Species of the month – January 2023 – Wood Frog

The brumating Wood Frog (Lithobates sylvaticus)
By Emily Ratch, Stewardship & Outreach Technician

Winter is a very deceptive season. At a glance, it seems like not much is happening in the natural world; it’s almost like nature is put on pause. But behind the scenes, life is still thriving – just in an incubation period.

small Wood Frog on rock
Wood Frog

One of the most fascinating phenomena during the winter is hibernation. Many species endure a hibernation period, including bears, groundhogs, bats, bees, snakes, frogs, and many others. There are three types of hibernation – true hibernation, brumation, and torpor:

  • True hibernation (mammals): Deep state of sleep for the duration of winter. Reduced mobility, heart rate, body temperature, and metabolic rate. Animals undergoing true hibernation will not eat, drink, or defecate while asleep. Hibernating animals usually find refuge in rock, tree, or debris cavities.
  • Brumation (reptiles and amphibians): Deep state of sleep induced by nearly freezing solid for the duration of winter. Reduced mobility, body temperature, and metabolic rate, stopped breathing and heart rate. During brumation, animals will not eat, drink, or defecate. Colder average temperatures and decreasing daylight hours trigger a brumation period. Animals will brumate underwater, in leaf litter or soil.
  • Torpor: (some birds and mammals): Unlike the other two types of hibernation, torpor is involuntary. When the animal is inactive, it will enter a deep state of sleep, only lasting for a couple of days or nights. Feeding schedules usually determine sleeping patterns. When sleeping, the animal will experience reduced heart rate, body temperature, and metabolic rate.
Frozen Wood Frog
Brumating (frozen) Wood Frog. (Photo: Janet M. Storey)

Species such as the Wood Frog endure brumation every fall to spring. Unlike other frogs, the Wood Frog is usually found hibernating under a thick layer of leaf litter or soil. It has specially adapted features that allow it to withstand freezing temperatures up to -6 ℃. Some of their cold temperature adaptations include an ‘anti-freeze’ like substance found in their blood made from glucose and glycogen called cryoprotectant solutes. This chemical prevents ice from forming in their cells and protects their organs from being damaged. Up to 70% of their fluids can freeze without harming the frog.

When in this frozen state, the frog’s heart will stop beating, and it will also stop breathing, eating, and drinking. It can live in this state for up to 5 months!

When temperatures rise in the spring, the Wood Frogs slowly start to thaw along with their surroundings. The first thing on the Wood Frog’s mind is reproduction! As they regain their consciousness, they begin to call for one another. As soon as sufficient pools of water begin to melt, these frogs get down to business. Females will lay egg masses of up to 2000 eggs and attach them to submerged vegetation in fishless ponds. Mating and egg laying happens all within about a week, earning themselves the name ‘explosive breeders.’ It takes about 2-3 years for the frogs to reach sexual maturity.

Frog eggs in water
Wood Frog egg cluster in water

The most northern range of Wood Frogs reaches to Alaska, where they’ve adapted to withstand temperatures up to -16℃. They also live as south as Georgia, USA. They are native to this range, including Ontario. Fortunately, this frog is not a species at risk. Although, it does face a lot of challenges, such as habitat loss, deforestation, polluted or dried-up waterways, road salt decimation, loose forest litter, and acidification.

So what can we do to help the Wood Frog to continue thriving? We can continue land conservation efforts and restoration of significant wetlands, reduce road salt near waterways (when possible), leave leaf litter alone to provide sufficient brumation shelter and report sightings to iNaturalist or other wildlife reporting initiatives. 


Wood Frog in pond
Wood frog in pond in Spring









Brianne Curry

Outreach & Fund Development Manager