A Gradient Written in Genes
Along the scrublands and Pampas grasslands of South America lives Leopardus geoffroyi, or Geoffroy's cat, a small wild feline roughly the size of a domestic cat. Its coat is typically a gray or tawny-yellow fur covered in small, black spots, providing excellent camouflage in its native habitat. Yet, this cat displays a remarkable pattern of variation. In the northern parts of its range, such as Bolivia and northern Argentina, nearly all Geoffroy's cats have this spotted coat. But traveling south through the grasslands of central Argentina, a dramatic change occurs: a progressively higher percentage of the cats are entirely black.
This phenomenon is a clinal gradient of melanism. The frequency of black-furred, or melanistic, individuals increases steadily with latitude. In the humid, open grasslands of the southern Pampas, populations can be composed of more than 70% black individuals. This change is not random. Genetic investigation revealed that melanism in Geoffroy's cats is caused by a mutation in the Melanocortin 1 Receptor (MC1R) gene. This is a different gene than the one responsible for melanism in domestic cats, which is a mutation in the agouti signaling protein (ASIP) gene. The independent evolution of the same trait through different genetic pathways in separate species points to strong environmental pressures at work.
The Purpose of Darkness
The southward increase in black fur is not an accident; it is the result of natural selection. Scientists have proposed several hypotheses to explain what advantages a black coat might provide in the southern portion of the cat's range. One possibility is thermoregulation. The colder, more temperate climate of southern Argentina could favor cats with black fur, which is more efficient at absorbing solar radiation and retaining heat.
Another strong possibility relates to camouflage. The environment of the southern Pampas is different from the arid northern scrublands. The southern grasslands are often wetter, with more open spaces and different light conditions. In this habitat, a solid black coat may offer superior concealment for these nocturnal and crepuscular hunters, making them less visible to their prey, which consists of small rodents, birds, hares, and even fish. Finally, some evidence suggests that the genes controlling pigmentation can also influence the immune system. It is possible that melanistic cats have a greater resistance to certain pathogens prevalent in the more humid southern environments. The combination of factors, the gradient shows evolution, adapting the appearance of a species to the specific demands of its environment.