• Andy Davis

New evidence that migration keeps monarchs from inbreeding

Hello blog readers,

I have a very cool new study to talk about today, and it's all about sex - monarch sex! Yes, today I'll be discussing, in detail, the nitty gritty details about monarch mating, and a new paper that was just published, that focuses on inbreeding in monarchs! Or I guess you could say that today I'll be talking about the birds and the bees of monarchs (ouch).

Before getting started, let's start with some basic "birds and bees" stuff, in case any readers don't know how this works in monarchs. Below is a picture of a male and female that are, ahem, in the middle of things...

This is a screenshot from a youtube video, that I encourage everyone to check out - https://www.youtube.com/watch?v=8pR79T0ZfD4. It appears to be an amateur video where someone filmed a successful mating event in their yard. The footage is excellent, and there are even some factoids throughout.

I'm sure most readers of this blog have seen this happen at some point too. In the picture above, the male and female are attached at the back end - the male private parts includes a clasper that connects to the female. Once they are together, the male basically carries the female around for up to a day - that's how long it takes for them to finish! But let's rewind a bit, because what happens before this is important for our discussion today. It is the male that actually initiates things, and with monarchs, this is a little disturbing. While many males in the animal kingdom seek to entice their potential mates with gifts, elaborate dances, or other displays, male monarchs simply pounce on potential mates. They will literally grab them out of the air, or a tree, and fall to the ground with them, all tangled and flailing. There is usually a struggle on the ground, and then finally, they pair. All of this is clear from the video I mentioned above too. This is the way monarchs do the deed. In research parlance, we call it "forced copulation." I know, no matter what you call it, it sounds disturbing when described.

The interesting thing here, is that even though it looks like the males are the ones who are "choosing" which females to tackle, if you look close during the wrestling part, you can see that females do have a say in the matter too. They can curl their abdomen up, and essentially refuse the male's advances, and wait for him to give up, or they can just out-wrestle him. In other words, the females could also be "choosing" males based on some cues they perceive while struggling. So they can choose to resist, or just go along with it.

From a scientific perspective, we actually don't know which sex is doing the choosing with monarchs, and this has long been an unanswered question in the research world. Researchers have looked at this question in many ways, and in the lab, and even at the winter colonies (there is lots of mating near the end of the winter phase - I'll come back to this). I don't think we've ever really figured this out, although I did work on a project once that found evidence that male color seems to matter. Another story...

OK, let's talk about this new paper. It was just published in the journal, Behavioral Processes, and here is a link. The study was conducted at Emory University, in the lab of Jaap de Roode, and was spearheaded by a postdoc there, Dr. Scott Villa, who is the lead author. The paper is titled, "Lack of inbreeding avoidance during mate selection in migratory monarch butterflies." And really, this title states the entire take-home message of the paper.

The researchers here wanted to find out if monarchs "know" when they are mating with their siblings, and if they show any sign of avoiding this, for obvious reasons. In prior work from this same lab, they had monitored what happens if monarchs do inbreed (this is an importance scientific question). Inbred monarchs don't turn out to be deformed or anything like that, but they did find that inbred monarchs tend to have shorter lives, and inbred females lay about 25% fewer eggs. And this was just from the few traits the researchers did measure, and there are probably more deleterious effects. So, it seems like in theory, monarchs should want to avoid this whenever possible.

Here's the thing, if anyone has watched monarchs in a cage, or in the field for a while, you'll know that a lot of the time, males don't seem very discriminatory. Males can be seen pouncing on other males sometimes, or even pouncing on other butterfly species! So, given that, it almost seems like they aren't too bright to begin with! Some would say this is typical of us males...

Anyway, the paper describes a series of very interesting and elaborate experiments that were designed to test if males or females actually avoid inbreeding. I'll just cover the beats here. These were all done in a greenhouse, and an environmental chamber, and using monarchs reared specifically for this. The researchers used wild-caught males and females and had them mate and lay eggs in captivity (on swamp milkweed), then these offspring were reared to the adult stage and numbered (on their wing) to keep track of who is who. This is a standard procedure in laboratory research with butterflies. It sounds like they reared several hundred monarchs for their experiments, so sample sizes were quite good.

The experiments included separate tests for males and females, to see if either show any sign of inbreeding avoidance. Recall that both males and females have a degree of choice. In one experiment they placed a focal monarch (a male or a female) in a standard netted cage, along with a monarch that was not related to it, plus one that was its sibling. They then watched to see what happens over the course of a few days, and they kept track of who mated with who. There was another round of experiments where they placed focal males or females in cages either with only siblings, or with only unrelated monarchs, and they compared the mating outcome of either scenario.

I'm going to cut to the chase next. This is a direct quote from the paper - "In the female choice experiment, the first mating in the 44 mixed relatedness trials was effectively random, where 52% chose their brother, and 48% chose the unrelated male." Also, "we found no significant differences in mating performance between cages with only siblings and cages with only unrelated monarchs."

For the male experiments, the results were similar. In these trials, 45% chose their sister, and 55% chose the unrelated female.

There were other results too, but the conclusion from these experiments was exactly what the title of the paper indicated - that monarchs don't seem to be able to tell if they are mating with their brother or sister. Or if they can tell, they don't seem to care.

There was another interesting, yet unrelated, finding from the experiments. The authors reported that there was a tendency for females to be more accepting of smaller males, regardless of relatedness. This is quite intriguing. I recall hearing about a related finding years ago from the mating period in Mexico, where a researcher made a similar observation - smaller males seem to be more successful. At the time, not much was made of that finding, but now it looks like this is a real thing.

Anyway, the rest of this paper has an interesting discussion of how these results fit in to the bigger picture with monarchs and the monarch population. The authors describe how the migration in North America is under threat right now, for a variety of reasons, but they specifically discuss the growing issue of "residency" - where monarchs are forming permanently-resident pockets in places like Texas, the Gulf coast, Florida, and southern California, and essentially choosing (or being enticed) not to migrate. This is being driven largely because of the growing problem of tropical milkweed, which remains in leaf year round (if not cut back), and this entices monarchs to stick around.

Here's the thing - the researchers point out that monarchs probably haven't evolved any means of discriminating their relatives, because they have never needed to. Their long-distance migration, which has been ongoing for eons, has always served to "mix" the population, and disperse the monarch relatives far and wide. If you think about it, by the time the monarchs have travelled down to Mexico, then their progeny has made the return trip back, there is very little chance that when a when a male and female monarch meet up in the next summer, that they will be related to each other. In this way, the migration basically protects the monarchs from inbreeding.

Now you can see why this would matter for resident monarchs, or those that do not migrate. In places like southern California for example, there is a growing non-migratory cohort of monarchs that hang around people's backyards and neighborhoods. With no migration to disperse them, there is a strong chance that these resident males and females will be related to each other, perhaps not as direct siblings, but maybe cousins. The same would be true for Florida.

I guess you could add this to the (growing) list of beneficial things that the migration does for our monarchs in North America. I've blogged about these other issues too - it keeps the population healthy of disease, it improves the gene pool, it leads to larger body sizes, and now it looks like it prevents inbreeding too.

Kudos to these researchers for a fine study.

That's all for now.


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