Late November may be a strange time of year to write about an animal with “spring” in its name, but among the absolute truths in the natural world are these: Seasons change, and wildlife is preparing for the next season even before the current season has fully taken hold.
And in any case, animals we associate with a particular season don’t just come into existence spontaneously to entertain us at a particular time of year; they’re here, or somewhere, the rest of the time, too, and the question of how water-based life forms make it from November to April without fatally freezing is a vital one.
The spring peeper, Pseudacris crucifer, surely peaks in prominence in spring, when males of this tiny tree frog species blend their individual peeps into a deafening chorus. But they’re here, and can sometimes be found with patient searching, throughout the year.
And an off-season call given by males, less piercing and more croaky than their spring song, can be heard when warm enough weather occurs at any point in the winter. I’ve heard or seen this species in every month of the year on Martha’s Vineyard, and as the climate continues to warm, I expect the “off-season” for this tiny amphibian to continue to shrink.
Despite averaging only an inch or so in length, spring peepers are incredibly tough. Their habits and their physiology both evolved to allow adults to overwinter successfully, and compared with surviving winter at the northern limit of the species range (Hudson Bay and Labrador!), doing so on the Vineyard must be a piece of cake.
Part of the secret is selecting a good place to hibernate. Spring peepers typically winter underneath a thick layer of leaf litter in a wet area. Even when the air temperature plunges, the debris (and sometimes snow cover) on top of the frog offers a measure of insulation.
But the key strategies for surviving freezing have to do with body chemistry and physiology. A 1995 article in the journal Climate Research outlined a complicated set of changes triggered by the onset of cold weather. The frog’s liver literally begins producing antifreeze — concentrations of glucose and glycogen can increase 200-fold in a freezing frog, lowering the freezing point of water and thereby preventing ice crystals from forming.
Organs in the frog’s body extrude water, shriveling in size and becoming less susceptible to damage from freezing, with the surplus fluid simply stored in body cavities, or at least in space among cells, where it can freeze without causing harm. Thus fortified, the frog can endure a body temperature well below the freezing point of water with no ill effects. Physical processes, such as heartbeat, grind virtually to a halt, conserving energy.
Just as remarkably, the freezeproofing process reverses upon the onset of warmer temperatures: Concentrations of chemicals within the body return to normal, organs and other tissues rehydrate, and the ability of muscles to contract gradually returns over the course of hours or days.
Interestingly, how quickly a frozen spring peeper thaws out depends on how deeply, and for how long, it was chilled. This may be a predictable outgrowth of the biology of freeze protection, but it serves a useful purpose by customizing a frog’s cold resistance to local conditions. In colder regions, or during a particularly cold winter, a frog takes longer to rouse and hence is less likely to make the mistake of resuming activity prematurely. In milder areas, frogs tend to resume activity more readily, allowing them to take advantage of an early spring to begin mating. The rest of the spring peeper life cycle is better known: the loud choruses around ponds and wetlands from late winter to spring are followed by fully aquatic tadpoles, and ultimately by another generation of amphibious adults. Spring peeper tadpoles, as you’d expect, are tiny, feeding on algae and tiny insects, and fed upon in turn by nearly everything, from fish to birds to other frogs.
The sheer abundance of this species is a tribute to fecundity: Females simply lay huge numbers of eggs, and while mortality is high, all the predators in the world can’t find and eat the huge number of young produced.
Even adult peepers must be highly vulnerable to predation. But they’re canny little critters. As spring progresses, individual frogs peel off from the huge mating concentrations around ponds, and the relatively low frog density once the population has dispersed into surrounding woods and fields limits the ability of predators to find them.
Moreover, spring peepers are masters of concealment. While they climb readily (and even have tiny suction cups on their toes to assist this), peepers spend most of their adult lives close to the ground, hidden in dense vegetation where the frog’s nondescript brown color aids concealment.
In short, these are year-round frogs, present and potentially active any month of the year. Spring peepers: Not just for spring!