Carnegie Museum of Natural History

One of the Four Carnegie Museums of Pittsburgh

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A Match Made by Coevolution

Darwin once predicted the existence of a pollinator after examining the star-shaped flower of the orchid Angraecum sesquipedale, a flower whose nectar is at the end of a 30 cm tube. Darwin wrote that “in Madagascar there must be moths with probosces capable of extension to a length of between ten and eleven inches [25.4–27.9cm].” Twenty years after Darwin’s death, his prediction was proven correct with the discovery of a moth, Xanthopan morganii praedicta, which boasted a proboscis 20 cm in length. In 1992, natural history observations of the moth feeding on the extreme flower and transferring pollen provided even more evidence that this plant and insect were tangled in a coevolution that resulted in their extreme morphology.

Coevolution is now a cornerstone of biology and has been well developed through examples of flowering plants and insects, parasites and hosts, predators and prey, and even gut microbiomes and human health. In fact, the influence of closely associated species on each other in their evolution is so ubiquitous one could argue that evolution is coevolution—as the boundary between what is an individual versus a consortia of different species blends as we dive deeper into the units that natural selection is acting upon. The microbiome and human health example helps illustrate the problem of defining an individual, specifically because scientists now think that microbial cells outnumber human cells in your body. Moreover, there is growing evidence this diversity of symbionts on our bodies complete metabolic pathways and serve other physiological functions. Coevolution crisscrosses the natural histories of organisms, creating nuances that sometimes complicate things.

With so much excitement and work surrounding coevolution, it is romantic to stumble across an example of coevolution fit for a kindergarten class. In the collection of birds at the Carnegie Museum of Natural History, we recently finished an analysis of 24 Costa Rican hummingbirds and the pollen types found on their bodies and were reminded of Darwin’s predictions of coevolution over 100 years ago with orchids and moths. The White-tipped Sicklebill (Eutoxeres aquila) is a hummingbird with an extreme bill curve, with an appearance that would remind kindergarteners of Jim Henson’s Gonzo Muppet. Putting this bird next to its favorite food, Centropogon granulosus, illustrates coevolution in an exciting way that doesn’t tangle you up in learning about microbes or imagining other complex ecological relationships. Like Darwin’s orchid and moth, this hummingbird and its preferred flower allow us to see coevolution is all around us.

sicklebill hummingbird and its preferred flower

In an ongoing study at the Carnegie Museum of Natural History’s Section of Birds, we were reminded of the natural history observations and predictions that led to an explosion in the field of coevolution. By studying pollen types collected from hummingbirds in Costa Rica we confirmed that the White-tipped Sicklebill (Eutoxeres aquila) feeds mostly on Centropogon granulosus, a match made by coevolution.

Chase Mendenhall is Assistant Curator of Birds, Ecology, and Conservation at Carnegie Museum of Natural History. Museum employees are encouraged to blog about their unique experiences and knowledge gained from working at the museum.

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Unscrambling the Science of the Egg

By Chase D. Mendenhall

eggs in a nest

What came first, the chicken or the egg? The answer to this riddle is the egg. Eggs are universal among all vertebrates, including humans, but reptiles are responsible for the development of the eggshells typical of terrestrial birds and early mammals. Eggs are virtually self-contained life support systems that freed the first reptiles to wander away from water for reproduction, separating them from amphibians. Eggs are packed with most of the ingredients needed to grow the animal inside. All they require for the embryo to develop properly are warmth and gas exchange.

Bird eggs vary tremendously. Shape, size, coloration, and contents have often been associated with life histories of the species that lay them. For example, asymmetrical eggs with one pointed end were thought to be the result of nesting on a cliff—these eggs roll in tight circles instead of straight off the edge. Similar stories have been written about extensively to explain the jelly bean shape of the hummingbird eggs, elongated ellipses of swifts, and the spherical nature of owl eggs—but new work done in museum collections may have answered the riddle of egg shape definitively. Specifically, scientists now have evidence to suggest that selection for flight adaptations is most likely to be responsible for most of the variation.

chart of egg shapes

Measurements of nearly 50,000 eggs in museum collection from 1,400 bird species by Dr. Mary Stoddard and colleagues revealed stunning evidence that egg shape is related to flight. Dr. Stoddard’s star variables for testing her hypothesis were egg asymmetry and ellipticity. Symmetric eggs have similar shapes at each end, like the hummingbird’s jellybean shaped eggs, and asymmetric eggs are pointed at one end, like a sandpiper egg. Ellipticity is related to length and volume of the egg—for example, owls lay spherical eggs, while Orioles and Swifts lay long zeppelin-shaped eggs. The two variables of asymmetry and ellipticity interact with one another, allowing scientists to categorize egg shape across two axes that provide information about the way the egg was shaped in the shell gland after passing through the uterus.

Stoddard discovered that mother birds shape their eggs mechanically, apply pressure to the egg membranes as layers of calcium carbonate crystals form the eggshell. The shape of the egg determines the space in which the young bird completes the process of building its body for flight. Like all multi-cellular vertebrates, one cell divides into many—differentiating into trillions of cells with specialized architecture and function. According to Stoddard’s analysis of egg shape in relationship to phylogenetic history, she was able to demonstrate that egg shape explained wing shape. Spherical eggs, like those of the owl, are symmetric and score low on the ellipiticity scale and tend to belong to birds who spend little time flying. Elongated, asymmetric eggs—like those belonging to sandpipers, are associated with champion flyers who might spend many days airborne.

Chase Mendenhall is Assistant Curator of Birds, Ecology, and Conservation at Carnegie Museum of Natural History. Museum employees are encouraged to blog about their unique experiences and knowledge gained from working at the museum.