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Monarch Adaptations: Migration and Mating

Posted on Sunday, January 1, 2006 at 3:27 pm in Monarch Biology

Some adaptations, once they appear in a population, quickly become successful and are asimilated in to the species' overall genotype. Here are a few adaptations that have been very important to the evolving monarch and which make Danaus plexippus the butterfly we all know today!

Coercive Mating in Monarchs

Monarch mating attempt. Note how the female is holding her abdomen to prevent the male from coupling with her. (Photo: MITC)

(based on research by K. Oberhauser, M. Solensky, and D. Frey)

Mating behavior in monarchs presents a puzzle, and evolutionary biologists love puzzles! Monarch males either pursue females in flight or pounce on resting females, and then attempt to couple with them in struggles that can last many minutes (we’ve watched them struggle for almost an hour!). During this struggle, females show characteristic, convoluted postures with names like “abdomen curl” and “arch”. Successful attempts result when the female stops using resistance behavior or when the male succeeds in coupling with an actively resisting female. Once they are paired, copulation lasts up to 16 hours. This behavior represents a puzzle because most female Lepidoptera, including monarchs’ closest relatives, can reject courting males successfully and quickly. While other Danaid males secrete pheromones (chemical signals) and engage in complex courtship rituals, ritual behaviors and chemical cues appear to be unnecessary in the take-down strategy employed by male monarchs.

We used a cost/benefit model to suggest how this mating strategy may have evolved. First, we thought about what is the most different between monarchs and their close relatives. We decided that it was the fact that monarchs migrate and then overwinter in huge congregations. Monarchs mate in both California and Mexico colonies. The payoff of winter mating, in terms of future offspring, is probably low for both sexes; females will probably remate before they lay any eggs, and last-male sperm precedence means that sperm transferred during winter mating is likely to be superseded by subsequent males. From the female perspective, costs of winter matings (possible physical damage during the struggle, susceptibility to predation, and exposure to diseases) are probably greater than the benefits. However, these costs are unlikely to be very high; the probabilities of physical damage, predation or venereal disease are relatively low. From the male perspective, we found that skinny males with lots of wing damage are most likely to be involved in winter mating attempts. These poor condition males would probably not be able to survive until the end of winter. While winter matings have relatively low payoff, they may be the only chance for males on their last legs (or wings). We propose that male coercion in monarchs evolved in the context of overwintering. At overwintering sites, males with low prospects for future reproductive success co-occur with females that have less to lose from unwanted matings than summer females who face the pressure of needing to maximize egg-laying time. We hypothesize that, once coercion evolved in monarchs, it became a mating strategy that was used in both overwintering and summer generations, and the dependence on chemical cues was lost. 

Evolution of Migration: Why go the distance?

A cluster of migrating monarchs. (Photo: Tom Johnson)

By: Reba Batalden

Monarch butterflies east of the Rocky Mountains complete their incredible long distance migration each year, much to the fascination of scientists and non-scientists alike. But, how did this migration evolve? Monarchs are tropical butterflies that capitalized on the abundance of their host plant, milkweed, in the prairies of North America. They cannot withstand freezing winter temperatures, thus they must migrate to warmer climates.

There is some debate in the scientific community over the evolution of the monarch migration. The most commonly held view is that it developed over 10,000 years ago. Thus, as the glaciers retreated, monarchs simply flew further north each summer to newly available prairie habitat. Gradually, natural selection increased the prevalence of long distance flight behavior in the population. in addition, as the glaciers retreated northward, the southern US continued to warm into inhospitable habitat. Thus, monarchs were forced to move north through summer. These conditions likely encouraged the evolution of the monarch migration.

Another possibility is that the migration evolved more recently. The mechanisms are similar, but the increase in available habitat is due to European settlement. Milkweed thrives in disturbed areas, as is evidence by its presence in roadside ditches. With colonialism, agricultural lands bean to dominate the North American landscape as the pioneers cleared the forests. This created high light environments, perfect for the monarch host plant, milkweed. This increase in availability could also have precipitated the evolution of the monarch migration, although this is not as favored an explanation as the previous one. 

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