to check for the dispersal of the powder. Finally, they looked at the fruiting behavior of plants with and without the modifications made to the white triangles, counting the number of viable seeds.
Changing the markings did not seem to influence fly approaches; there was no difference in the number of approaches for flowers whose triangles had been blotted out compared to those that weren’t. However, whether or not the insect inserted its proboscis in the tube successfully did depend on the manipulation. While nearly all of the insects on the unmodified flowers probed successfully, fewer than 10% of the insects alighting on the flowers without markings did so. Similar results obtained for the other measures. The authors write: “Only one out of 20 flowers without arrow-markings exported dye grains, to only one neighbouring flower, whereas 11 out of 21 flowers with arrow-markings exported dye grains.” From these and other results, the authors conclude:
Our results clearly demonstrate the importance of the white arrow-markings on L. oreogena flowers for proboscis insertion by their pollinator. These arrow-markings could thus be considered to be functional nectar guides ‘sensu stricto’—i.e. visual markings on tepals of nectariferous flowers that guide flower visitors towards a concealed nectar reward… our results provide clear evidence of a strong causal link between presence of nectar guides and both male and female components of plant fitness in a natural system.
Roughly, insects seem to use the white triangles as local guides for their proboscis, rather than as a global cue to the flower, more like runway lights – which is used by pilots on final approach – rather than as a beacon getting the pilot to the airport itself.
This paper is not obviously about evolutionary psychology. I’m writing about it for a couple reasons. First, it’s cool. I think it’s pretty neat that patterns on flowers serve a nose-guiding function for insects with long proboscises. Second, I think it’s important to continually examine the standards of evidence used to evaluate functional claims by those who study species other than our own. From this study, it’s clear that the researchers think that behavioral experiments (well, the behavior of another organism, really) are relevant to evaluating a candidate functional explanation. Generally, I think it’s worth monitoring in the primary literature how claims about function – in this case about increasing pollen uptake through proboscis-guiding – are tested by researchers in the biological community. In this case, it’s through behavioral experiments, using dependent measures that are plausible proxies for fitness. That seems perfectly reasonable…
Citation
Hansen, D. M., Van der Niet, T., & Johnson, S. D. (2012). Floral signposts: testing the significance of visual ‘nectar guides’ for pollinator behaviour and plant fitness. Proc Roy Soc B, 279, 634-639.
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