Shape matters: corolla curvature improves nectar discovery in the hawkmoth Manduca sexta

Abstract

1. We measured the effects of variation in corolla curvature and nectary aperture radius on pollinator foraging ability using the hawkmoth Manduca sexta and 3D-printed artificial flowers whose shapes were mathematically specified. 2. In dimorphic arrays containing trumpet-shaped flowers and flat-disk flowers, hawkmoths were able to empty the nectaries of significantly more trumpet-shaped flowers regardless of nectary aperture size. Interestingly, trumpet-shaped flowers needed to deviate only slightly from the flat-disk morphotype in order to significantly increase hawkmoth foraging ability. 3. Whole-flower three-dimensional shape, particularly corolla curvature, has the potential to act as a mechanical guide for Manduca sexta, further implicating direct flower-proboscis contact as an important contributor to foraging success during flower handling in hawkmoths.

Figure 1

Manduca sexta, the pollinator used in this study, probing a flower with its proboscis. Photo by Armin Hinterwirth.

Figure 2

(A) Bird's eye view (top) and lateral section (bottom) of generic artificial flower depicting what each of our four shape parameters represent. R is the linear distance between the nectary aperture edge and the edge of the “corolla” and r₀ is the radius of the central aperture. Note that this means that whole-flower radius is equal to r₀ + R. (B) Illustration showing how altering the value of the corolla curvature parameter, c, in our flower shape equation affects the lateral profile of our artificial flowers. When c is equal to 0, the corolla forms a straight line between the base of the flower and its outer lip, forming a truncated cone if revolved around the vertical axis. Negative values produce curved corolla profiles that lead to trumpet-shaped flowers if revolved around the vertical axis. As c approaches negative infinity, the corolla approaches a vertical path that turns suddenly at a 90-degree angle, producing a flat-disk flower with cylindrical nectar tube if revolved. Positive values of c produce bowl-shaped flowers if the resulting path is revolved. The given values of c are only for illustrating the trend. (C) A 3D computer rendering of what the flower in (A) would look like.

Figure 3

Bird's eye view (top) and lateral section (bottom) of the five different artificial flower morphs used in our experiments. They are labeled M1 through M5 and referred to as such in the main text for convenience. All are drawn to the same scale.

Figure 4

Mean number of emptied flowers of each flower morph per foraging bout (± SEM) in each of our three experiments. Parts A-C correspond to Experiments 1-3 respectively. Significant differences are denoted with an asterisk.