Take a close look at the photo above, and look closely at the monarchs, which are presumably all migrants. This picture was taken by a keen Journey North observer this week in Michigan who made a very interesting observation - of all the monarchs seen that day (the observer estimated 200), very few of them were females. The actual field observation from the Journey North website is here, and you can link here to see a full-size image. It's hard to know the exact numbers since not all of the monarchs were counted and identified to gender, but it sounds like there were fewer than 15% females. That's crazy. Especially when you consider that whenever people rear monarchs in their homes, or when we rear them in the lab (even in the fall), the sex ratio is always pretty much 50-50. And in the summer when you catch breeding monarchs, the sex ratio is also pretty even.
This is actually not a new thing and it's something I've been following with interest for a few years now. For some reason, when you look at collections of migrating monarchs (either tagged ones or from photos like the one above), you see far more males than females! In fact, in most large collections of migrants these days, males outnumber females by 3 or 4 to one. What makes the Michigan observation even more interesting is that the ratio there was more like 10 to 1. As scientists, we have no idea why this is ocurring, or even if it's a bad thing! With observations like this one though, maybe it's time we looked into this further.
One thing that we do know is that whatever is happening to female monarchs during migration, it's happening more so now than it used to. A few years ago, I worked with Eduardo Rendon, of WWF Mexico (who coordinates their overwintering program) on a paper that described how the sex ratio of monarchs overwintering in Mexico has been slowing shifting in the past 30 years, going from 55% females in the late 70s to 45% females in recent years. We also reviewed the scientific literature on migrating monarchs, and found a similar trend was happening - in the last decade, males far outnumber females in migratory collections.
Here is the main figure from our paper, showing the decline in females at the overwintering site:
This paper got a fair amount of press, of course because it had the word 'declining' in the title:
Davis, A.K.* and E. Rendon-Salinas. 2010. Are female monarch butterflies declining in eastern North America? Evidence of a 30-year change in sex ratios at Mexican overwintering sites. Biology Letters 6: 45-47.
I know what you're thinking, and we already have an answer - this trend does not seem to be because of different 'catchability' of males vs females during migration or at the overwintering sites. In a paper I did in 2010 with Billy McCord, a dedicated tagger from North Carolina who has loads of tagging records, we found that only 35% of his 10,000+ tagged monarchs were females, and importantly, that number was the same whether you looked at monarchs he caught flying, or caught roosting. So because females still were scarce in roosting clusters, this ruled out the catchability thing. In addition, previous researchers also showed females do not preferentially roost on different spots on trees than males do in overwintering clusters in California.
One thing I find interesting about this Michigan observation is that it came from a location near the upper end of the flyway, where the migration essentially has only just begun. This is very important, since it helps to figure out what's going on, or at least, to rule some explanations out. At the time I was working on this issue, I had an idea that maybe the females simply experience higher mortality along the flyway than do males (for unknown reasons). But, since these monarchs in the picture had presumably not been travelling for long, there should have been little mortality experienced thus far. In other words, if my idea held water, then you would see a 50-50 sex ratio near the upper end of the flyway, and a shrinking proportion of females as you go southward along the route. So I think the Michigan observation rules this out. In fact, it not only rules that explanation out, it really suggests that whatever is happening to the females, is happening before the migration even begins - like when the migratory generation is developing, perhaps.
One other thing to consider is that male and female monarchs do have inherently different flight ability (not flight behavior, per se). In a project I worked on a few years ago where we tracked flight performance of monarchs on a flight mill (basically a butterfly 'treadmill'), we found females tended to fly for longer stretches than males - link here to see the paper. That COULD mean that females don't need to stop as much as males to rest, and therefore we'd see fewer of them at ground level during migration. However, this inherent flight difference between males and females would presumably have been always present (i.e. ever since monarchs have been migrating), and it would not explain why this is a more recent phenomenon.
A more ominous explanation for this may be some inherent male-female differences in suceptibility to diseases and parasites as larvae. This is a stretch, but female caterpillars could be more suceptible to these natural enemies, and thus they may not survive to become adults in the migratory generation. We have a little bit of data from various laboratory experiments that supports this (females can differ in immune function and/or parasite load), but not a lot. We do know though that rates of OE always tend to be highest in the late-summer, so if this was the issue, it makes sense that it would have the strongest effect in the migratory generation. And, we also know that rates of OE have been increasing in recent years...hmmm...
As you can see, there are a lot of unanswered questions surrounding this very wierd phenomenon. I don't know of any scientists who are currently or planning to work on this question either, so this is something that may remain a mystery for a while. Until then, folks should continue to submit their observations of males and females to Journey North - one of my favorite citizen science programs - so that these can help us (scientists) figure things like this out!