- Andy Davis
Direct evidence that captive-reared monarchs are not as good as wild ones - an UNPUBLISHED study
In the most recent blog post on MonarchScience, I highlighted two statements just released by the Monarch Joint Venture and Xerces Society that condone the practice of captive-rearing monarchs. This post appeared to be well-read and widely-discussed, which tells me that this is a topic that has some importance to readers of this blog and folks who have an interest in monarchs and their conservation. Given this, I thought that this would be a good time to show some results from a study I conducted a few years ago relating to this issue.
Before I begin, let me first say that these are results from an unpublished study I conducted. That means that these results are not yet peer-reviewed and published in a scientific journal. It does NOT mean that they are wrong. It just means that I never got around to submitting the manuscript to a journal - I recall getting slammed with other, more time-sensitive, matters when I did this project. I guess this paper just fell through the cracks then and it's been sitting on my desk ever since. However, given that this is a subject that everyone is talking about right now, I figured I'll forgo the publishing part, and just relay the results directly to the users, so to speak.
So, the original goal of this project was to directly compare monarchs that had been reared in captivity, to those reared by mother nature, to see which ones are "better". This is a really important question - recall that in the previous post about rearing, I pointed out that by hand-rearing all the eggs and larvae you find on your milkweed, you essentially bypass natural selection, and allow EVERY monarch to survive - even the lowest-quality individuals. Meanwhile, in the cruel wild world of mother nature, only the best monarchs ever make it to adulthood. Thus in theory, mother nature's monarchs should be better than those you rear in your kitchen. However, other than the studies of tagged monarchs, we really don't have hard evidence in support of this idea, until now.
I should point out here too that this project was done in a very unique way. This project compared photographs of monarchs that had been reared by citizens to those found in the wild. These days, there are so many photographs of monarchs on the internet, it seemed like it would be easier to use these rather than try to rear monarchs myself. In fact, Journey North contains loads of photographs of monarchs (embedded within the monarch observations), and this is where I obtained all of the photographs for this project. I extracted photos of monarchs from two of the categories of observations: 1) the observations of reared monarchs, and 2) observations of roosting monarchs. I figured that the roosting monarchs would be a good group to use for comparison purposes, since these represent wild, migrating monarchs. The photos of reared monarchs were taken by people who had just released a monarch they had reared. I downloaded 50+ photographs from each group (52 reared vs 61 wild). I remained as objective as possible with the downloading too - my only criteria was that the photo had to show a monarch with wings spread, and had to be in good lighting. Also, I only used photos that had been taken after Aug 1, so that each collection covered a similar time frame.
Next, let me explain what I did with the photos. Recall that the goal was to determine which monarchs are "better". To do this, I set out to compare the wing color of the butterflies in the photos!
One of my personal research interests is in determining how animal coloration is related to their health and fitness. Over the years I've researched a lot of critters with this question in mind, including our beloved monarchs. This research has led to a number of publications on the subject, which are listed below. In each of these projects, my colleagues and I examined how individual monarchs differ in wing color - by that I mean how they differ in the degree of redness. Just so we're on the same page here, you need to know that monarchs come in a variety of subtle shades of orange and red - it's really like a spectrum - going from a yellow-orange, to orange, to a brick-red color. You need to look really close at the wings to see this. One thing that all of the the monarch research has shown is that this subtle variation in "redness" is very meaningful to monarchs. Their black wing margins and veins also vary a little, but this is less important.
Based on the studies, the important thing to know here is that redder is better for monarchs. The degree of redness in males is correlated with mating success - we've done the experiments to test this (Davis et al 2007). Redness is also correlated with how much fat reserves monarchs have at eclosion (Davis 2014), which itself is a key predictor of adult longevity (fat reserves, that is) in the lab. So by extension, it follows that redness is correlated with adult lifespan. Finally, and most importantly, redness is directly correlated with flight ability - this has been shown with a laboratory apparatus (Davis et al 2012) and by examining wild, migrating specimens (Hanley et al 2013). Basically, the redder the monarch, the better its flight ability and migratory success. This result also helps to explain a curious pattern I found some years earlier, that the migratory generation appears to be redder than the breeding generation (Davis 2009). This seems to be a key feature of the migratory generation - they have dark red wings. So to sum up, redder monarchs have better fat reserves, they live longer, they mate more, and they fly better. Remember this for later. Now, full transparency here - we have no idea why redness is so meaningful. It just is.
So I guess you can see where this is going. My plan was to use the photographs to determine if reared monarchs are as red as wild ones. If so, that would tell us whether they are as fit, from an ecological standpoint, as those reared by mother nature.
Now, normally when I measure wing color in monarchs I use computer software to extract the exact color shade. However, these photos were all taken by people under different conditions, which the software doesn't like (I actually did run the software - see below). So for this part I used people - undergraduates to be specific. I had five students sit down at a computer screen (one at a time, and on different days) and look at each photo. I then asked them to score the wing color of the monarch in the photo using a very simple color scale, which is shown below. This is a figure that shows the different shades of orange that monarchs can have - I made this based on my own research. If you want, you can copy this chart and use it for your own reference - it's kind of handy.
So the student gave each monarch a score of 1 to 4, flipping through the images until all had been scored. And then the next student did the same thing, and so on.
Now, this part is very, very, very, very, very (how many verys is that?), VERY important - the students were blind to the groups - they did not know which group the photos came from, because I presented them in random order each time, and there was no way the students could see the group ID in the photo. Nor did they even know why they were doing this! I purposely did not tell them what the goal of this project was, because I wanted a completely objective assessment. I simply told them to look at these photos and score the wings of the monarchs in the photos.
For transparency, I'm going to next show two thumbnail collages of the actual photos used in this experiment.
After all of the students had assessed the wing colors, I then calculated the AVERAGE color score for each photo, which was the key piece of information for this project. Next, came the actual comparison of the two groups.
I'm now going to show you the result of this comparison in a graph below. This graph shows the AVERAGE wing color score of all hand-reared monarchs, compared to the wild monarchs. This graph represents the main result from this whole study. And sure enough, it shows that hand-reared monarchs have a lower color score than wild migrants do.
Another way to demonstrate this is by looking at the frequencies of the various color scores in the two groups. If we assume that the best colors are 3 and 4 (which the prior research demonstrates), then we can simply look to see how many of these are in each group. Doing this, I determined that 56% of the wild migrants have a color score of 3 or above. Meanwhile, 40% of the reared monarchs are a 3 or higher.
If you want, you can go back up and look at the two thumbnail collages again and see if you can tell which group tends to be more red.
And by the way, just for grins, I did use my computer software to extract the redness scores from each of the photos, and I then calculated the average score for each group - it showed the same thing. Reared monarchs are not as red as wild ones.
OK, let's do a quick recap here. Recall that these two groups represented monarchs that were either reared by hand by citizens, or reared by mother nature. And both groups were from the same time frame - late summer and fall. And, the butterfly wing colors were assessed objectively by 5 people who were blind to the identity of the monarch and even the nature of this project (it doesn't get much more objective than that). And the results showed that reared monarchs were less red than wild.
Now, let's talk about what this means. Based on all of the prior research on monarch color (showing the importance of redness), we can pretty confidently conclude that reared monarchs are not as good as wild ones, based on the shade of their wings - their orange colors are more pale, which means they will be less fat, they will not live as long, they will not mate as much, and most importantly, they will not be as good at flying. In short, they are less fit.
There is something else to consider too - remember that there is evidence from tagging data that hand-reared monarchs have a much lower rate of recovery at the overwintering sites than wild monarchs (Steffy 2015). This implies that reared monarchs have a lower migration success rate. These new results concerning wing color are also telling us (indirectly) that reared monarchs have a lower flight ability and migration success. The fact that these two studies arrive at the same conclusion is significant.
Now, before I end here, let me explain that the study here used a random subset of reared and wild monarchs to compare. There was a statistically significant difference found, which means the groups tended to be different, on average. It doesn't mean that all reared monarchs are less red, it means they tend to be, on average. In either group, there were some pale monarchs and very red monarchs - just more red ones in the wild sample. So, if you reared a monarch that looks red to you, don't use that one butterfly as evidence that this study is wrong. Also, I would hesitate to guess that if you reared that monarch, then you are probably not objective regarding its redness.
The moral of this story is that reared monarchs are a poor substitute for the real thing. As I indicated in the last blog, mother nature knows best.
References of papers cited above:
Davis, A.K., Cope, N., Smith, A. & Solensky, M.J. (2007) Wing color predicts future mating success in male monarch butterflies. Annals of the Entomological Society of America, 100, 339-344.
Davis, A.K. (2009) Wing color of monarch butterflies (Danaus plexippus) in eastern North America across life stages: migrants are ‘redder’ than breeding and overwintering stages. Psyche, 2009, DOI: 10.1155/2009/705780.
Davis, A.K., Chi, J., Bradley, C.A. & Altizer, S. (2012) The redder the better: wing color predicts flight performance in monarch butterflies. Plos One, 7, e41323. doi:41310.41371/journal.pone.0041323.
Hanley, D., Miller, N.G., Flockhart, D.T. & Norris, D.R. (2013) Forewing pigmentation predicts migration distance in wild-caught migratory monarch butterflies. Behavioral Ecology, 24, 1108-1113.
Davis, A.K. (2014) Intraspecific variation in wing colour is related to larval energy reserves in monarch butterflies (Danaus plexippus). Physiological Entomology, 39, 247-253.
Steffy, G. (2015) 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. Annals of the Entomological Society of America, 108, 718-728. ***************************************************************************************************************************
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