Amphibians
Leopard Frog
Lithobates pipiens and relatives · also called meadow frog, grass frog
The leopard frog is the frog a whole generation of students met on a dissection tray. Slim, green or brown, and scattered with dark halo-ringed spots, it was once so abundant in northern wet meadows that it was netted by the million for classrooms and laboratories. Across much of the western half of the continent it is now hard to find at all.
Overview
Leopard frogs are a cluster of similar North American species — northern, southern, plains, Rio Grande, and others — built on the same slim, long-legged plan with dark rounded spots and a raised ridge of skin down each side of the back. They are carnivores of pond and meadow edges, hunting any small animal they can swallow, and their tadpoles graze algae. The names meadow frog and grass frog come from their habit of feeding in damp grassland well away from water.
The dissection frog
For most of the 20th century the northern leopard frog was the default animal for school and university dissection and physiology labs, taken from the wild in huge numbers. That collecting, on top of everything else, helped thin out many accessible populations. Leopard frogs are also the athletic jumpers used in county-fair frog-jumping contests.
Winter underwater
In the north, leopard frogs overwinter on the bottom of ponds and streams that stay oxygenated, breathing through the skin for months in near-freezing water. They cannot survive in oxygen-poor mud, so they need good permanent water as well as temporary breeding pools.
Status
The northern leopard frog is Least Concern overall but decreasing, common in the east and largely gone from the west. Some southwestern species, such as the Chiricahua leopard frog, are federally threatened. The drivers are wetland drainage, the chytrid fungus, a water mould that kills eggs, introduced fish and bullfrogs, and agricultural runoff.
Leopard Frog: common questions
What is the difference between a leopard frog and a pickerel frog?
They are close relatives that share the slim build, the pale-edged back ridges, and a spotted pattern, and they often live in the same wetlands. The clearest difference is the shape and colour of the spots. A leopard frog has rounded spots with pale halos, scattered somewhat irregularly, and yellow or white on the underside of the thighs. A pickerel frog has squarish spots arranged in two fairly neat parallel rows down the back, and bright yellow or orange on the hidden surfaces of the thighs that flashes when it jumps. The pickerel frog also has mildly toxic skin secretions that make it distasteful to many predators and can harm other frogs kept in the same container, whereas leopard frogs are not toxic.
Why have leopard frogs become less common?
Leopard frogs were once one of the most abundant vertebrates in northern wetlands, so their decline has been striking. Several pressures act together. Wetlands have been drained for farming and building, which removes breeding sites and the damp corridors frogs use to move between them. The amphibian chytrid fungus and a water mould called Saprolegnia that attacks egg masses have caused die-offs. Introduced sport fish and American bullfrogs eat tadpoles and adults in waters that were once frog nurseries. Decades of large-scale collecting for the dissection and biological-supply trade removed adults from many accessible populations. Pesticide and fertiliser runoff adds chemical stress. In the east the frog is holding on, but across much of the west it is now scarce.
Are leopard frogs the frogs used in jumping contests?
Usually, yes. The frog-jumping competitions held at agricultural fairs in North America, and especially the Calaveras County Jumping Frog Jubilee in California inspired by Mark Twain's 1865 story, are typically run with American bullfrogs or leopard frogs because both are large-legged, athletic jumpers that are easy to obtain. A leopard frog escaping a predator jumps in a rapid zigzag of leaps many times its body length, heading for water, and a startled frog on a contest stage does much the same thing. Contest frogs are generally released afterwards, and some events now require using locally caught frogs that are returned to the wild.
Where do leopard frogs go in winter?
Northern leopard frogs hibernate underwater. In autumn they move to ponds, lakes, or slow streams that are deep enough not to freeze solid and that keep some oxygen in the water through winter, and they settle on or just under the bottom sediment. Their metabolism drops to almost nothing, and they take in the small amount of oxygen they need directly through their skin. They do not bury deeply in mud, because mud holds too little oxygen and a frog can suffocate there during a long winter. Getting this choice of overwintering site right is one reason the species needs good-quality permanent water as well as the temporary pools it breeds in.