The Evolutionary Origins Of The Weasels

Weasels belong to the canoid group of placental carnivores, which originated in the New World (Flynn & Wesley-Hunt 2005). The first predatory mammals with characteristics of the weasel family, and clearly different from their closest relatives the Procyonids (Bininda-Edmonds et al. 1999), appeared in North America in the early Miocene, some 28 to 30 million years ago (Figure 1.4). Throughout the Miocene period, these animals were forest-dwelling hunters, probably somewhat like martens. Some mustelids must have also have reached Eurasia by the beginning of the Miocene, because diverse mustelids were by then living in both North America and Eurasia. The oldest fossil of a mammal clearly belonging to the genus Mustela comes from eastern Eurasia, dated to late Miocene, and an independent lineage of mustelids very similar to the first weasels appeared in North America at roughly the same time. Extensive movement back and forth between North America and Eurasia (Wederlin & Turner 1996) set the scene for an active evolutionary radiation within the Mustelidae over the next 15 to 20 million years. Among the early (by mid-Miocene at least) migrants to travel west from America must have been the far-distant ancestors of the stoat and common/least weasel (Hosoda et al. 2000), while the distant ancestors of the longtail remained at home.

By the early Pliocene (5 million years ago), at least three separate lines of true Martes were well established, as well as other species intermediate between Martes and Mustela. Throughout the Pliocene, extensive open savannahs began to develop in both North America and Eurasia, as the climate cooled toward the approaching glacial periods and the grasses evolved and progressively replaced the forests. The grasslands were soon populated by the early species of voles and lemmings, and then by the ancestors of the weasels. It seems likely (King 1983a; King 1984a) that those early weasels, descended from the larger,

P Mustela lutreola

- Mustela lutreolina r- Mustela nudipes

- Mustela sibirica

- Mustela strigidorsa

C Mustela eversmannii Mustela nigripes

— Mustela putorius j- Mustela altaica 1— Mustela erminea

— Mustela frenata

— Mustela nivalis Mustela africana Mustela felipei

— Mustela kathiah Mustela vison

Martens, African and S.Am."weasels" Skunks, Otters, Badgers Raccoons and allies Seals, Sea Lions and Walrus Bears Canids

60 50 40 30 20 10 Million Years Before Present

Figure 1.4 A phylogenetic tree for the weasels developed from morphological, fossil, and biochemical data. This and most other phylogenetic trees made using different data agree consistently that Mustela erminea is most closely related to M. altaica and not most closely related to either M. nivalis or M. frenata. (Redrawn from Bininda-Edmonds et al. 1999.)

marten-like mustelids already existing, discovered the advantage in becoming small enough to exploit the new niche for predators able to get into the burrows and runways of voles, mice, and lemmings.

The living forms most similar to ancestral weasels are probably either the modern stoat or the Asian mountain weasel (M. altaica). An ancestral stoat, M. plioerminea, appeared in Eurasia in the Pliocene, some 4 million years ago (Kurten 1968). During the long transition between the end of the Pliocene era and the beginning of the Pleistocene, and throughout the first of the four great glacial epochs conventionally recognized and the first (Cromerian) interglacial epoch, an intermediate species called M. palerminea was common. Around that time, the evolutionary lineages leading to stoats and mountain weasels separated

(Bininda-Edmonds et al. 1999: Kurose et al. 2000). Definite specimens of the modern species M. erminea date only from the time of the third major glaciation, which started about 0.6 million years ago, and are common in European deposits dated to the last full glaciation (Sommer & Benecke 2004). M. erminea returned to its ancestral homeland via the Bering Land Bridge into North America roughly half a million years ago, and now occupies about 13 million km2 of the United States and Canada (Fagerstone 1987).

The earliest form of the common weasel was Mustela praenivalis (probably derived from M. pliocaenica) (Kurten 1968), which appeared in Eurasia some 2.6 million years ago (Bininda-Edmonds et al. 1999) and survived for nearly 2 million years alongside M. palerminea, until the Cromerian interglacial epoch. The transition to M. nivalis was gradual, but was completed by the time of the second major glaciation (less than half million years ago). The modern form is found among a forest fauna of that age at West Runton, Norfolk, eastern England, and is common in European cave deposits dating from the last glaciation (Yalden 1999; Sommer & Benecke 2004). In the late Pleistocene, during the last glaciation (van Zyll de Jong 1992; Abramov & Baryshnikov 2000), M. nivalis also crossed the Bering Bridge, and followed M. erminea back into North America. It now occupies about 7.5 million km2, mostly in Canada.

M. frenata appeared quite abruptly in North America more than 2 million years ago, before the first glaciation. It has therefore survived in its present form much longer than either of the other two modern species, and it has the longest strati-graphic record and widest fossil distribution (>30 locations) of any North American weasel. Its present range is about 8.5 million km2, mostly in the United States. Its ancestor is unknown: The only suggested candidate so far is M. rexroadensis, a medium-sized weasel, known only from two fossil sites containing mammal faunas dating from around 3 million years ago (Kurten & Anderson 1980). The immediate origins of M. rexroadensis are unknown.

During the cold phases of the Pleistocene, the weasels found themselves already adapted to live under snow, finding there both food and shelter from the killing cold above. They were already the right shape to burrow through soft powder snow on the surface, dive through the deeper layers, and follow the tunnels made by rodents on the ground underneath, just as they do today (Chapter 2). Most of the huge ranges of the stoat and least weasel, and the northern end of that of the longtail, still lie within climatic zones having a severe winter with prolonged snow cover (Figure 1.5). Weasels still live all year round at elevations of 2,000 to 3,000 m or more in the snows of the high mountain ranges of, for example, the Sierra Nevada of California, the Alps of Europe, and the Caucasus, Altai, and Tien Shan of Asia.

The interesting implication of these family histories is that, although the stoat and the longtail look and behave alike, they not closely related. Their far distant common ancestor came from America more than 4 million years ago, before M. plioerminea existed. The modern forms evolved separately, one in Eurasia

Figure 1.5 The approximate distribution of snow cover in countries inhabited by weasels, shown as the average number of days per year with snow lying on the ground in the morning. (Note: New Zealand, inset, is slightly enlarged relative to the Northern Hemisphere.)

and the other in North America. Combined phylogenetic analyses of the contemporary species show that the most closely related pair is the stoat and the Asian mountain weasel (Bininda-Edmonds et al. 1999); the longtail is about equally distant from these two as from the common/least weasel. The next closest relatives are the New World tropical and Colombian weasels. Yellow-bellied weasels are more distant relatives of the central group of four, and the back-striped and barefoot weasels appear to be closer to minks and polecats than to the rest of the weasels. Other authors (Sato et al. 2003; Abramov 2000) have produced phylogenies differing slightly from this one, because the details depend on the time scale of interest, the methods used, and which species are included, but all agree on the main outlines of mustelid history.

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