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Following in Darwin’s footprints: Hau unlocks secrets of tropical birds through field study on the Galápagos

From the Feb. 20, 2006, Princeton Weekly Bulletin

The Galápagos Islands hold a unique place in the history of science. It was here, in the 1830s, that Charles Darwin gathered the clues that led to his theory of evolution.

It is here, today, that Princeton’s Michaela Hau continues Darwin’s intrepid scientific tradition. Her studies of tropical birds may shed light on the mysteries of human behavior and could lead to better models for ecosystem conservation.

Birds display a remarkable variety of behaviors, including reproductive behavior such as defending territories and mating, said Hau, an assistant professor of ecology and evolutionary biology. “I study the mechanisms that underlie the control of behavior and how they have evolved over time,” she said. Since she can’t go back in time to study the ancestors of her present subjects, she compares old lineages of birds with more recent lineages. This is where the tropical birds come in.

During the Ice Age, from 1.6 million to 10,000 years ago, much of the earth was entombed in a sheet of ice. As the glaciers advanced from the poles, many species of birds evacuated their usual habitats for the tropics, where slightly balmier temperatures still sustained life. Once the ice melted, those that survived followed the glaciers’ retreat and recolonized the emerging landscape. Thus, many of today’s temperate-zone birds have evolved from tropical ancestors.

In fact, this repopulation is at the heart of a bit of a disconnect in bird science. While more than 60 percent of the world’s bird species live in tropical latitudes, and the number increases dramatically with the inclusion of tropical migrants — those that breed in the higher latitudes but spend most of their lives in the tropics — much of what is known about bird behavior comes from studies of mid-latitude, or temperate, species. Biologists, it turns out, have been studying the specialists that have evolved to fit particular environmental niches and have long viewed the tropical birds as evolutionary oddities. They may have it all backward, said Hau.

“Ninety-five percent of what we know about just about anything comes from temperate latitudes,” said John Wingfield, a professor of biology at the University of Washington, and Hau’s postdoctoral adviser. “To a great extent, our entire concept of physiology, endocrinology and control mechanisms is based on humans and animals of mid-to-high latitude origin. Most organisms actually come from the tropics, and we know much less about them. So, in terms of fundamental information and how the world works, we need to know a lot more about tropical species.”

Comparing bird behavior

Hau, an animal physiology and behavior specialist, is a leader in tropical bird research, said Wingfield. She’s pioneering integrative studies at the interface of ecology, behavior, evolutionary biology and endocrinology (the study of how hormones work), he said.

Her “laboratory” on the windswept islands off the coast of Ecuador is a far cry from her German roots. She was a “city girl — not one of those kids who grew up hugging every frog,” she said, and is the first scientist in her family.

The decision to study biology was motivated more by intuition than any overarching life plan. Once she started though, she was hooked. She earned her Ph.D. from Ludwig-Maximilians-University Munich in 1995, conducting pre-doctoral and postdoctoral research at the Max Planck Institute for Behavioral Physiology. Her husband, Martin Wikelski, also is a faculty member in Princeton’s ecology and evolutionary biology department.

Both have held postdoctoral positions at the University of Washington in Seattle and conducted field work at the Smithsonian Tropical Research Institute in the Panamanian rainforest. At Princeton since 2000, Hau teaches courses in topics including comparative physiology, the biology of populations, and hormones and social behavior, and involves students in her work far from campus. In between teaching and raising their 2-year-old daughter, both Hau and Wikelski conduct research on the Galápagos — he on marine iguanas and she on tropical birds.

Hau studies groups of birds that have remained in the tropics throughout their evolutionary history and compares their behavior to related species in the temperate latitudes.

One of Hau’s most compelling research questions is: How do birds know when to reproduce?

In higher latitudes, the links between mating and longer, warmer days and new plant growth, which leads to greater food abundance, seem obvious. Move to tropical latitudes, however, and the environmental signals to mate are much less evident. Temperatures vary by only a few degrees from season to season and day length remains the same throughout the year at the equator.

In Panama, Hau and her students study spotted antbirds, a small bird with black spots on a white breast and brown and black striped wings and tail. They discovered that the antbirds are remarkably efficient at discerning very small changes in day length.

The Smithsonian Tropical Research Institute is located approximately 600 miles north of the equator and day length, also called photoperiod, varies by only one hour over the year. Hau discovered that an almost imperceptible change in day length by only 17 minutes is sufficient to trigger a cascade of hormones that head straight to the gonads causing them to grow and produce testosterone — flipping the reproductive “on” switch. Antbirds, it appears, follow the same broad hints that it’s getting seasonally close to breeding time as their more northerly neighbors.

Bird behavior on the Galápagos tells a different story. The islands sit almost exactly on the equator and the length of day varies by all of six minutes. “It’s very unlikely, although I can’t exclude it, that the birds could be able to discern the minute differences in day length from one day to the next,” said Hau. Day length changes in a predictable way, yet Darwin’s finches, the birds Hau studies on the islands, reproduce unpredictably.

So what puts birds in the mood? Hau theorizes that it might be rain.

The islands, normally bleak and almost devoid of vegetation, can experience an explosion of growth following infrequent rainfall. With the plants come insects, an important source of protein. “Food abundance is what the adults need to sustain the energetically expensive breeding efforts and to feed the young,” she said.

Further implications

Hau, whose work builds upon the landmark studies of Darwin’s finches by colleagues Peter and Rosemary Grant, said that her efforts are ongoing and that she still hasn’t fully solved the puzzle of finch breeding behavior.

“I think there might be a very complex use of environmental cues and I’m sure that rainfall is stimulating, but I’m not saying just yet that it makes the birds reproduce,” she said. “Other things have to come into play. We know a lot about day length, how it’s perceived and used; we still don’t know much about these non-photoperiodic cues.”

What the behavior does tell her is that birds that rely on the breeding signal of lengthening days in temperate climates have evolved to do so. They also have evolved a link between photoperiod and the regulation of testosterone.

Testosterone plays a role in both reproductive and aggressive behavior, like defending territory. Hau’s research shows that tropical birds use testosterone differently than temperate zone birds. Spotted antbirds, for example, have very low levels of testosterone, even during the mating season, yet still become sufficiently testy to fight off birds that invade their territory. At the other extreme, North American red-wing blackbirds have high levels of testosterone coursing through their bodies.

One reason for the difference, Hau believes, is that birds that migrate to breed must arrive in their breeding region, establish a territory and defend it from other competitors, advertise for and attract a partner, mate and then continue to defend and protect the young. For these birds, reproduction and aggression go hand in hand and they’ve evolved the testosterone levels to keep them fired up for the job.

Yet there’s a trade-off. “The theory is that high levels of testosterone suppress the immune system, making the birds more susceptible to infections and decreasing their lifespan,” she explained. Why some birds have high testosterone levels compared to others is still a bit of a mystery — one that Hau plans to tackle next.

In the future, Hau’s endocrinology studies linking behavior with testosterone levels in birds may also lead to a better understanding of human behavior. While it might seem like a big jump from bird brains to humans, we share many common physiological chemicals, like testosterone.

“We’re not going to cure cancer or find the solution to AIDS,” said Wingfield. “But I think overall we’re contributing to basic biology, which humans are part of too. And understanding the basic concept of life on earth is understanding something basic about ourselves as well.”

“Michaela’s work also has conservation biology implications,” he added. “The more we know about these tropical birds, the better we will know how to conserve their species within tropical rain forests. I feel that conservation biology, although it does not relate to human well-being directly, is relevant because if we let the ecosystems of the world degrade, then eventually we’ll be degrading our own lives. So I think her research will have relevance to human welfare on many different fronts.”

Further unraveling the complexities of evolution in Darwin’s backyard is a tall order for some small birds and an assistant professor who juggles teaching and research with motherhood and marriage. Yet, Wingfield is confident that Hau is up to the task. “She’s still very early in her career, but she’s showing all the signs of great leadership.”

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