Although Charles Darwin hypothesized that some sexual dimorphsims in
animals were the result of ecological causation in the form of resource
partitioning, little unambiguous evidence exists in support of his
hypothesis. My studies of the purple-throated carib hummingbird,
Eulampis jugularis, provide strong evidence for Darwin's
ecological causation hypothesis by linking sexual differences in trophic
morphology to sexual differences in resource use. The purple-throated carib
hummingbird exhibits one of the most pronounced sexual dimorphisms in bill
size and shape of any avian species. Males are 25% heavier than
females, yet bills of females are 20% longer and 40% more curved. My field
studies indicate that this sexual dimorphism is maintained by specialization
of each sex on one of two species of Heliconia, whose flowers match
the size and shape of either the male (H. caribaea) or female (H. bihai)
bill. Presently, I am investigating the roles of sexual and natural selection
in the maintenance of sexual dimorphism in the purple-throated carib, and
the degree to which these hummingbirds drive natural selection in the
floral traits of their Heliconia food plants.
Pollination syndromes are suites of floral traits proposed to reflect
adaptations for pollination by animal groups that differ at the level of
order (bees, beetles, and moths) or higher (bees, bats, and birds).
My earlier studies focused on the hummingbird pollination syndrome.
Although bird-pollinated flowers are typically thought of as long, red
tubes that match the size and shape of the bills of their avian
pollinatiors, my studies suggest that researchers have overlooked key
aspects of flowers when drawing generalizations about hummingbird-flower
coevolution. By filming hummingbirds at flowers my students and I have
identified a number of "new" floral characteristics that play a role
shaping hummingbird-flower interactions, such as mobile flowers, flexible
pedicels, and flower lips as nectar
guides.
For example, studies that I have made in a temperate system in Amherst,
Massachusetts, have revealed that flowers of jewelweed (Impatiens capensis)
exhibit significant variation in the angle of their nectar-containing spur,
with some flowers have spurs that curve back along the tube of the flower,
and others having spurs that hang straight down, perpendicular to the
floral tube. In collaborations with Dr. Steven Travers, an Amherst Copeland
Fellow of 1999, honors student Irvin Pan (‘99), and student Jed Horwitt (‘01),
I determined that this variation is heritable and is apparently maintained by
divergent selection pressures exerted by hummingbirds and bees.
Amajor
goal of evolutionary ecology is to understand how ecological interactions
such as competition and mutualism shape evolutionary patterns within and
between species. I have approached this topic through studies of the role
of competition in the evolution of sexual differences within species and
the role of mutualism in the coevolution of plants and pollinators. My
current research looks at both questions in a hummingbird - Heliconia
system in the Lesser Antilles. My past research examined aspects of the
hummingbird-pollination syndrome in plants of eastern North America,
focusing on jewelweed, Impatiens capensis.
Past research: Spur Variation in Jewelweed (click on title for more info)