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Field Report from Mexico by Dr Lincoln Brower

March 25, 2009

I just got back from Mexico and I was able to get to four colonies. I wanted to check up on them to see how they are doing and also to take some close-up photographs of the beautiful arrangements of the monarch clustering on tree trunks. We just finished up a (scientific) paper which has shown that the tree trunk is like a hot water bottle. During the night the tree trunk is warmer than the outside temperature and during the day its cooler. So the monarchs that are sitting on tree trunk get two advantages. 1) If it really drops below freezing at night then they're going to be warmed a little bit, 1-2 degrees and 2) during day they're going to be 1-2 degrees cooler which means they're not going to be burning up their fats as fast.

This led me to think about what the old forest was like. We went up into one of the areas on the beautiful south side of Cerro Pelon which has had colonies on it ever since Professor Urquhart discovered the butterflies in 1975.

There weren't any big trees there now but there were these enormous stumps. They were 3,4,5 feet in diameter. They probably were cut 40 or 50 years ago. It's just mind-boggling to think of what that forest must have been like originally, before anybody got in there and cut it.

With what we call our "hot water bottle effect" (with tree trunks being warmer because they have a larger volume), I started thinking about if we went back to the forests the way they were 100 years ago. When the trees were really big, how much surface area does a large tree trunk have compared to a small one? They were at least 3 feet in diameter 100 years ago. Now they're somewhere between maybe 8 inches and 1.5 feet (and there are a few 2 foot diameter trees). But if you calculate the difference in area of a large versus a small diameter tree the area is enormously greater. So monarchs would have had the advantage of this 'hot water bottle effect' to a much great degree. Here is another very, very strong argument for conserving an old growth forest and promoting the return of these old big trees.

But anyway, we went to an area on Pelon which had a colony last year. It's in a marshy spot on the south side of Pelon. This really fascinated me because if you walk up to Pelon there's this huge open field called a llanos (which means big open area). It's more than 15 acres in extent and, when butterflies are in their colony on west side of this big flat llanos, they'll fly by the millions eastwards. I always wondered where they were going. It's like a blizzard of butterflies flying across that llanos, and I think they were going to this wet area in order to drink water.

[Chuckles as he begins...] We had this Ford Explorer that's pretty old. It was supposed to be 4-wheel-drive but the 4-wheel-drive wasn't functioning very well. But I drove 5 of us up on this road which got increasingly rutted to the point where there were 3-4 foot-deep ruts. We were actually able to make it to the top which really surprised me without getting hopelessly stuck in the ruts. But anyway, it was very exciting to see the place where monarchs have overwintered in the past—where they were last year—and where there probably is a critical place for them to get water as the dry season advances from January through March.

One of my objectives on this trip was to see what the status of the forest is on the ground because we've been doing all this aerial reconnaissance. Chivati-Huacal was one of the major colony areas before 1986 when they cut it down. There's one little area of forest left on the southwest side and monarchs have been hanging on in that area. It was really interesting because we found the original guide that had gone up there with Calvert, Pedro Silvea, a nice old guy. He took us up and we found the colony and also walked through all this devastated forest.


Lincoln Brower, Sweet Briar College

Five people fit inside the remnant trunk of this giant-sized tree. The monarch's forest had huge trees many years ago and they offered the butterflies more protection. (More...)

Lincoln Brower, Sweet Briar College

Monarch butterflies clustering on the boughs and the trunks of Oyamel fir trees. (More...)

More pictures of trunk clusters from Dr. Brower's collection.

Lincoln Brower, Sweet Briar College

Lincoln Brower, Sweet Briar College

Lincoln Brower, Sweet Briar College