Woolly mammoths (Mammuthus primigenius) are perhaps the most charismatic of the extinct Pleistocene megafauna and have become emblems of the last ice age. Unlike extant African and Asian elpehant species, which live in warm tropical and subtropical habitats, woolly mammoths lived in the extreme dark and cold of the dry steppe-tundra where average winter temperatures ranged from −30° to −50°C. The Proboscidean lineage origintated in a warm, tropical environment indicating that the suite of derived traits in mammoths evolved after their divergence from Asian elepahnts approximately 5 million years ago (MYA) but before they colonized the steppe-tundra 1-2 MYA, suggesting that their cold-adapted traits evolved relatively recently.
Mammoths evolved a suite of traits that allowed them to survive the long, cold and dark ice age winter including:
- Small ears and short tails
- Thick subcuteaneous fat
- Long, thick fur
- Dense sebaceous glands
- Adult brown fat deposits
- Molecular and physiological adaptations to extreme cold
Identifying the genetic changes that underlie morphological differences between species is challenging, particularly when reconstructing how the genotype-phenotype map diverged in non-model organisms. Identifying the specific function alterting nucleotide substitutions from among the many millions of non-functional substitutions is duanting, and this challenge only grows more difficult as species diverge and assumlate neutral substitutions. Woolly mammoths are an ideal model system in which to identify the genetic basis of cold adaptations because they diverged from Asian elephans relatively recently thereby preserving a footprint of the genotype-phenotype map that can no longer be eroded by continued divergence.
Figure 1. Woolly Mammoth Phylogeny, Ecology, and Extinction. (A) Phylogenetic relationships among recent elephantids. Branches are drawn proportional to time. The ancestor of Asian elephants and mammoths is labeled (AncGajah). (B) Mammoth ecology and extinction. Irradiance at 60°N (top) in June (orange) and December (blue), arctic surface temperature (middle), and estimated mammoth abundances at three localities (bottom) during the last 45 ka are shown. Data modified from MacDonald et al, (2012).