U. ARIZONA (US) — Just as there is no free lunch, nearly every evolutionary trait comes with a trade-off in energetic demands.
New research is looking into the priority rules that govern how organisms use a continuously changing and limited set of resources.
“For example, any organism that is growing, has to make a decision—physiologically speaking—where to allocate those resources,” says Goggy Davidowitz, assistant professor of entomology at the University of Arizona.
“You can allocate them to growth, to keeping yourself alive, to reproduction, locomotion and various behaviors like foraging and so on.”
Some traits affect reproductive success more than others. For example, in some animals, larger body size can lead to higher mating success. Larger females can produce more eggs. A fast growth rate helps a vulnerable juvenile shorten the time it is exposed to predators and parasites.
“All these traits determine the reproductive success of an organism,” Davidowitz says. “The trick is to find out which ones are the most important and how they trade-off with each other.”
In Davidowitz’s lab, a giant hawk moth flies in circles inside a container about the size of a passenger car tire, attached to a wire boom, that causes it to spin on a hub at the center.
Tubes and cables snake in and out of the container and connect it to electronic recording devices.
“This setup was designed specifically for our lab,” Davidowitz says.”As far as we know, it is the only one of its kind that exists for insects.”
The flight arena allows researchers to precisely keep track of which resource an individual moth uses throughout its life, from the time it hatches from an egg as a caterpillar munching on its favorite host plant, Datura wrightii, also called jimson weed, to when it emerges from its pupa, about to embark on the final week of life, most of which is spent searching for a mate, nectar sources, and places to lay eggs.
The answers Davidowitz is hoping to find could also help solve questions that apply to other living creatures, including humans.
“What we do is not specific to these insects. We still don’t know how resource allocation changes as an individual grows and how the trade-offs shift over the course of a lifetime.”
“Take athletes, for example,” he says. “They invest a lot of nutrients into muscle mass and a lot less into reproduction, to the point where some female athletes no longer have a menstrual cycle. Reproduction requires a minimum amount of fat reserves. When those athletes no longer compete, they are able to develop those fat reserves and they can have babies.”
Traditionally, evolutionary biology has taken a look at these processes at the level of whole populations and in the context of an end product, such as body size or number of offspring.
“But the current ability of an organism to function and reproduce is based on its past history, what kind of nutrients it has acquired and how much and so forth,” Davidowitz says.
“In addition, resource availability changes throughout its life. This is the first time we’re able to study this on an individual level and not a population level. And we can do this in real time. We can see the changes in allocation as they happen.”
While the moth turns its circles in the flight arena, Davidowitz and colleagues measure the oxygen it takes in and the carbon dioxide it gives off. By analyzing the ratio of the two gases, researchers can tell whether the moth is burning fat it stored from the plant material it ate as a caterpillar or carbohydrates it took in while sucking nectar from flowers.
“We can tell when they switch between carbohydrates and lipids,” Davidowitz says. “This allows us to quantify how much energy they’re using and from what source. The caterpillar eats and eats and eats, and then what does it do with what it’s eating? Does it store it all for later use as an adult? Does it use everything up now to become as large as it can as fast as it can? It’s things like these that we’re after.”
Using a carbon isotope gas analyzer, the team can distinguish between the carbon isotope signatures of the caterpillar and adult foods.
“We can tell whether they’re burning nutrients we fed them when they were caterpillars or from when they were already adult moths.”
Combining the flight arena and the isotope analyzer, the team studies two traits that are among the most energetically costly ever to evolve: flight and reproduction.
“Imagine a moth that uses nectar to fuel flight and provide carbohydrates to its developing eggs. Now imagine it emerges from the pupa and we make sure there is no nectar around. It would immediately start to use the fat stored when it was a caterpillar as flight fuel. But if we allow it to feed on nectar, it probably will shunt some of it to making eggs and some of it to power its flight. If more flowers are available, it probably will shunt most of the nectar to the eggs.”
Giant hawk moths are powerful and enduring fliers. Davidowitz once observed a moth that flew circles in the flight arena for 50 miles (80 kilometers) non-stop.
“Insect flight muscle is considered to be the muscle system with the highest energy demand of anything,” he says. “These hawk moths hover to suck nectar from flowers, and hovering is by far the most energy-intensive behavior. When they do that, their metabolic rate is about 100 times higher than when they’re at rest.”
The moths have to find nectar, find mates and a plant to lay the eggs on. These plants are dispersed so the moths have to fly considerable distances to find them. Fuel for flight comes from nectar initially but then the moth switches to lipids to fuel flight.
“After we fly the animals, we can look at the eggs and the flight muscle to see where the animal allocated the resources we fed them during their lives’ history, and what they used them for.”
To conduct the research, Davidowitz received a CAREER Award from the National Science Foundation.
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