Skip to Main Content
San Diego Zoo Wildlife Alliance logo
San Diego Zoo Wildlife Alliance Library logo

Mountain Yellow-legged Frog (Rana muscosa) Fact Sheet: Reproduction & Development


Reaching sexual maturity

  • Females mature between 1.73 and 1.97 inches (44 and 50 mm) in length (Zweifel 1955)
  • First breeding at about 2 years of age


  • At lower elevations, breeding occurs between April and May
  • Eggs laid late spring to early summer (Natalie Calatayud, personal communication, 2016)
    • Often laid on undersides of leaves or other horizontal structures

Life Stages

Egg (The Mountain Yellow Legged Frog Site 2015; Vredenburg 2005; Zweifel 1955)

  • Number of eggs: 40-300 per egg mass
    • Egg size about 1.9-2.3 mm
  • Time to hatch
    • Depends on environmental conditions
    • In the wild: not well-understood
    • In managed care: 5-6 weeks (Natalie Calatayud, peronsonal communication, 2016, citing Michelle Curtis and Nicole Gardner, personal communications, 2016)
    • Eggs hatch into free-living larvae
  • Timing of metamorphosis (time elapsing from fertilization of egg to frog)
      • At lower elevations: same season as eggs are laid
      • At higher elevations: usually occurs the next summer
        • May not occur until the fourth summer at the highest elevations


  • One of largest tadpoles of any North American frog
    • Up to 3 inches long
  • Produces mucous to anchor itself to plants
  • Feeds off residual yolk from yolk sac
  • Develops long tail, external gills, working mouth parts, and long digestive tract
  • Metamorphosis in the wild
    • May occur within 6 months, but may take up to 2-3 years, depending on environmental conditions
      • Most frog species only need one season
  • Metamorphosis in managed care
    • Occurs before winter
  • Metamorphosis: changes in body structure
    • Tadpole increases in size
    • Eyes more noticeable, mouth is wider
    • Hindlimbs and forelimbs develop
    • Tail becomes shorter; reabsorbed
    • Gills become smaller; fold of skin grows over external gills
      • Breathes exclusively through internal gills as external gills disappear
      • Lungs develop
        • Tadpole breathes by gulping air from surface
    • Young frog can move to land
    • Diet becomes carnivorous

Typical Life Expectancy

Wild populations

  • Approx. 12–15 years (Natalie Calatayud Crump, personal communication, 2023)

Managed care

  • No AZA estimates

A New Generation

female Mountain Yellow-legged frog and eggs

Female Mountain Yellow-Legged Frog (Rana muscosa) with eggs.

Image credit: © San Diego Zoo Wildlife Alliance. All rights reserved.

Mountain Yellow-legged frog tadpole

Development of a tadpole.

Image credit: © San Diego Zoo Wildlife Alliance. All rights reserved.

Page Citations

Zweifel (1955)
Livezey and Wright (1945)
Stebbins (1951)
Mathews and Miaud (2007)
Vredenburg (2005)
The Mountain Yellow Legged Frog Site

SDZWA Library Links