- Andy Davis
Thoughts on Gayle Steffy's new study on monarch tagging
We're back already! As mentioned in the previous post, the collection of monarch papers that just came out in the Annals of the Entomological Society contains a wealth of new information on monarch biology. One of those papers was a neat study conducted by a very dedicated monarch tagger - Gayle Steffy. Gayle's paper was titled "Trends Observed in Fall Migrant Monarch Butterflies (Lepidoptera: Nymphalidae) East of the Appalachian Mountains at an Inland Stopover in Southern Pennsylvania over an Eighteen Year Period", and there are some things about this paper that deserve mention.
First, kudos to Gayle for putting this together. Gayle is a long-time independent tagger of monarchs, and someone who has some background in science writing. By all accounts, this was a difficult climb for her to get this paper ready for prime time. She had to do all of the statistical analyses and writing herself, and it was her first paper. But more importantly, a real kudos to her for being so diligent with her monarch tagging activities. Over the years, she kept detailed records of all of the monarchs she tagged, and she even went above and beyond and measured the monarchs she caught too, which not everyone does. Because of this, it all ended up being a very impressive data set.
What the paper was about was trying to figure out which of the monarchs she tagged were most likely to be recovered in Mexico (we sorely need more papers like this!). She did some cool analyses breaking up the fall migration into three time periods, early, middle and late. She found that of all the monarchs she captured and tagged, those tagged in the early part of the fall were by far the most likely to make it to Mexico. Those migrating late had a terrible recovery rate (i.e. none made it!). So the message from this is that the front-runners of the migration are the fittest monarchs, and the stragglers are most likely to perish during the journey. This bit is consistent with some other studies, so it makes sense.
Another very interesting thing Gayle did was to compare the recovery rate of her wild-caught tagged monarchs to those that she reared in her house and subsequently tagged. This was the first-ever such comparison in the scientific literature, and it's a very big deal. A HECK OF A LOT of people rear and tag monarchs, and if you're one of them, you should listen up right now - Gayle found that those reared monarchs had an absolutely terrible Mexico recovery rate - only 2 monarchs out of 3000 reared ones ever made it. This is compared to 50 monarch recoveries out of 11,000 wild ones. In other words, reared monarchs were about 20 times less likely to reach Mexico than wild ones. Why was this? Gayle did find that her reared monarchs were significantly smaller than the wild monarchs, and wing size is a sure predictor of migration success. She also speculated that the reared monarchs came out later in the fall, and the late ones, as mentioned earlier, have a terrible time reaching Mexico.
So what can we learn from Gayle's study? Well, rearing monarchs for release into the migratory pool probably doesn't have as much of a positive effect as people might think. Another way to think about this is that if you really want to have one of your reared monarchs be recovered in Mexico, you need to release about 1500 to make that happen! An even more stingy interpretation is that reared monarchs are not-well-suited for release into the migratory generation - they just don't have a good migration success rate. This may not be what people who rear monarchs want to hear, but it's hard to argue with Gayle's 18 years of data.
In the end, this paper should end up being a valuable contribution to the monarch literature.