Effect of floral orifice width and shape on hummingbird-flower interactions

Abstract

Nectar guides are common among insect-pollinated plants, yet are thought to be rare or absent among hummingbird-pollinated plants. We hypothesize that the lower lips and trumpet-shaped orifices of many hummingbird flowers act as nectar guides to direct hummingbirds to the flowers' nectar and orient the birds for pollination. To test this hypothesis we conducted laboratory experiments using flowers of Monarda didyma (bee balm) and M. fistulosa (wild bergamot), which have orifice widths of about 4 mm and 2 mm, respectively, and latex flowers with orifice widths of 4 mm and 2 mm and three orifice shapes (trumpet, lipped, and lipless). Rubythroated hummingbirds (Archilochus colubris) made fewer errors during bill insertion and spent a smaller proportion of their feeding visit in error at M. didyma flowers than at M. fistulosa flowers, and at unaltered flowers of both species than at flowers with lower lips removed. Handling times were longer at both lipped and lipless flowers of M. didyma than at those of M. fistulosa, and at lipped than at lipless flowers of M. didyma. The average duration of contact between a hummingbird and a flower's anthers and stigma was longer at M. didyma than at M. fistulosa for both lipped and lipless flowers, and at lipped than at lipless M. didyma flowers. Hummingbirds missed the openings of latex flowers with their bills more frequently and spent a greater percentage of their total feeding visit in error at (i) 2-mm than at 4-mm flowers of all three shapes, (ii) lipless flowers than at trumpet or lipped flowers, and (iii) lipped flowers than at trumpet flowers of both widths. The duration of hummingbird/anther contact was longer at (i) 2-mm than at 4-mm flowers of all shapes, (ii) lipped than at trumpet or lipless flowers, and (iii) lipless than at trumpet flowers for both widths. No significant differences in handling times of hummingbirds were observed among any of the latex flower shapes or widths. Our results demonstrate that orifice shapes can act as guides by reducing the frequency of feeding errors by visiting hummingbirds, and that effects of orifice shape on pollination must be considered in conjunction with flower widths and locations of anthers and stigmas.

Island–mainland difference in Nicotiana glauca (Solanaceae) corolla length: a product of pollinator-mediated selection?

“…Differences in floral dimensions less than 2 mm have been shown to have significant effects on pollination success (e.g. Campbell et al 1996;Smith et al 1996), and corolla length is a likely target for selection by long-tongued or -billed pollinators (e.g. Nilson 1988;Johnson and Steiner 1997).…”

Floral signposts: testing the significance of visual ‘nectar guides’ for pollinator behaviour and plant fitness

“…In some cases this contrast occurs mainly in the ultraviolet (UV) spectrum, making the guide patterns indistinct or invisible to humans [3,4]. Floral spatial cues that have been studied include contrasting patterns created by pollen, anthers or mimic anthers [5][6][7], flower shape [8], different petals with contrasting colours [9], scent [10], surface structure [11 -13] and even acoustics [14].…”

Down the Tube: Pollinators, Predators, and the Evolution of Flower Shape in the Alpine Skypilot, Polemonium Viscosum

“…Because flower shape is relatively constant within populations, tests for stabilizing selection often require experimental manipulations of corolla shape (Wilson 1995) or use of artificial flowers (Temeles 1996) to evaluate the performance of extreme versus representative phenotypes. Results suggest that flower dimensions operate in concert to influence pollinator posture and contact with reproductive parts (e.g., Grant and Temeles 1992;Smith et al 1996;Te-meles 1996), but that such contact may not strongly affect pollen transfer dynamics (Campbell et al 1991;Wilson 1995;Kobayashi et al 1999). The ambiguity of these results implies that stabilizing selection for pollinator fit provides a weak mechanism for the canalization of flower shape.…”