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  • Andy Davis

Why the winter storm in Mexico is not all that bad for monarchs

Hello folks,

This week there was a major winter storm in central Mexico that appeared to encompass the monarchs' overwintering sites, and that's all everyone is talking about. By all accounts, the temperatures dropped below freezing, and snow and ice fell on the sanctuaries and surrounding towns. The photo above is one of many that were taken by locals and which have been circulating around cyberspace this week. It shows monarchs clustered on a tree trunk with snow on it, which illustrates the cold temperatures and freezing precipitation.

This event has understandably generated a lot of concern - there has been a flurry of emails on dplex, many facebook postings, and I've seen a few news articles. As one might expect, the tone of these comments and posts are of concern and worry - even prayers for the monarchs. People are worried that this storm will cause a great deal of mortality. But in this post I'd like to point out some interesting observations that could emerge from this storm, and which should be discussed equally along with the worry.

First, the bad news - yes, a lot of monarchs will die in this storm - either from the sleet, snow, or from getting wet and chilled. It's not clear how many will die. Some scientists have estimated between 30 to 50% mortality. If that's the case this will definitely be a major mortality event, as that would translate to millions of deaths, and monarch bodies everywhere at the sanctuaries. In fact, I expect in the next few days to see pictures circulating of the forest floor littered with dead monarchs.

From a different perspective, this event reminds us that extreme weather such as drought, floods, and cold can offer an opportunity to study these as selection events operating on natural populations. In other words, while many monarchs will die, we can also ask which ones survive? Is it the largest ones that had ample fat stores to buffer against the cold, or ones that had some type of biochemical antifreeze in their bodies? Are the monarchs that are roosting deepest within the clusters more protected from the cold, or are those roosting centrally as opposed to the periphery of the colonies more likely to survive? If these traits are heritable and can be transmitted from parent to offsping, then this selection event could possibly lead to future generations of more cold-tolerant monarchs, which might be important if the frequency or severity of winter storms were to increase in the future.

To put this another way, mortality is an important part of the process of natural selection, which likely operates on monarchs in every generation. As I've talked about in past blogs, we have good scientific evidence that natural selection has shaped the monarchs' wings and bodies to increase their flight performance during their long-distance journey. Their epic long journey selects for individuals with larger, more angular wings that are more likely to make it to Mexico. What is happening now at the overwintering sites could be part of a similar process if monarchs that survive the freeze produce more freeze-tolerant offspring in future generations. As the earth warms, climate scientists predict that we will see more frequent extreme weather events such as winter storms.

There is another reason to temper concern about the impacts of this storm. As many monarch enthusiasts remember, there was a previous winter storm at the colonies back in January 2002 that killed an estimated 80% of the monarchs. At the time, this was major news and there was a lot of worrying, praying, etc, just as there is now. But owing to favorable spring and summer conditions, the numbers of monarchs later that year was not abnormally low, but rather close to average. If you look at any population graph from the MonarchNet data page (, and go to the Data Tool page), it is very hard to see an effect of that 2002 winter mortality on the summer (or fall) numbers that year. In other words, the monarchs rebounded from the winter mortality during their breeding period, which makes sense given that a single female monarch can lay several hundred eggs. To be fair, monarch scientists feel that the conditions that summer were very good for breeding, which might not always be the case, and if monarchs had encountered a cold, wet spring or summer drought their numbers might not have rebounded as readily.

So here are a couple of bullet points to sum up this post:

- as a scientist, I think the winter storm offers the opportunity to ask interesting questions about monarch biology

- Many large-scale mortality events in animal populations represent natural selection events that might increase the frequency of traits that protect against future mortality

- winter storms are known to happen to monarchs, and monarchs can rebound quickly under favorable breeding conditions

Let me end by saying that I think we should refrain from too much worrying until more data are provided on the extent of the winter mortality, and we should also keep in mind that monachs have weathered similar storms in the past, and this might be an important part of their natural cycle.



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The science of monarch butterflies

A blog about monarchs, written by a monarch scientist, for people who love monarchs

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