The albatross and the hummingbird have something in common – both are very slow egg producers for their size. A new study by UNM researchers shows how and why wild bird species vary in their rates of egg production.
The study was a collaboration between Distinguished Professor James H. Brown, Assistant Professor Chris Witt, and Ph.D. candidate Natalie Wright, all of the UNM Biology Department. Other members of the research team included: a longtime collaborator of Brown's, Professor Richard Sibly, University of Reading (United Kingdom); a former postdoc of Brown's who is now a professor at Yale, Walter Jetz; and an expert on evolutionary statistical analyses Chris Venditti, University of Hull (United Kingdom).
"An animal's productivity is the rate of production of biomass (babies, eggs, etc)," explained Witt. "Darwin's theory tells us that individuals should seek to maximize this rate in order to maximize their contribution of genes to the next generation (i.e. their fitness).
"However, lower productivity sometimes leads to higher fitness – it's slightly counterintuitive, but think of the potential advantages of producing a few high quality offspring versus many low quality ones."
"As you get smaller, you get more productive – exactly what we would predict from Brown's metabolic theory – that smaller things, because of their disproportionately higher metabolic rates, have more excess energy to spend on reproduction," said Witt. "What Brown's theory didn't predict was that the relationship would be curvilinear – meaning that at the very smallest end of the spectrum of bird sizes, there is no additional productivity gained by becoming smaller – there may be a most productive body size, somewhere between 10-100g."
Birds that feed on abundant, energy rich food resources don't just convert it into more productivity.
"Actually the opposite is the case: birds with the best food have the lowest productivities," said Witt. "Why? We think it's because these particular good food resources are also associated with escape from predators. If you can obtain your food quickly without exposure to predators, you have high survival rates. The result is that natural selection favors quality over quantity."
Hummingbirds can get a lot of energy quickly from nectar, and don't have to be exposed to predators for long. Likewise, nighthawks feed on flying insects that are abundant at dusk and dawn, allowing them to hide from predators most of the day and night. Albatrosses and other birds that spend all their time on the open ocean or on remote islands rarely encounter predators. Under these conditions, evolution favors slow growth and reproduction, probably because strong immune systems and high longevity become paramount to evolutionary fitness.
The researchers also saw direct effects of energy limitation on bird productivity. "If the female alone cares for the young, she produces less egg biomass – the parental care contributed by the father or by cooperative groups translates into higher productivity," said Witt.
Years ago, Brown and Sibly analyzed the causes of variation in productivity across mammals. They found that mammals with abundant, rich food resources such as marine and large grazing mammals tended to have higher productivity – meaning that when more energy was available, more energy was being allocated towards reproductive biomass. This new study on birds found the opposite effect. Marine birds like albatrosses have among the lowest productivities of all birds.
The researchers faced a few challenges during the study. "There was no evolutionary tree available for birds that encompassed all of the species in our dataset, and to answer questions about the evolution of traits, you really need a hypothesis about how the species you're interested in are related to one another," said Wright, who's interested in how and why certain avian traits evolve.
So Witt and Wright estimated the evolutionary tree or phylogeny of the 980 species by using DNA sequences available in an online database called GenBank and a previously published tree.
"The phylogeny or evolutionary tree was really very interesting because it showed that the connections between lifestyle and productivity that we found are things that have evolved multiple times in the history of birds," said Brown.
"There's enormous variation among birds," said Witt. "Having an estimate of the tree allowed us to determine how much of the variation in productivity was due to current lifestyle versus evolutionary history." It turned out that both evolutionary history and current lifestyle were important in determining bird productivity.
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