We have two states of metabolism (on this graph). We have a state of metabolism which is quite high, and that's the reprodutive phase. So for April through August, in this particular figure, you see butterflies which have a fairly low amount of lipids in their systems. The reason is that as they emerge from the chrysalis they have a moderate level of lipids and, instead of acquiring lipids during this period, they actually consume them in the process of reproduction. They consume them because they're trying to maintain an active lifestyle and they're trying to reproduce. A lot of that fat eventually gets converted to eggs, and some of it is used to provide energy just to sustain the reproductive butterflies.
A different condition pertains when we are looking at migratory butterflies. So if you look at September you can see that you're looking at 60 miligrams rather than 20 mg, and then it goes up to well over 100 mg and then eventually up to November where you're seeing about 140 mg.
On the migration you've got a really remarkable thing happening and that is that the butterflies are actually acquiring lipids. The reason they're able to acquire lipids during this period is that their metabolism is quite low. Somehow they've been able to shift into a physiology which allows them to convert the sugars they obtain from nectar into fat which is stored. It is one of the few organisms that actually gains in mass as it migrates.
Then you see the loss of lipids as they go through the winter. This allows us to talk about how they live off their fats through the winter, and that's pretty neat as well. You can see that they're actually losing lipids as you go from November, December, January and so on. That's perfectly understandable. They're not feeding and they're not particularly active; they have a metabolim that's pretty low but they also are still consuming energy and that energy is provided by converting a lot of those fats that they have stored on the migration to Mexico. They're converting those fats into sugars, particularly a sugar called trehalose which is used by most insects to maintain their metabolic state. That conversion of fat also releases a little water, so they have a little metabolic water that's provided under those conditions and that also helps them get through the winter.
That's it in a nutshell. You have two phases, a high metabolic rate during reproduction and a low metabolic rate and fat acquisition that takes place during the migration. The important point of the migration --and really for the entire year--is that to maintain any status of monarch butterflies you've got to have those nectar sources. They can't reproduce without nectar, they can't go through migration without nectar, and they can't survive the winter without having a very large amount of lipid stores.
So nectar is important all around. We can't just plant milkweed, we have to plant the nectar plants too.
Data courtesy of Dr. Lincoln Brower, Sweet Briar College.
Figure adapted from: Alonso-Mejia, A., E Rendon-Salinas, Brower, L.P. 1997. Use of lipid reserves by monarch butterflies overwintering in Mexico: Implications for conservation. Ecological Applications, 7(3), 1997, pp 934-947.