Melanism in Monarchs: The Use and Importance of Museum Collections in Scientific Research

Original Paper: Echevarría Ramos, M. and C.M. Hulshof. 2019. Using digitized museum collections to understand the effects of habitat on wing coloration in the Puerto Rican monarch. Biotropica51(4), pp.477-483. DOI: https://doi.org/10.1111/btp.12680

Featured Image Source: A monarch butterfly on butterfly weed at the Lenoir Preserve Nature Center in Yonkers, New York, USA. Credit: Don Sutherland, 2010. Flickr

Have you ever been to a natural history museum and seen the animals and plants on display? All kinds of species, from mammoths to moths, are put in exhibits to be observed by admiring crowds day after day. But are these specimens just there for our enjoyment, or can we also use them to further scientific research?

The Secrets of Specimens

Aside from those on display, museums and other institutions with natural history collections often have an extensive catalog of specimens that aare kept away from the public. These specimens are organized, annotated, and documented in a way that allows researchers to find, identify, and compare specimens both in the museum’s collection and with collections at other institutions.

Why would it be useful to maintain collections that might never be shown to the public? One big reason is because of the importance of collections for scientific research. Collections can offer a chance for scientists to study aspects of a species that are difficult to analyze in the field or laboratory with limited resources. Specimens can also be loaned between institutions, and there are specific curators that have the job of maintaining these collections. This is a common practice at both small and large institutions, as collaboration and documentation are very important aspects of working with collections. For instance, the American Museum of Natural History in New York City receives about 900 research visits from international scholars and makes about 300 collections loans every year. The breadth of natural history collections in institutions around the world allows for collaboration between researchers and access to more data, so it opens the door to new studies that would not have been possible, or at least as feasible, without this network of collections.

Antique bug collection from the Horniman Museum in the UK. Credit: Andrew Moore, 2016. Flickr
Melanin in Monarchs

A group of scientists led by Mariangelí Echevarría Ramos and Dr. Catherine Hulshof decided to use some of these museum specimens to study the drivers of variation in color of the Puerto Rican monarch. In particular, they wanted to look at melanization in wings because studies have linked this to adaptations resulting from environmental change. To study this melanization, the researchers looked at 265 butterflies from a collection of monarch species gathered across Puerto Rico. They identified two contrasting monarch habitats – coastal dry forests at lower elevations and montane wet rainforests at higher elevations – and grouped specimens by habitat types, season collected, and sex. Then they digitized each specimen, which meant they took a picture of each butterfly on a grey background and included a color card and scale bar with the specimen. These methods help keep the digitization process standard across all specimens so different specimens can be compared to each other. After digitization, they measured wing size and noted wing color of each specimen as percent melanization.

What is Melanin?

Melanin is a natural pigment found in most animals and is responsible for determining color, with darker colors often having more melanin. The drivers of melanization in wings can be abiotic or biotic, with abiotic being non-living aspects of the environment, like wind and light, and biotic being living (or once living) aspects, such as plants and animals and their responses.

Research suggests that biotic factors affecting wing melanization generally result in changes in variations in the phenotype, or physical appearance of an organism resulting from the expression of its genes, across space or time. There are many biotic factors that can affect selection of wing melanization, including predation pressures relating to camouflage, aposematic signals (when an animal “advertises” to predators that it’s not worth eating), and mimicry. In butterflies, when predation is dependent on appearance, darker individuals will often be found in darker environments and open areas.

There are two main hypotheses on how abiotic changes in the environment affect wing melanism. One is referred to as the “thermal melanism hypothesis,” which is related to how animals change their internal temperatures. Butterflies are ectotherms – they are “cold-blooded” and cannot regulate their own internal temperatures, and so they rely on the environment to do so for them. This hypothesis states that darker animals are expected to occur in cooler habitats because they can heat up faster and, thus, they will reach higher equilibrium temperatures than lighter animals. Melanin also reduces cell damage from UV radiation. Based on this, the researchers expected to find darker butterflies where UV radiation is high, or at high elevations and low latitudes. This also means butterflies could become darker over time due to climate change in areas where UV radiation is increasing. Another hypothesis starkly contrasts the thermal melanism hypothesis, as it states that darker individuals are expected to occur in warm, humid regions with higher parasite and pathogen densities because melanin can enhance disease resistance. Resource limitation and environmental stress can also decrease the production and expression of wing melanization, so lighter individuals could be expected in these conditions.

Orange and brown butterflies in the Vanderbilt Museum on Long Island, New York, USA. Credit: Curious Expeditions, 2008. Flickr
What Did They Find?

Based on this knowledge, the researchers expected darker animals to be found in montane forests and lighter animals to be in lowland, coastal regions. However, the results did not recapitulate other studies which found support for the thermal melanism hypothesis. Instead, they found that only habitat had a significant effect on percent melanization, and coastal populations of monarchs actually had greater percent melanization than montane populations. This surprising finding indicated that melanization in butterflies and other species may be a result of tradeoffs between different abiotic and biotic factors, and some factors may be more important in determining melanization in different populations. Though they cannot tell based on the results of this study which specific factors led to higher percentages of melanization in these coastal monarch populations, this study emphasizes the importance of studying various species and populations around the world, and it recommends against generalizing results from one study across systems. 

In addition, without museum collections, this study would have taken a long time and may not have even been possible. In the field, there are techniques used to scan butterfly wings, but they are destructive and time consuming, so sample sizes of these studies are often low and limited by the extent of field work. New technological advances in the digitization of specimens has made it easier to standardize images and access data on specimens from collections around the world. Global collaborations increase the sample size for studies like these and makes it possible to conduct analyses on a wide variety of available data. The researchers who conducted this study emphasize the importance of using museum collections and developing higher digitization and image analysis standards for future studies.

Can Anyone Explore Museum Collections?

Though the collections not on display aren’t immediately accessible, many museums give dedicated “collections tours” to the public. These are often by appointment or special access, however, or they are held at certain times during the year. If there are collections you are interested in seeing, you may be able to visit them by attending one of these or by contacting the head of the collections department that you’re interested in. Even if you can’t get an in-person look, some museums, especially now, are offering virtual tours or have put up image of their collections online. For instance, the Natural History Museum in London offers a database of their online collections which you can peruse at your own leisure. So get out there and explore!

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Jessica Espinosa

Jessica Espinosa

Hi! My name is Jessica Espinosa and I am a PhD student at UConn in the Ecology and Evolutionary Biology Department interested in the bird conservation. I received my masters degree in Conservation Biology from Columbia University where my thesis focused on the effects of coastal pollution on the behavior and morphology of hermit crabs in Fiji. I am also a Mount Holyoke College and City Year Alum. In my free time, I enjoy reading, writing, hiking, doing martial arts, and playing music.

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