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