Dark Mice on the Lava Field

Black Rock, Tan Mice

1Yusra Halloran spent a week of her summer at a desert field station in the Southwest, helping biologist Dr. Paloma Etsitty study rock pocket mice. These small mice come out at night to gather seeds among the rocks. In most of the area the rocks and sand are light brown, and most of the mice there have sandy tan fur. But a few kilometers away is an old lava flow, a wide field of nearly black rock, and many of the mice that live on it are almost black.

2Dr. Etsitty explained that fur color comes from pigment made by pigment cells at the root of each hair. These cells can make a dark pigment or a pale, yellowish pigment. A protein called a receptor sits on the surface of each pigment cell and works like a switch. When the receptor sends its signal, the cell makes dark pigment. When it does not, the cell makes pale pigment. The instructions for building the receptor are in one gene. A mutation is a change in the DNA of a gene, and a change in a gene can change the shape of the protein it codes for. Because a protein's shape decides how it works, a changed shape can change how the protein does its job.

3The team caught mice in live traps, took a few hairs and a tiny skin sample from each one and then let it go. A lab read the receptor gene from each sample. The team found three versions of the gene in these mice. Yusra looked at the hairs under a microscope and recorded where the pigment was.

4To test how fur color affects survival, the team made 160 clay mice the size of real ones. Half were painted nearly black and half sandy tan. They set out 40 dark and 40 tan models on the lava field and 40 dark and 40 tan models on light sand nearby, all in open spots. Owls and other predators that hunt by sight sometimes strike at the models. After three nights, the team counted the models with beak or claw marks.