The long-toed salamander (Ambystoma macrodactylum) is a mole salamander in the family Ambystomatidae. This species, typically long when mature, is characterized by its mottled black, brown, and yellow pigmentation, and its long outer fourth toe on the hind limbs. Analysis of fossil records, genetics, and biogeography suggest A. macrodactylum and A. laterale are descended from a common ancestor that gained access to the western Cordillera with the loss of the mid-continental seaway toward the Paleocene.
The distribution of the long-toed salamander is primarily in the Pacific Northwest, with an altitudinal range of up to . It lives in a variety of habitats, including temperate rainforests, coniferous forests, montane riparian zones, sagebrush plains, red fir forests, semiarid sagebrush, cheatgrass plains, and alpine meadows along the rocky shores of mountain lakes. It lives in slow-moving streams, ponds, and lakes during its aquatic breeding phase. The long-toed salamander hibernates during the cold winter months, surviving on energy reserves stored in the skin and tail.
The five subspecies have different genetic and ecological histories, phenotypically expressed in a range of color and skin patterns. Although the long-toed salamander is classified as a species of Least Concern by the IUCN, many forms of land development threaten and negatively affect the salamander's habitat.
Taxonomy
Ambystoma macrodactylum is a member of the Ambystomatidae, also known as the mole salamanders. The Ambystomatidae originated approximately 81 million years ago (late Cretaceous) from its sister taxon Dicamptodontidae. The Ambystomatidae are also members of suborder Salamandroidea, which includes all the salamanders capable of internal fertilization. The sister species to A. macrodactylum is A. laterale, distributed in eastern North America. However, the species-level phylogeny for Ambystomatidae is tentative and in need of further testing.
Systematics and biogeography
Evolutionary origins The ancestral origins for this species stem from eastern North America, where species richness of ambystomatids are highest. The following biogeographic interpretation on the origins of A. macrodactylum into western North America is based on a descriptive account of fossils, genetics, and biogeography. The long-toed salamander's closest living sister species is A. laterale, a native to northeastern North America. Ambystomatidae was isolated to the southeast of the mid-Continental or Western Interior Seaway during the Cretaceous (~145.5–66 Ma).Milner AR. (1983). The biogeography of salamanders in the mesozoic and early Caenozoic: A cladistic-vicariance model. In Sims, RW, Price JH, Whalley PES. (Eds.), Evolution, Time and Space: The Emergence of the Biosphere. (pp. 431–468) Vol. 23 of The Systematics Association, Special Volume. Academic Press, London. While three other species of the Ambystomatidae (A. tigrinum, A. californiense, and A. gracile) have overlapping ranges in western North America, the long-toed salamander's closest living sister species is A. laterale, a native to northeastern North America. It has been suggested that A. macrodactylum speciated from A. laterale after the Paleocene (~66–55.8 Ma) with the loss of the Western Interior Seaway opening an access route for a common ancestor into the Western Cordillera. Once situated in the montane regions of western North America, species had to contend with a dynamic spatial and compositional ecology responding to the changes in altitude, as mountains grew and the climate changed. For example, the Pacific Northwest became cooler in the Paleocene, paving the way for temperate forest to replace the warmer tropical forest of the Cretaceous.Nussbaum RA. (1974). Geographic Variation and Systematics of Salamanders of the Genus Dicamptodon Strauch (Ambystomatidae). 94 pp. Miscellaneous Publications of the Museum of Zoology, University of Michigan, No. 149. A scenario for the splitting of A. macrodacylum and other western temperate species from their eastern counterparts involves Rocky Mountain uplift in the late Oligocene into the Miocene. The orogeny created a climatic barrier by removing moisture from the westerly air stream and dried the midcontinental area, from southern Alberta to the Gulf of Mexico.
Ancestors of contemporary salamanders were likely able to disperse and migrate into habitats of the Rocky Mountains and surrounding areas by the Eocene. Mesic forests were established in western North America by the mid Eocene and attained their contemporary range distributions by the early Pliocene. The temperate forest valleys and montane environments of these time periods (Paleogene to Neogene) would have provided the physiographic and ecological features supporting analogs of contemporary Ambystoma macrodactylum habitats.For the original source describing the paleoenvironmental analogs that was cited by Thompson (2003), see: Heusser C, Minneapolis (1983). Vegetational history of the Northwestern United States including Alaska: The late Pleistocene. In: Wright H, Porter S. (Eds.). Late-Quaternary Environments of the United States. (pp. 239–258) University of Minnesota Press.Brunsfeld S, Sullivan J, Soltis D, Soltis P. (2001). Comparative phylogeography of northwestern North America: A synthesis. In: Silverton, J., Antonovics, J. (Eds.), Integrating Ecology and Evolution in a Spatial Context. The 14th Special Symposium of the British Ecological Society. British Ecological Society, Blackwell Science Ltd., Ch. 15, pp. 319–339. The Cascade Range rose during the mid Pliocene and created a rain shadow effect causing the xerification of the Columbia Basin and also altered ranges of temperate mesic ecosystems at higher elevations. The rise of the Cascades causing the xerification of the Columbia Basin is a major biogeographic feature of western North America that divided many species, including A. macrodactylum, into coastal and inland lineages.
Subspecies
There are five subspecies of long-toed salamander. The subspecies are discerned by their geographic location and patterns in their dorsal stripe; Denzel Ferguson gives a biogeographic account of skin patterns, morphology; based on this analysis, he introduced two new subspecies: A. macrodactylum columbianum and A. m. sigillatum. The ranges of subspecies are illustrated in Robert Stebbin's amphibian field guides.
Physical appearance (phenotypes)
Summary of distinguishing skin patterns and morphological features for the subspecies include:
A. m. croceum Orange dorsal color on tail breaking into patches along black body and into tiny dots on head, often absent anterior to eyes. Sides have whitish flecks. Number of costal grooves equals 13. A. m. columbianum Yellow to tan dorsal stripe on black body, continuous blotches to spots along body ending in narrowed blotches with spot patterns distributed on the head. White flecks along the sides and underside remaining as separate small flecks. Number of vomerine teeth greater than 35. A. m. krausei Yellow to tan dorsal stripe, continuous blotches to spots along body ending in widened blotches with spot patterns distributed on the head. White flecks along the sides and underside remaining as separate small flecks. Number of vomerine teeth equaling 32. Number of costal grooves equals 12. A. m. sigillatum Wax yellow to tan dorsal stripe forming spotty to irregular shaped blotches along body ending in dots or specks of dorsal color on head. Number of vomerine teeth equals 44. Number of costal grooves equals 13. A. m. macrodactylum Citrine, dull citrine, to tan dorsal stripe that is diffuse and continuous along grayish body. Pattern ending in diffuse specks of stripe color or absent on head and snout. White flecks on sides sometimes coming together to form larger flecks. Number of vomerine teeth equaling 33, forming a distinguished transverse arc. Number of costal grooves equals 13.
Biogeography and genetics
Mitochondrial DNA analysis identifies somewhat different ranges for the subspecies lineages. The genetic analysis, for example, identifies an additional pattern of deep divergence in the eastern part of the range. The spatial distribution of populations and genetics of this species links spatially and historically through the interconnecting mountain and temperate valley systems of western North America. The breeding fidelity of long-toed salamanders (philopatry) and other migratory behaviours reduce rates of dispersal among regions, such as within mountain basins. This aspect of their behavior restricts gene flow and increases the degree and rates of genetic differentiation. Genetic differentiation among regions is higher in the long-toed salamander than measured in most other vertebrate groups. Natural breaks in the range of dispersal and migration occur where ecosystems grade into drier xeric low-lands (such as prairie climates) and at frozen or harsher terrain at high elevation extremes ().The height of elevation extremes varies with climate, but > is likely to be an impediment to dispersal across most of this species range north of Oregon. See also: Giordano AR, Ridenhour BJ, Storfer A. (2008). The influence of altitude and topography on genetic structure in the long-toed salamander (Ambystoma macrodactulym). Molecular Ecology 16(8): 1625–1637. PDF
A. m. columbianumGenetic evidence for the 'central' subspecies (A. m. columbianum) suggests that it does not extend north into British Columbia, but is restricted to the Blue and Wallowa Mountains of central to northeastern sections of Oregon. Populations are restricted to these areas by the Snake River Canyon (Idaho) to the east and low dry or xeric lands in the Madras basin to the west. A. m. macrodactylumThe 'coastal' or 'western' subspecies (A. m. macrodactylum) lineage extends north from northeastern California, across the Klamath Siskiyou Range, through the Willamette Valley, along the coastal mountain ranges, including the Cascade Mountains, and continuing north through British Columbia and up into Alaska. A. m. croceumThe Santa Cruz long-toed salamander (A. m. croceum) is most closely related to the 'coastal' or 'western' subspecies. This conclusion is the most parsimonious biogeographic explanation with nearest populations of A. m. macrodactylum separated by approximately 300 km across the Sacramento-San Joaquin River Delta, California. The isolated endemic populations are listed as an endangered subspecies. Based on the biogeography and molecular clock calibrations, this subspecies may have been separated from the remainder of the distribution since the Miocene, molecular clock calibrations estimating 13.9 million years of separation. A. m. krauseiThe 'eastern' subspecies (A. m. krausei) range is distributed throughout the interior mountains, with the western extent of its range encroaching into the low-land areas of the central interior plateau of Washington and British Columbia and the eastern extent of its range pushing through Rocky Mountain valleys into the lowland foothills and prairies of Montana and Alberta. A. m. sigillatumThe traditional 'southern' subspecies (A. m. sigillatum) does not register a mitochondrial genetic identity. This subspecies was identified by Ferguson as forming an intergrade with A. m. columbianum in south central Oregon.