FROM THE VICE PRESIDENT
This University, amidst this natural laboratory, is a special place.
A rundown of science stories during the past year.
UNLOCKING THE SECRETS OF FLIGHT
UM releases new theory of bird evolution.
ONE HOWL OF A GOOD IDEA
Scientists create innovative listening device to track wolves.
Researcher provides tools to track their habitat.
Young researcher studies raptors impacted by metal contamination.
Wildlife biologists snare two prestigious grants.
HARE TODAY, GONE TOMORROW?
The snowshoe hare may become a climate change poster child.
Projects tackle dams, invaders, hybrids and more.
CRACKING A MYSTERY
Researcher suggests inattentive bird parents may produce larger eggs.
While studying how chukar partridges grow and develop the ability to fly, UM researcher Ken Dial developed an original theory for the evolution of flight.
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|This Andean solitaire thrush, found in the Venezuelan rainforest, has larger eggs than its counterparts in more temperate regions. (All wildlife images by Tom Martin)
Cracking a Mystery
Research suggests inattentive bird parents produce larger eggs
By Jeremy Smith
Both the spotted barbtail and blue-gray tanager are tropical songbirds native to Venezuela. One weighs, on average, 16.5 grams. The other weighs 32.5 grams. Why, then, does the much smaller species — the barbtail — bear eggs nearly 10 percent larger?
This question led Thomas Martin, assistant leader of UM’s Montana Cooperative Wildlife Research Unit, to a surprising discovery: At least in birds, parental neglect may require the parents to invest more energy in eggs to produce stronger, healthier offspring.
For most animal taxa, Martin explains, egg size and offspring number are inversely related: Bigger eggs mean fewer offspring; more offspring mean smaller eggs.
“The classic theory is that long-lived things put more energy per offspring to increase the quality of that offspring,” he says. “The idea was that they’re putting more energy into each young.”
Scientists long believed that comparisons of tropical and temperate songbirds supported this explanation because tropical songbirds generally bear fewer but larger eggs than their northern counterparts.
“From long-standing theory, it has just been assumed that this reflects that trade-off,” Martin says. “But people didn’t look deep enough.”
Refuting the theory of a quantity-versus-quality trade-off are examples such as the barbtail and the tanager. Each tropical species produces the same number of offspring — two. Yet average egg size in relation to body mass is dramatically smaller in the tanager, a seeming anomaly repeated across dozens of different tropical species comparisons in Martin’s data sets.
“The truth is, in the tropics most bird species have little variation in the number of eggs they produce,” he says. “Most species produce two eggs. So you’ve got a relatively invariant number of eggs, yet egg size varies dramatically across those species.”
Addressing this enigma required more than a decade of on-the-ground observation by Thomas and his field teams in tropical Venezuela, subtropical Argentina and north temperate Arizona. Within and among those regions, he discovered that differences in parental care — far more than fertility or any other factor — explained which species produced relatively larger offspring.
But the trend line runs opposite to what one might anticipate. The less time and attention parents provide their eggs, Martin found, the relatively bigger each member of their brood will be.
When parents leave their nests for long periods, it significantly lowers the temperature of their eggs. Lower temperatures, in turn, mean longer embryonic periods. Not unlike hibernation, this deferred birth date requires that eggs be provisioned with more energy — literally bigger yolks and greater albumen. Hence the eventual larger offspring.
“One of the things many people don’t really realize, including many scientists, is that birds are actually ectothermic as embryos — they rely on the parents to provide the heat by sitting on the eggs,” Martin says.
In addition, birds are extremely hot-blooded, with an average body temperature of about 40 degrees Celsius (104 degrees Fahrenheit), he says. Each minute parents spend off the nest, therefore, has the potential to slow embryo development dramatically.
“The ambient temperature at the elevation where we work in Venezuela only gets up to 25 degrees Celsius and drops to 15 Celsius at night,” Martin says. “Even at its maximum, it’s still much colder than the optimum.”
Nonetheless he found that parents in the tropics sit on their eggs for shorter periods than in the temperate zones.
“The fact that the tropical birds spend less time on the nest means that, even though the tropics are warmer, the eggs are actually experiencing colder temperatures than their counterparts in the temperate regions,” Martin says. “It’s the opposite of what you might expect.”
Martin first came to the topic in an effort to understand how such differences in temperature affected the rate of embryo growth. When he realized colder eggs would cause embryos to use more energy, his next thought was to study the possible effects on egg size.
“If they need more energy, they have to be larger,” he says. “So I would expect species that let their embryos get particularly cold to need even larger eggs.”
Such is the case of the spotted barbtail, “a little guy,” as Martin describes the species, but one with relatively enormous eggs. Field teams observed every day that barbtail parents take a four- to five-hour foraging recess from the nest. Within minutes, eggs drop to ambient temperature — about 20 degrees Celsius.
“That’s about half of the optimum range,” Martin says, “and it’s going to sit there for four to five hours at that temperature until the parents get back on.”
He contrasts this situation with that of the blue-gray tanager, whose parents seldom leave the nest for more than 40 minutes. “Average egg temperature is much warmer for the tanager than for the barbtail,” Martin says. “So the tanager can survive with a smaller egg size.”
Of 36 species studied in Venezuela, the barbtail had the longest incubation period at 27 days. By contrast tanagers hatched after 14 days.
What’s especially exciting about Martin’s novel embryonic temperature hypothesis is its ability to explain egg-size variation not only within the tropics but also between latitudes.
“The traditional theory is that development is slow in the tropics because that allows enhanced development of intrinsic qualities like neural development and immune functions, which enhance longevity,” he says. “Some of that theory is true, but there’s a lot of variation that’s explained by temperature caused by parental behavior. I’m arguing development is slow in the tropics because parents [spending more time off nest] are allowing their embryos to experience these cold temperatures.”
Earlier this year the Proceedings of the National Academy of Sciences accepted for publication a paper by Martin explaining his new theory and the data that supports it. This follows another major publication last year in the journal Ecology about how climate change affects the ecosystem of his longtime north-central Arizona study site — a 20-kilometer area near the edge of a long, steep slope that forms the southern border of the Colorado Plateau.
“My research is two-pronged,” Martin says. “One is looking at all the long-term components of this major ecosystem and how they work together and how they’re affected by things like climate change and resource management. The other prong is trying to understand wildlife demographic and reproductive strategies across the world.”
He’ll explore another avian mystery next year when research begins at a new field site in Borneo’s Kinabalu National Park.
“Birds in tropical South America and tropical Africa have similar clutch sizes — generally two eggs,” Martin says. “Asia is completely different. Most of those species have three or four eggs. The little information we have suggests that they develop very fast but are still long-lived there, again in contrast to South America and Africa. It potentially breaks the traditional view that you have to develop slowly to be long-lived. Nobody’s recognized that previously.”
The umbrella organization of all cooperative wildlife research units, the United States Geological Survey, employs 10,000 scientists. Only 20 may be designated a “senior scientist” of high international stature. This April Martin was so recognized.
“I’m one of very few people in the world looking at tropical nesting biology,” he says. “Early in my career, reviewers of my grant proposals would say, ‘Everyone knows you can’t find nests in the tropics.’” His solution? “A lot of area and a lot of training. I’ve always had a knack for finding nests. The trick is to then train other people to do that.”
Recent successes, he says, only make him impatient to return to the field.
“I’m running out of years to start a new project,” Martin says with a laugh. “Both the Argentina and Venezuela study sites were in the Andes. In Borneo, it’s Mount Kinabalu, which is a very steep mountain. Running up and down these mountainsides is getting harder.”
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|UM researcher Thomas Martin has become adept at locating the nests of the songbirds he studies, such as that of the plain antvireo (middle) and a hummingbird. Both nests were found in the tropical rainforests of South America.
|A chlorophonia songbird nesting in the Venezuelan rainforest
|A white-eye songbird feeding three hungry chicks