Brighter Future for Yellow-legged Frogs
The mountain yellow-legged frog Rana muscosa has declined to a population of less than 200 adults in southern California. The future of the mountain yellow-legged frog is not all doom and gloom, though, and we have made significant progress towards recovery of the species.
In 2006, a group of tadpoles were collected from the wild and were housed in two small tanks in the corner of our ecology lab. As the population of tadpoles began to metamorphose into hungry frogs we needed more and more room as they began overtake the entire lab. Today the frogs are housed in a dedicated amphibian conservation laboratory at the Arnold and Mabel Beckman Center in Escondido.
Our primary goal with the captive colony is to produce captive bred offspring for reintroduction efforts. Achieving successful breeding was a challenge and we had limited reproduction in the initial years of breeding.
In order to improve reproductive output I designed an experiment to test the effect of hibernation on breeding. A subset of frogs was hibernated for 60 days in modified beverage coolers. Immediately after pairing the frogs together following hibernation I was excited to find that the hibernated males were making advertisement calls to entice the females and soon all the hibernated couples were in amplexus (fancy word for amphibian love hug). Six clutches of eggs were laid within two days. I was so happy to solve the fertility problems of our frogs because, with a supply of captive-bred animals, I was able to shift my focus to our field reintroduction efforts.
In order to increase the probability that mountain yellow-legged frog reintroduction efforts are successful, I conducted research to understand more about the natural history of the frogs in the wild.
For example, why are frogs only found in certain sections of a stream and not in others? Are there certain habitat features that allow the frogs to thrive in certain areas of streams? To answer these questions I hiked the remote mountain streams where the frogs are found in order to determine where the frogs occur and why they occur there. I found that frogs prefer living in deep pools with a lot of leaf litter and an open canopy. The acquisition of this information allows me to make informed decisions about which stream areas are best suited for frog reintroductions.
The mountain yellow-legged frog is found in beautiful habitats and I am always excited to conduct my research in the frogs’ mountain stream habitat where colorful butterflies and wildflowers add a hint of color to the lush green landscape. This year’s reintroduction efforts are focused on the release of juvenile mountain yellow-legged frogs that were raised in captivity through the tadpole life stage. In the wild, tadpoles have naturally high level of mortality because the streams are teeming with aquatic predators such as dragonfly larvae. By releasing juvenile frogs I hope to reduce predation levels following reintroduction.
To further improve the survival of frogs I am also testing the benefit of holding frogs in acclimation cages prior to their release into the stream. One group of frogs will be held in acclimation for 30 days, another for 15 days and another group will be released without acclimation. In order to measure the benefit of these different treatments we will track the movement and settlement patterns of the frogs after they are released in order to determine if there is a benefit provided by the use of acclimation cages.
With all of this exciting research I am optimistic that the mountain yellow-legged frog will once again thrive in the mountain streams of Southern California.
Frank Santana, Research Coordinator, Applied Animal Ecology
San Diego Zoo Institute For Conservation Research