I am waiting for a particular combination of meteorological variables. One night very soon, air temperatures will be warm and there will be the deep smell of water everywhere. This night may come at the end of or maybe even in the middle of a day (or three) of steady rain. The ground will be wet and muddy. Low swales will be full of new water. This perfect storm of warm air and water will shake the spring peepers from the last of their hibernation hangovers and stimulate them to start their nighttime choruses in ponds and puddles all over Western Pennsylvania. .
Spring peepers (Pseudoacris crucifer) are small tree frogs that live around marshes, ponds, and temporary pools throughout the United States. Peepers have sticky footpads that enable them to climb up the trees, shrubs, and tall grasses that surround their “home base” water sources, and it is from these perches that the male peepers sing out their distinctive, spring mating songs,
These mating choruses begin in early spring (around here usually in mid-March, but I am writing this during the first week of April and still have not heard them calling). They usually start up fifteen minutes or so after sundown and then call for a four hour period. The calls require a very large expenditure of energy which may explain why the males bunch together to form large, high volume ensembles (even though these groupings greatly intensify mating competition between individuals). The mating/calling season lasts for four to eight weeks. Male peepers begin to sing when they are three years old and the age, size, and overall health of the frog greatly affect their calling frequency. Vigorous or dominant frogs may take the lead in these group choruses and thus stand out among the crowd for the attracted females. The temperature of the evening also affects calling patterns. On warmer evenings the frogs call much more frequently.
A paper published this past January about some tree frogs in Europe (Conservation Biology January 11, 2017) showed that human generated noise (like road noises) can increase stress levels in tree frogs and cause a decline in both their immune functioning and chances for reproductive success! Elevated levels of stress hormones can increase susceptibility to disease (the major cause of the on-going, worldwide extinction of amphibians (see Signs of Fall 5, October 6, 2016)). It can also cause a diminished coloration of the male’s vocal sac (a prime clue to females of health and vigor of the male).
Females, attracted to the calling of the males, enter the calling area and select the individual with whom they want to mate. The male then clasps himself onto the female’s back and remains there as the female return to the water source to deposit her eggs. The attached male prevents other male from mating with the female and insures that all of the female’s eggs will be fertilized by his sperm. The female can lay between 800 and 1000 brown-colored eggs either singly or in clusters. The eggs may be set afloat in the pond water, attached to submerged vegetation, deposited in the muddy bottoms of pools, or even put into fluid filled tree hollows or many other types of available micro-pools.
The eggs hatch in six to twelve days. The emerging larvae (the “tadpoles”) will typically remain in their aquatic form for ninety to one hundred days. This larval incubation period, however, can be as short as forty-five to sixty days depending upon weather conditions, time of egg deposition, and conditions in the tadpole’s pool. Populations of peepers developing in temporary pools may be undergoing selection for shorter and shorter larval incubation times. The devastating impact of a tadpole pool drying up is an unforgiving selection force!
The tadpoles eat a wide variety of foods (including algae, dead vegetation, bacteria, fungi, zooplankton, flesh from animal carcasses, and even inorganic materials like sand). The tadpoles are, in turn, preyed upon by almost any organism that is larger than they are. Fish are especially significant tadpole predators in ponds, but predaceous beetles, salamanders, and water snakes also readily consume the tadpoles. Further, pesticides and other pollutants (including acid rain) are significant agents of mortality in spring peeper tadpoles.
The metamorphosis of the tadpole into a frog begins with the appearance of hind limbs which is followed by the emergence of the forelimbs and the shrinkage of the tail. Jaws with teeth, eyelids, mucous glands in the skin, and finally the transformation of the light cartilaginous skeleton of the tadpole into the denser, bony skeleton of the frog complete the metamorphic transition into a tiny frog. The emergence of the frog onto land then exposes it to even more predators and environmental dangers.
The frogs are readily eaten by snakes, turtles, birds, and mammals like chipmunks and muskrats. They are also frequently killed on roadways by passing cars and trucks. Peepers are also susceptible to many viral and bacterial illnesses, exhibit a wide range of benign and cancerous skin and mucous membrane tumors, and are beset by a wide range of endo- and ectoparasites (including tapeworms, flukes, nematodes, protists, and larvae of several dipteran species.
Destruction of their aquatic habitats and even more subtle alterations of the forest cover around their wetland breeding sites can also negatively affect the breeding potentials and survival of these frogs.
Hibernation is even a dangerous time for the peepers! While they are hibernating under soil and leaf litter, in and under rotting logs, and even under rocks, they are often exposed to sub-freezing temperatures. They are able to survive these freezing events by generating large quantities of blood glucose from their livers. This sugar (like we have seen in wood frogs and tree frogs) acts as a natural anti-freeze in their blood and other body fluids. Survival during these sub-freezing events is inversely related to the duration of the exposure (85% of the frogs survive after three days of freezing, about 50% survive after seven days of freezing, and 0% survive after twenty-eight days of freezing). While in hibernation, the peepers may also be preyed upon especially by small mammals active in the subnivian space beneath the snow cover. Shrews in particular readily consume the inactive, hibernating frogs. Long, cold winters, then, via extended lethal freezing and increased predation opportunities can have significant impacts on the population of this important sign of spring!
All of this tells me that it is very hard to be a small frog!
I am at this writing still waiting for the first sound of this spring’s peepers! If anyone has heard them already, please let me know. Let’s get this Spring going!