In the bird world, flashy colors and boisterous songs are generally reserved for males. Females tend to be comparatively dull-colored and often stay quietly hidden in the background. Charles Darwin himself noted this difference between the sexes and proposed that it was mainly caused by past evolutionary changes in males, in a process he called sexual selection—a theory still widely upheld today.

Yet, these understated female birds have a hidden past that overturns something researchers have assumed for over 150 years.

In a recent study, biology researchers J. Jordan Price of St. Mary’s College of Maryland and Muir Eaton of Drake University provide evidence that, despite current appearances, most past evolutionary changes in color have occurred in females, not males. Many of today’s drab females had brightly colored ancestors, and current differences between the sexes are largely due to females losing bright colors rather than males gaining them.

“I think Darwin would have been very surprised by this idea,” said Price. “Darwin was a strong advocate for the idea that sexual dimorphism is due to sexual selection. Many male traits undoubtedly are the products of sexual selection, as he suggested, but our evidence suggests that differences between the sexes are largely due to selection on females.”

Price and Eaton focused on a group known as the New World blackbirds, which includes a variety of songbird species ranging from Canada to Argentina. In New World blackbirds some females are dull in color compared to their male counterparts and some are just as brightly colored as males. Using relationships based on DNA sequence data, they were able to reconstruct the likely evolutionary histories of each sex, which revealed the surprising differences in the speed with which males and females have evolved.

“We mapped out differences using a phylogenetic tree to figure out what happened in the past and to look at color differences over time, and we found that although evolution occurs along the same period of time for both sexes, females have covered a lot more evolutionary ground,” said Price.

To paint the picture of the female birds’ rapid, teeter-totter evolution—away from and toward male colors—Price said to think of something many do every day. “It’s like when you are out walking your dog and you cover a distance and the dog covers three times the distance by running back and forth,” he explained.

“What’s often assumed is that sexual selection operates mainly on male appearance, and the result is that males then look different from females,” said Eaton. “Our results strongly suggest the opposite. Females, with their dull colors, are under strong natural selection to not stand out, thus they look very different from males. Perhaps this is because they spend more time on or near the nest and must be inconspicuous.”

Eaton also noted that the color differences in male and female birds go beyond what the human eye can see. “Our use of objective measurements of feather coloration, and quantification of color differences from the perspective of how birds see color differences, allowed us to uncover these complex evolutionary patterns that might otherwise go unnoticed,” he said.

The study, entitled “Reconstructing the Evolution of Sexual Dichromatism: Current Color Diversity Does Not Reflect Past Rates of Male and Female Change,” is available online today in the journal Evolution.

Professor Karen Crawford of the biology department was published in the March 2014 issue of Nature Protocols, an online journal of laboratory protocols for bench researchers, and was awarded the publication’s cover shot. Featured was a paper that she co-authored during her sabbatical, entitled “Optimized Axolotl (Ambystoma mexicanum) Husbandry, Breeding, Metamorphosis, Transgenesis and Tamoxifen-mediated Recombination.” With a title like that, we were curious to know what it all means. We recently sat down with Professor Crawford to find out.

What was your paper about?

It is a methods paper for anybody who wants to work with the model salamander system that I study, which is the Mexican axolotl.

What is the Mexican axolotl?

The Mexican axolotl is a native to two crater lakes near Mexico City and was first brought from Europe by the Spanish in the 1500s. It is an aquatic salamander that retains its gills throughout life and fails to undergo metamorphosis. It is a vertebrate tetrapod, meaning that it has a backbone and four limbs, much like we do. It is special in that it has a remarkable ability to regenerate its arms, tail, spinal cord, and parts of its forebrain. It can also regenerate its liver and parts of its heart. It does things that we can’t do as humans. Therefore, by looking at how the Mexican axolotl   regenerates at the cellular and molecular level, we can become better informed on how to enhance regeneration and healing in ourselves.

Sounds like a really remarkable animal. Can you tell us more about the paper?

What’s important about this paper is that it is the new ‘gold standard’ for how to work with this model system. People can take the fundamental methods explained and apply them to their own genes of interests, in their own labs. It’s a cookbook, if you will, on how to turn genes on, off, and insert them. With this you don’t have to mate animals to get a genetic line; you can make an animal transgenic. Anybody who publishes on salamander work, axolotl work, transgenic work, or on how to get genes into a vertebrate model system, such as a salamander or a frog, can cite this paper. So this is one of those papers that’s going to live for a long, long time and be an important, critical resource for people.

What made you want to write about this subject?

The making of the paper goes back years ago right here at St. Mary’s. I did a study with a student on regeneration and metamorphosis. And in that paper, which was published in the Journal of Experimental Zoology, we outlined a very reliable protocol on how to drive axolotl into metamorphosis chemically with thyroid hormone. It’s known that after metamorphosis happens for some animals regeneration slows down.

So how exactly does the process that you detail in your paper work?

These salamanders [the Mexican axolotl] are so precise. If you amputate them at the wrist or the elbow or the upper arm, they’ll replace just what’s missing and they’ll do it perfectly and you would never tell the difference. You can amputate the animal and allow a bud to form, called a blastima, and then you can actually remove that blastima and graft it to a different animal. You can rotate it and change the pattern of the limb that’s going to form on the animal. With this, you can start to ask really fundamental questions about how cells interact and how pattern is established. We would like to know the molecular ‘players’ that allow the salamander to do that. With this knowledge we can apply it to quiet the scarring mechanisms or slow the healing process in ourselves, which would allow for a better healing process to occur after spinal cord transection or injury.

You mentioned that this research can help humans. Can you tell us how?

In the last decade there’s been a huge resurgence in regenerative medicine, and we’re now looking at how to push undifferentiated cells like cells from your bone marrow. We can harvest bone marrow from you and then culture that marrow in a dish. It’s got your genetic marker, your genetic label. And then we can try with transcription factors and growth factors to tweak those cells into different pathways: neuro pathways, cardiac muscle pathways, skeletal muscle pathways, liver pathways, pancreas pathways. If we can do that, then we can take those cells that have your genetic makeup and put them back into you and rescue things like type 1 diabetes. Regenerative medicine right now is not just fashionable, it’s really important, and many, many labs are looking at trying to create therapeutic agents through regenerative medicine studies to facilitate better human health and disease amelioration.

Is there anything else you would like to add?

This paper empowers any lab working with salamanders, or encourages labs that aren’t working with them to work with them. The more people that are actually involved in pushing forward these studies the faster we’re going to have important outcomes in regeneration and therapeutics for ourselves.