McGuire's Lab:

IB professor Jim McGuire is a herpetologist and evolutionary biologist, and not surprisingly his work on the Indonesian island of Sulawesi focuses on reptiles and amphibians--fanged frogs, skinks, bent-toed geckoes, and flying lizards. Across the globe, high in the Andes of Peru, McGuire has a separate field program focused on hummingbirds. These two projects concern distantly related taxa, but the underlying questions are similar.

"I'm interested in diversification and speciation," says McGuire, "and the various factors that play into that, including biogeographical processes, adaptation, and morphological innovation."

Herp research in Sulawesi is still in the discovery phase: "How many reptile and amphibian species occur on this island?" McGuire asks. And what are the processes that led to Sulawesi's current state of diversity? As for the hummingbirds, McGuire is interested in learning how birds with sky-high metabolic rates evolved to tolerate high-elevation, low-oxygen conditions in the Andes, and how they became so diverse.

Let's start in Sulawesi, where McGuire spends two months each summer. Sulawesi is a huge island, shaped like the letter K, roughly 1,000 kilometers from north to south. It is separated from Borneo by the Makassar Strait, an extremely deep channel that at its narrowest spans 120 kilometers. Running through Makassar Straight is the famous Wallace Line, a biogeographic boundary named for 19th Century naturalist Alfred Russel Wallace, who noticed the differences between the biotas of Asia and Australia.

Sulawesi is a composite island, formed from at least five separate paleo-islands, which over the past 25 million years slowly smashed into each other as tectonic plates collided. Because of this complicated geologic history, the flora and fauna of Sulawesi are not only unique relative to the rest of Southeast Asia, but also exhibit tremendous regional variation within Sulawesi itself.

"On every expedition we find several new species, and I'm sure that we've not come close to surveying the entire fauna," says McGuire.

There are at least eight areas of endemism on Sulawesi--areas with species found in that particular locale and nowhere else on earth. For some species, these areas of endemism seem to correspond to the five paleo-islands. For other taxa, the story is more complicated.

On research trips, McGuire and a team of grad students, undergrads, and local Indonesian student and faculty researchers pile into trucks and drive along the dirt roads, stopping frequently to search for animals along streams and in forests and fields. It's exhausting: Some of the animals are active during the day, and some are active at night, so McGuire and his team are active pretty much all the time.

"We sample during both the day and night," says McGuire. "We don't want to take a single day off because our time in these remote localities is so precious."

They collect hundreds of specimens, and carefully record where they were obtained. Half of the specimens are deposited in the National Museum of Indonesia and half are brought back to Berkeley for phylogenetic and population genetic analyses, in order to understand the patterns and processes that have generated Sulawesi's incredibly rich herpetofauna.

Many of the species are part of "species complexes"--groups of species that do not interbreed, yet look almost identical to each other and are difficult to tell apart without examining their DNA.

Phylogenetic analyses allow McGuire and his team to locate the boundaries between closely related species. Sometimes, those boundaries correspond to geological history. For example, the lines separating several flying lizard species are the same lines where individual paleoislands abut one another. "Many congruent boundaries have turned up in virtually every taxon that we study--but a few oddball taxa exhibit idiosyncratic patterns that are difficult to reconcile with our knowledge of the geological history of the island," says McGuire.

Once the locations of these boundaries have been determined, there are more questions: "What is the timing of divergence?"--when did the species on either side of the boundary last share a common ancestor? "Is there gene flow--migration--across the contact zones now?" asks McGuire. Basically, how long have the species boundaries been in place, and are how permeable are they?

McGuire and his team are looking at the species boundaries and population genetics of nine different species complexes: flying lizards, two kinds of skinks, several groups of frogs, bent-toed geckoes, and even macaque monkeys.

This is probably enough to keep him busy for a while, but McGuire can't resist a cool side project. And the Andean hummingbirds are "a side project that's become very large in scope," says McGuire.

Hummingbirds are astoundingly diverse: there are at least 330 species, and this is probably a substantial underestimate. Hummingbirds are most diverse at mid to high elevations in the Andes Mountains of South America, which is surprising because hummingbirds in the Andes face several daunting challenges.

To start with, their hovering locomotor strategy requires that they have high metabolic rates--the highest among vertebrates. Maintaining these metabolic rates is a challenge in cold, high-altitude settings. Furthermore, the air is thin at high altitude, and so hovering is even harder work in these upland areas. Finally, there is less oxygen available at high elevation, and for animals with high metabolic rates (and therefore high oxygen consumption), this is a particularly difficult environmental barrier. Despite these challenges, hummingbirds have invaded these high elevation habitats no fewer than 11 separate times. Hummingbirds likely have experienced convergent evolution, allowing these invaders to adapt to the extreme conditions independently.

McGuire and his colleagues, Berkeley IB professor Robert Dudley, UC Riverside professor Doug Altshuler, and University of New Mexico professor Chris Witt, have been studying how hummingbird morphology and flight kinematics enable them to fly at high elevations.

On average, high elevation hummingbirds are larger than their lowland relatives (probably to keep warm in the cold climate), and their wings are larger in proportion to their bodies, when compared with low elevation species. This decreases their wing loading (the amount of body weight per unit of wing area), making it easier for the birds to remain in the air.

When faced with progressively reduced air densities in experimental conditions, hummingbirds flap their wings with increasingly greater stroke amplitudes, such that their wingtips ultimately contact at the top and bottom of their stroke. Once the birds have hit this wall, any further reduction in air density results in aerodynamic failure--the birds slowly fall to the ground. Understanding these morphological and biomechanical constraints and adaptations was the first phase of the research.

Next, they wanted to know how hummingbirds deal with life at high elevation on a physiological level. Oxygen is severely limited at high elevations, because of reduced barometric pressure. Oxygen is carried in the blood by hemoglobin, so McGuire and his collaborators hypothesized that hemoglobin in high-elevation hummingbirds might have been a target of natural selection. Modifications in hemoglobin could allow high elevation specialists to occur in low oxygen environments that lowland hummingbirds could not tolerate.

McGuire and his colleagues are investigating the genes that code for hemoglobin. They are comparing these genes across many species of hummingbirds, and finding that there are amino acid substitutions specific to high-elevation species--substitutions in the same regions of the gene that have been shown to influence hemoglobin oxygen binding efficiency in other birds. Preliminary data shows a signal of positive selection--adaptation--in these hemoglobin genes.

There is also an experimental arm (wing?) to the project in which McGuire and his collaborators perform physiological experiments in the field to determine how well hummingbirds living at different elevations tolerate reduced oxygen availability. In one experiment, they place individual hummingbirds in an airtight chamber and progressively reduce the available oxygen, by replacing oxygen with nitrogen, until the birds are no longer capable of sustained hovering. With these experiments, they have found that high elevation species are able to handle much lower oxygen availabilities than are lowland species, just as they predicted based on the results of their hemoglobin molecular evolution studies.

In a separate set of experiments, they obtain blood samples from free-living birds, to determine if they exhibit altitude-specific characteristics such as higher hemoglobin concentrations or higher red blood cell volumes, which are expected if there is a high premium placed on oxygen transport to the tissues.

One of their more interesting findings is that lowland hummingbird species that lack hemoglobin adaptations can have extremely high red blood cell concentrations when they are at the upper altitudinal limit for the species. This is presumably because they need to transport as much oxygen as possible, but lack important evolutionary specializations present in their high elevation relatives. Some of these birds, when compared with humans, have twice the volume of red blood cells in a given volume of plasma.

"These hummingbirds can evidently deal with having syrup for blood," marvels McGuire.


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