I get to wander many of Providence’s wilder places on a pretty regular basis, and now that I have been taking wildlife videos in the North Burial Ground for several years I have become more knowledgeable about the wildlife in our city, as well as simply much more observant of the world around me.
With the dryness of this spring my usual haunt of the drainage swale is cracked mud and unless we get significant rains in the next week or two, it is unlikely that Fowler’s Toads or Gray Tree Frogs will breed there this year. Luckily the other wet spot in the Burial Ground is a permanent pond that seems almost completely unaffected by the dry weather. I have recorded two types of herons, many smaller birds, bats, at least two kinds of turtles, and a variety of other life this spring. Many of these creatures are already posted on Moshassuckcritters, or will be by the time the fall rolls around.
This week i have been focusing on one of my favorite phenomena, the jumping Bullfrog tadpoles in the pond. I have noticed this before, I have posted videos of it both of the last two years, but as is the case with all of this work, each year I probe a little deeper.
What I have learned, mostly from experience, followed by a bit of research, is that Bullfrog tadpoles tend to jump out of the water more the closer they get to becoming frogs. Bullfrog tadpoles overwinter in the pond, with breeding in the late spring, development over the course of the summer, winter under the ice, and a spring growth spurt. Last year for the first time I was able to capture in pixels first year Bullfrog tadpoles in the fall. They are much bigger now.
As I did research I found a variety of things on why frog tadpoles jump out of the water, breeching like mini whales. There is no one definitive answer, but what appears to be the case is that for some species of frogs the breeding ponds can become oxygen depleted, and as the frogs get closer to metamorphosis their lungs start to develop so they can breathe in the air, and need the oxygen. This begs the question of why they need to expend so much energy jumping almost completely out of the water rather than just swimming to the surface of the pond. For some species the jumping is a prelude to moving away from oxygen depleted and drying up ponds (always a problem for amphibians) and searching for new ponds.
The Bullfrogs I observe are in no danger of drying out, the pond is permanent, so while it may be oxygen depleted (I am guessing it is from the constant murk of dead algae blooms that color it coffee brown) the frogs are not going anywhere, and there are no other nearby waterbodies to move to except the Moshassuck River 200 yards away.
Some observers have suggested that tadpoles jump to escape predators. This is entirely possible in some places and with some amphibian species, but in this case that seems most unlikely. The only predators in the pond that this strategy would work on are snapping turtles, and there are at most 1 or 2 snappers in the pond. Since the Bullfrog tadpoles are often simultaneously jumping all over the pond, clearly this is not the answer.
The final suggestion I have read is that this is innate behavior in the tadpoles that is preparing them for life as frogs. As I write this I have no observations of legs on Bullfrog tadpoles yet this year, but that is probably as much due to the difficulty of observing tadpoles in the water in the murk, as anything, as with metamorphosis only 5 weeks away it is likely that legs are developing. Based on my observations of leg development in Fowler’s Toad tadpoles, which i have studied intensively (check out “6 weeks in 90 Seconds” on the Moshassuckcritters Youtube channel) early stage legs in frogs are not much use, but s they develop the frogs use them more and more in locomotion, though as swimmers rather than jumpers until the front legs break out just as metamorphosis is starting.
Therefore it is most reasonable to think that as tadpole bodies are slowly preparing to be frogs that jumping would start to become a part of the behavioral repertoire even before the legs are quite ready to use. The exercising of muscles other than those in the legs that are part of the complex of muscles needed to jump as adult frogs could easily lead to jumping tadpoles with tiny legs.
My research is incomplete, and likely to be more complicated by the serious lack of knowledge/research in the field, so I will have to leave us pondering the relationship between low oxygen ponds and the development process in determing why Bullfrog tadpoles start jumping in May in Providence. But what ever the reason the breeching mini whales are fun to watch, and I offer up this video.