I camped at Teal Lake in the Park Range to photograph fall colors, but I also enjoyedthe ospreys that were hunting trout and the numerous leopard frogs at the lakeshore. TealLake is like many other lakes, ponds and marshes in that the leopard frogs, Rana pipiens (orLithobates pipiens), come in two colors, brown or green. This color polymorphism is foundthroughout the range of the species, from Hudson Bay to the Southwest, and from easternCalifornia through the upper Midwest to New England. The persistence of two colorseverywhere suggests a balance of evolutionary forces and raises the question: Does it makeany difference whether a leopard frog is brown or green? Phrased another way, are brownand green frogs adapted to different environments, or circumstances?
Color in leopard frogs is determined by a gene that has two forms, or alleles, G andg. Each frog has two alleles, and frogs with genotypes GG or Gg are green, while gg frogsare brown.Many subspecies of leopard frogs have been described, and in general subspeciesthat are geographically close are able to hybridize. However, as crosses between moregeographically distant subspecies were performed, reproductive success dropped to zero,indicating that they were reproductively isolated. This pattern of mating success isindicative of local adaptation of populations, so that frogs from similar and nearby habitatsare able to hybridize, but frogs from very different environments, such as New Mexico andVermont, perform as distinct species, unable to produce hybrid offspring.For many decades, biologists have wondered whether geographical patterns in thefrequencies of brown and green frogs would provide any insight to the adaptivesignificance of the color polymorphism. But studies of geographic patterns of thefrequencies of brown and green did not yield interpretable patterns. Brown and green donot show simple patterns from north to south, or from east to west, or with elevation, orwith habitat.Paul Stephen Corn, for his Ph.D. thesis research at Colorado State University, testedthe hypothesis that developmental rates differed between brown and green frogs. Hecompared data from Larimer County, where populations contained from 24% to 68%brown frogs. He studied developmental rate in two ponds, Prairie Divide (30% brown) andRed Mountain (68% brown) by measuring the time from mating to the time a frog wasmetamorphosing from a tadpole to a frog. He then compared developmental rates of brownand green frogs in both ponds. In Prairie Divide Pond, he saw a tendency for brown frogs todevelop faster, but the results were not quite statistically significant. In contrast, brownfrogs developed much faster than green frogs in Red Mountain Pond, and this result washighly significant.Why did these ponds have such disparate frequencies of colors, and why diddevelopmental rates differ in one pond, but not in the other? These data were difficult tointerpret until Corn considered two additional observations. First, developmental time was 16 days shorter at Red Mountain, where brown frogs developed faster than green. Second,in the data for Larimer County, brown frogs were more common in ponds with either troutor tiger salamanders, but green frogs were more common in ponds without predators.Vertebrate predators, such as trout and tiger salamanders, eat tadpoles and therebycreate an advantage for frogs with shorter developmental times — higher larval survival.Ponds with predators and selection for fast developmental time have higher frequencies ofbrown frogs, which develop faster and thus have higher larval survival.If brown frogs have such an advantage, why aren’t all frogs brown? Green frogsmust have an advantage in some environments, or in some circumstances. In placeswithout vertebrate predators, the frogs dawdle through development and in these ponds Isuspect that green frogs will reach slighter greater size because they have a tendency todevelop slower. Larger frogs would produce more eggs and therefore have a fecundityadvantage. In the six ponds in Larimer County, green frogs predominate in the absence ofpredators and brown frogs predominate where predators consume frog tadpoles. Note thatthe advantage of green frogs in environments without predators is my hypothesis — itneeds to be tested experimentally.At Teal Lake, which has a healthy population of trout, brown frogs are morecommon than green. Now we know why.