Wood Froglets

Less than three months after the eggs were laid, young wood frogs are emerging from ponds and wetlands around Duluth.  Wood frog metamorphosis occurs fairly quickly and can be easily observed in pond edges.  

Over the last two weeks wood frogs in Hartley Park transformed from 2 inch long tadpoles into froglets about 3/4" long. 

The rainstorms that have rolled through Duluth this week likely encouraged wood frogs to emerge from the wetlands. There are many froglets hopping along the wet forest floor - watch for them on the trails and your lawn. 

Wood frog at Gosner stage 40/41. Hindlimbs are fully developed, forelimbs are about to emerge from under skin.

Wood frog at Gosner stage 42. Forelimb just emerged, full tail remains.

 

Wood frog at Gosner stage 43. Tail somewhat reduced.

Wood frog at Gosner stage 44. Tail greatly reduced.

Wood frog at Gosner stage 45. Only small tail stub remains.

Another wood frog at Gosner stage 45.

Amphibian growth and development is dependent on many factors, including temperature, availability of food, water depth, and photoperiod (the amount of daylight).  Many amphibian species can accelerate or delay larval (tadpole) development and metamorphosis based on quality of their aquatic habitat. Although most wood frogs in a pond transform into froglets at approximately the same time (calling synchronous metamorphosis), multiple stages can usually be found during a visit.

 

Wood frogs resting on a floating board.  Top at Gosner stage 44 or 45, bottom at Gosner stage 42.

*The Gosner staging system was developed for Northern Leopard Frog (Rana pipiens) to identify key points in developmental progress from embryo through metamorphosis, also used for other Ranid species such as the wood frog (Rana sylvatica). citation: Gosner KL. 1960. A simplified table for staging anuran embryos and larvae with notes on identification. Herpetologica 16:183-90.

For Father's Day: Great Frog Dads

The Giant African Bulfrog (Pyxicephalus adspersus) made #3 on Animal Planet's Top 10 Animal Dads! Like some of our local frog species, the Giant African Bulfrog lays eggs in temporary ponds - but after the eggs are laid, male frogs remain in the ponds to protect the eggs and tadpoles.

This BBC video show the bulfrog dad protecting tadpoles from predators and desiccation:

In addition to the Giant African Bulfrog, some other amphibians provide parental care for eggs and tadpoles.  For example, male midwife toads (genus Alytes) carry fertilized eggs on their backs until they are ready to hatch. A good summary of parental care in anurans (frogs and toads) can be found here.

In case you are wondering, none of the anurans that breed in Duluth exhibit parental care.  However, female Eastern red-backed salamanders (Plethodon cinereus) guard their eggs until they hatch.  More information about this entirely terrestrial salamander can be found at the University of Michigan Museum of Zoology's Animal Diversity Web.

Distracted by non-frog signs of summer

It is easy to get distracted by the non-amphibian phenology, such as the flowers blooming around Duluth.  The lilacs in my yard are just starting to open:

In addition to these signs of spring (and summer!), frog activity continues.  The early spring breeders have growing tadpoles that will begin to develop hind limbs soon, while the late season species have not yet started to breed.  Mid-season breeders, such as the gray treefrog, are just finishing up calling. Gray treefrogs generally call from trees over and around wetlands (and thus are often mistaken for birds), and lay their eggs in seasonal and temporary wetlands.

gray treefrog in a wetland - notice the large toe pads

Gray treefrogs change in color from bright green to mottled gray in seconds to aid with camouflage. The below frog does not match it's surrounding in color, but is still hard to detect!

just grass

is that a frog?

gray treefrog in grass clump in wetland

A few years ago, I was fortunate to find many gray treefrogs while sampling a wetland with students.  Check out the variation in color.

gray treefrogs can change from bright green to dark mottled gray

The Eastern gray tree frog (Hyla versicolor) and the Cope's gray tree frog (Hyla chrysoscelis) are both found in this region and there is not a reliable way to tell these species apart in the hand.  The only way to distinguish these species is the number of chromosomes (Eastern gray treefrog is tetraploid while Cope's gray treefrog is diploid) or the males' calls (see below videos).  Additionally, there are may be habitat preferences and behavior that distinguish these species - I look forward to future research revealing the similarities and differences between the Eastern gray treefrog and Cope's gray treefrog.

Toads and Friends

Before this foggy thunderstorm rolled into Duluth, the amphibians were very busy.  In the last two weeks, three species have begun calling: Northern leopard frog (Rana pipiens), American toad (Bufo americanus), and gray tree frog (Hyla versicolor).

Around Boulder Lake, the Northern leopard frogs started and finished calling in just four days. This species is considered an early breeding species, but tends to breed at few weeks after wood frogs in the northern reaches of Minnesota.

Adult Northern leopard frog

I only heard this species calling for one day and found eggs on the same day. We got a frost the night after I found these eggs, which seems to have damaged more than 50% of the embryos.  Each leopard frog lays up to 3,000 eggs, so there were plenty of tadpoles hatching late last week despite this frost damage.  

Amercian toad calling 

American toads began breeding late last week.
One day after hearing this species calling for the first time, American toads were calling in a full chorus and multiple pairs were laying eggs in the backwaters off of Boulder Lake.




Listen to this chorus of American toads heard last Friday May 20th:

Multiple pairs of American toads in amplexus (and some males)

American toads lay long double strings of eggs, which hatch in 2-8 days depending on water temperature.

Pair of american toads in amplexus (male on back of female)
-notice the strings of eggs under the pair-

American toad eggs - freshly laid

Gray treefrogs started calling last week, and Western chorus frogs and spring peepers continued to call.

Although spring peepers were in full chorus in Duluth city limits in late April, in some locations this species did not start calling until last week. A full chorus of spring peepers is so loud that it makes my ears ring after standing by the wetland for five minutes.

Spring peeper

Listen to this chorus of spring peepers heard last Wednesday May 18th:

Developing Wood Frog Eggs

Finally, the wood frog eggs are starting to develop into tadpole shaped embryos.  I expect these eggs to hatch next week.

Fresh wood frog eggs are very easy to spot......

wood frog eggs masses - freshly laid

....but are harder to find as they develop. Egg masses lose their distinct shape and symbiotic algae turns them greenish, making the eggs blend in with the general pond vegetation and organic matter.

wood frog egg masses - newly hatched

close up of wood frog egg mass - freshly laid

Due to the transparent gelatin-like coating, embryo development can be followed from fertilization through hatch. The embryo (black ball at center of egg in left photo) begins to elongate and grow into tadpoles. The eggs hatch before the tadpoles are able to swim, so they remain in or on the egg masses for several days (see below).

close up of newly hatched wood frog tadpoles

Wood frog eggs hatch 4 to 28 days after being laid.  Embryonic development is temperature dependent, so timing of hatch varies with weather conditions. 

Frozen Wetlands

Monday morning update: the wetlands appear to have iced over last night.  I will be checking the wood frog eggs later today, hopefully they were spared from freezing.

Monday afternoon update:  the wood frog eggs in the wetlands I regularly check seem to have survived the cold weekend and some have even started to develop.

Confused About the Season

Although I am a little confused about the season, I cannot imagine what it would be like to be a frog during the recent winter-to-spring-to-winter flips. By the end of last week three species were calling - chorus frogs, spring peepers, and wood frogs. On Friday afternoon, I saw (and heard) around 100 wood frogs calling in a pond in Hartley Park, along with many egg masses and several females.

Four male wood frogs calling in vicinity of multiple egg masses, with a pair in amplexus in the middle

male before calling

Wood frogs are fun to watch calling.

 

swimming while calling

after calling

female wood frog on edge of wetland

The colder weather this weekend has quieted the calling, but these three species should be out again as it warms up over the week.

Why it would be hard to be a frog in this weather:
Male wood frogs must be almost out of energy after 3 weeks of starting and stopping calling. These early season breeders emerge from winter hibernation soon after the snow melts and the males begin calling almost immediately, fueled by fat storage from last fall. Calling is the most demanding activity that many male frogs will ever do; in some species, calling is 10 times more metabolically demanding than moving at maximum speed. The trunk muscles, which power calling, can be 12% of body mass in a male frog, while only 3% in a female. Not only is calling a lot of work, males of early breeding species expend all that energy before spending much (or any) time foraging for food.

I learned most of this from a great (but heavy) book:
The Ecology and Behavior of Amphibians - Kentwood D. Wells