Amphibians
Jefferson Salamander
Ambystoma jeffersonianum · also called Jefferson's salamander
On the first thaw of late winter, while the woods are still bare and the pools still rimmed with ice, the Jefferson salamander is already on the move. For a week or two it fills the vernal pools of eastern North America, courts, lays its eggs, and vanishes back underground. And in many of those pools, alongside it, lives one of the strangest reproductive systems in the animal world.
Overview
Ambystoma jeffersonianum is a slender mole salamander, about 10 to 18 cm long, dark grey to grey-brown with a faint bluish speckling on the limbs and lower flanks. It lives across the deciduous forests of the north-eastern US and southern Ontario, spending nearly the whole year in burrows and root channels underground and emerging only to breed in fish-free woodland pools.
An early breeder
Jefferson salamanders migrate to their pools on the first mild, wet nights of late winter, ahead of almost every other amphibian, sometimes crossing snow. Breeding early gives their larvae a head start before the pool warms, fills with predators, or dries.
The all-female salamanders
Across much of the range, the same pools hold all-female salamanders of ancient hybrid origin that look almost identical to Jeffersons. Each carries two to five genomes from several species. They must mate with a male of a normal species to start their eggs developing, but usually discard his genes and clone themselves — a mode called kleptogenesis. This lineage is around five million years old, by far the oldest known all-female vertebrate line, which evolutionary theory says should be impossible.
Status
The Jefferson salamander is Least Concern globally but declining, and it is listed as threatened or endangered in several US states and in Ontario, because it needs both clean vernal pools and large blocks of surrounding forest, and loses out when either is broken up.
Jefferson Salamander: common questions
Why does the Jefferson salamander breed so early in the year?
Breeding early lets its larvae get a head start in the vernal pool before the water warms, before competitors and predators build up, and before the pool begins to dry in summer. Jefferson salamanders move to their breeding pools on the first mild, rainy nights of late winter, often in February or early March in the middle of the range, sometimes crossing snow and slipping into water that still has ice around the edges. The adults court and lay eggs over a week or two, attaching small egg masses to submerged twigs, then leave and return underground. The trade-off is the risk of a hard frost, which can kill eggs in a pool that refreezes.
What are unisexual Ambystoma salamanders?
They are populations of all-female salamanders, found across much of the Jefferson salamander's range and beyond, that arose long ago from hybridisation between mole salamander species. They look very like Jefferson or blue-spotted salamanders and share their ponds, but every individual is female, and each carries two, three, four, or even five complete genomes drawn from up to four species. To reproduce, a unisexual female must mate with a male of a normal sexual species living nearby, because his sperm is needed to trigger her eggs to develop. Usually his genome is then thrown away and she produces clones of herself; occasionally it is kept, adding another genome to the mix. This is why a pond survey may turn up far more females than males.
How can an all-female lineage survive for millions of years?
In theory it should not. Lineages that stop mixing their genes are expected to accumulate harmful mutations and to be unable to adapt, and most all-female animal lineages are young and short-lived. The unisexual mole salamanders appear to have lasted roughly five million years, which makes them a genuine puzzle. Part of the answer seems to be that they are not fully isolated. By occasionally incorporating a genome from the males they mate with, they can bring in fresh genetic material, swap genomes over time, and avoid the slow decay that dooms a strictly clonal line. They are, in effect, borrowing just enough sex to keep going.
Is the Jefferson salamander protected?
Globally it is listed as Least Concern, but its situation is patchier than that suggests. Several US states, including New Jersey, Ohio, and Kentucky, list it as endangered, threatened, or of special concern, and in Ontario it is protected as an endangered species. The reason is habitat. The salamander needs clean, fish-free temporary pools for breeding and large areas of intact forest around them for the rest of its life, and it will not cross wide roads or open ground to reach new sites. Housing development, road building, and the draining or filling of vernal pools break this system apart, and populations disappear even where some woodland remains.