The official website of the Elkhorn Slough Foundation and Elkhorn Slough National Estuarine Research Reserve
Elkhorn Slough Amphibians: American Bullfrog

Scientific Name:
Rana catesbeiana


Found at the Slough:
Permanent freshwater ponds

Did you know...
No American Bullfrogs have been seen at the Reserve since 2004. Of the Elkhorn Slough Foundation properties, very few ponds have this invader!

Adult American bullfrogs can be distinguished by their size – they are the largest frog in North America. Their tympanum (ear drum) is generally larger than their eyes and they lack the prominent ridges (dorsolateral folds) that run down the sides of our other sizable local frog, the California red-legged frog (Rana draytonii). Thier call is strikingly similar to a mooing cow, as their name suggests. "Jug-o-rum" is what some people hear when the adult male is calling for a mate. Juvenile bullfrogs make a cat-like meowing sound when they are startled and jump into the water.

The American bullfrog is not a native species to California but can be found in permanent freshwater ponds and lakes often coexisting with introduced fish species and crayfish. It primarily eats aquatic invertebrates and other aquatic organisms, including their own young and smaller frog species.

The native range of this aquatic invader is North America east of the Rocky Mountains. The bullfrog was introduced to California in the late 1800s and it now inhabits 6 of the 7 continents. It was introduced primarily because it's big meaty legs make good eating. It is listed by Conservation International as one of the 100 Worst Invasive Species and is considered a factor in global decline of native amphibians.

Not that long ago this frog inhabited the freshwater ponds of the Elkhorn Slough Reserve but happily they are no longer found there. No American Bullfrogs have been seen on the Reserve since 2004. Of the Elkhorn Slough Foundation properties, very few ponds have this invader.

Despite their widespread global invasion and apparent success in their introduced range, American bullfrogs are declining in many parts of their native range. Reasons for this decline include habitat loss and degradation, water pollution and pesticide contamination.

Elkhorn slough researchers have conducted a variety of research studies on American bullfrogs and specifically their effects on native amphibians.  They have found that invasive bullfrogs alter the way that California red-legged frogs use their habitat, that they differ in their response to human alteration of the landscape, and that they even interfere with successful reproduction of the threatened California red-legged frog.

Find out more about the Elkhorn Slough Reserve's Amphibian Research and Monitoring Program.


More links:


  • Antonia D'Amore, Erik Kirby, Michael McNicholas. Invasive species shifts ontogenetic resource partitioning and microhabitat use of a threatened native amphibian. Aquatic Conservation: Marine and Freshwater Ecosystems, Volume 19, Issue 5, Date: July/August 2009, Pages: 534-541.
  • Antonia D’Amore, Valentine Hemingway and Kerstin Wasson. Do a threatened native amphibian and its invasive congener differ in response to human alteration of the landscape? Biological Invasions, Volume 12, Number 1 / January 2010, Pages 145-154. ISSN: 1387-3547 (Print) 1573-1464 (Online). DOI: 10.1007/s10530-009-9438-z
  • Antonia D'Amore, Erik Kirby, Valentine Hemingway. Reproductive Interference by an Invasive Species: An evolutionary trap? Herpetological Conservation and Biology 4(3):325-330.'Amore_etal_2009.pdf

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