In the Late Pleistocene, the Don hare (Lepus tanaiticus Gureev, 1964), which became extinct in the Holocene, was widespread in Eastern Europe and Northern Asia [1, 2]. According to genetic data, this hare is now also considered as an extinct subspecies of the mountain hare (L. timidus tanaiticus Gureev, 1964) [3]. On the basis of bone remains of the Don hare, it was established that it had large sizes close to those of the largest extant subspecies of the mountain hare, more massive bones and teeth than the mountain hare, lower jaw with high dentition, high crowns of molars, and steeply curved upwards incisal part of the lower jaw, which indicates feeding on roughage [1, 4], apparently tough steppe herbaceous vegetation [5]. Don hares from Yakutia are described as an independent subspecies (L. tanaiticus vereschagini Averianov, 1995) [2], which differ from the nominative subspecies in their smaller sizes [2, 4].

In the summer of 2021, at the Ogorokha site, which is located on the right bank of the Ogorokha River (Indigirka River basin, Abyiskii district, Yakutia, 68°14.02′ N, 146°50.84′ E), local residents found a frozen mummy of a hare in permafrost deposits at a depth of 2–3 m (Fig. 1a). In the same deposits, next to the hare, bone remains of typical mammals of the Late Pleistocene mammoth fauna were found earlier: woolly mammoth (Mammuthus primigenius Blum., 1799), wolf (Canis lupus L., 1758), cave lion (Panthera spelaea Goldf., 1810), and woolly rhinoceros (Coelodonta antiquitatis Blum., 1799) [6]. Radiocarbon dates from the Ogorokha outcrop (30 900 ± 390 (OxA-38733), 30 970 ± 380 (OxA-38749) years ago (bones of cave lions) [7] and 36 500 ± 2000 years ago (ETH-122763) (wolf skull) [8]) testify that the bone-bearing deposits, in which the mummy of a hare was found, were formed here in the cold period of the Karginian interstadial of the Late Pleistocene, typical for Siberia in the time interval 38 (33)–30 kyr [9, 10]. Preliminary studies of the complete mitochondrial genome of the Ogorokha hare have shown that, on the one hand, it clusters with mitogenomes of fossil Don hares from Yakutia aged 28 360 ± 170 to 50 120 ± 1210 years; on the other hand, similarly to other Don hares studied, it exhibits phylogenetic relatedness with the mountain hare [11]. The frozen mummy of the hare (specimen no. 7133) is stored in the Geological Museum, Institute of Geology of Diamond and Precious Metals, Siberian Branch, Russian Academy of Sciences (GM IGDPM).

Fig. 1.
figure 1

Frozen mummy of the Don hare, specimen GM IGDPM no. 7133 from the Ogorokha locality in Yakutia: (a) general view, (b) brush hair on the plantar skin, (c) cross sections of specialized brush hairs. (a, b) Photo by G.G. Boeskorov, (c) scanning electron microscopy (SEM) data.

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The mummy of the hare is well preserved and has no damage on the body (Fig. 1a). This is an adult individual, as evidenced by the partially worn teeth, fused sutures on the lower jaw, and a fully adherent distal tibial epiphysis. The body length is large, close to that of large mountain hares from the Arctic zone and maximum for the Central Yakut mountain hare (Table 1). The length of the foot and ear of the mummy is disproportionately small compared to that of the relatively small extant hares from Central Yakutia (Table 1).

Table 1. Main measurements of the body of hares (limits and mean values are shown, n is the number of measurements)
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The surface of the mummy’s body is covered with hair by approximately 60%; on the dorsal and posterior parts of the body, a significant part of the hair is lost (Fig. 1a). The color of the fur is grayish brown due to its contamination with enclosing sediments. When washed from pollution, the hair has a white color, which is typical for the winter color of mountain hares. The tips of its ears are black, similarly to those of the mountain hare. The fur is thick and long: 20–25 mm on the ears, 15–22 mm in the occipital region, up to 50 mm on the withers, 33–44 mm on the chest, 60–80 mm on the belly, and 40–50 mm on the tail. On the back, the underfur is 15–20 mm long (the guard hairs are not preserved). The fur of the Ogorokha hare is longer (up to 80 mm on the abdomen), and the hairs are thicker (up to 114.5 μm on the withers) than the coat of the extant mountain hare, in which the longest hairs in winter also grow on the abdomen, but their length does not exceed 49.5 mm, and the thickness of the hairs is maximum on the sacrum, where it reaches 108.8 μm [12]. The coat of the Ogorokha hare reliably protected both the dorsal side of body (due to the thickening of rather short hairs with a highly developed medulla) and the ventral side (due to the considerable medulla of thinner hairs) against the cold, which is especially important for long-term lying in the snow. In addition, the specific structure of the hair shafts (longitudinal striation) (Fig. 2a) and the well-developed airy medulla (Figs. 2a, 1; 2b, 2), which is characterized by a great thickness, columnar structure, large cavities, and folded septa, ensured effective insulation due to the preservation of a significant amount of air in the fur, and also chevron cuticle (Figs. 2c, 3).

Fig. 2.
figure 2

Microstructure of the guard hair of the withers of the Don hare mummy from the Ogorokha locality in Yakutia: (a) transverse section of the shaft, (b) longitudinal section of the shaft, (c) cuticle ornament. Designations: 1, air cell of the core with corrugated partitions; 2, columnar core; 3, scale of a complex chevron cuticle. SEM data. Scale, 10 µm.

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The lower jaw of the Ogorokha hare is fairly large; some of its measurements correspond to the minimum or average measurements of the Don hares from Berelekh and Bykovskii Peninsula and minimum measurements for the large mountain hare subspecies L. timidus begitschevi Koljuschev, 1936 (Table 2). However, it is distinguished by a high dentition, which is typical for the Don hare. The jaw height at the third premolar (Table 2) corresponds to the limits of variability of this measurement in the Don hares (16.0–19.8 mm [1]) and exceeds its maximum value in the hare (16.3 mm) [2], including the Holocene one [5]. In addition, the Ogorokha hare has a relatively long diastema, which is typical for the Don hare and distinguishes it from the mountain hare. The index of the height of the lower jaw before p3 in relation to the alveolar length of the lower dental row is also indicative: in the Don hares, it is higher than 80% (in the Ogorokha hare, it is 86.5%), whereas in the mountain hare it is lower than 80% (on average, 70.8–75.7%).

Table 2. Sizes of some skeleton bones of fossil Don hares and extant mountain hares from Eastern Siberia (limits and mean values (mm) are shown, n is the number of measurements)
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The Ogorokha hare has a long and relatively powerful tibia, the dimensions of which correspond to or exceed the dimensions of the tibia in the Don hare from Berelekh and Bykovskii Peninsula and are greater than its maximum dimensions in the extant mountain hare from Central Yakutia (Table 2) and the Holocene L. timidus from Altai and southern Siberia [5].

The length of the hind foot of the Ogorokha hare is small, less than that of the mountain hare (Table 1). In July 2022, in the Karginian interstadial deposits on the Semyuelakh River (Indigirka River basin), the hind leg of an adult hare (apparently, the Don hare) was found, the length of which is even smaller (137 mm). Possibly, the small length of the hind foot of the Ogorokha hare reflects the peculiarities of the locomotion of the Don hares that lived under drier conditions with a lower snow depth than the extant hares. The comparison of specific hairs forming the stiff and dense brush on the plantar skin of the Ogorokha hare (Figs. 1b, 1c) with a homologous structure in the Holocene mountain hare of Chukotka [13] showed their complete morphological correspondence. The wide and long feet of the mountain hare are also covered with numerous coarse hairs, which together allowed it to move easily even on loose snow and swampy terrain. Apparently, in ancient hares, the hair brushes of the feet were similarly involved in locomotion: they ensure mechanical protection for the plantar skin and tight adhesion to the substrate when the hares moved through deep snow and the snow crust, as well as over marshy soil.

We have shown that the morphological characters of the Ogorokha hare correspond to the main diagnostic characters of the Don hare, which is also confirmed by genetic data. Taken together, the morphological characters of the Don hare suggests that this was an independent species highly adapted to the harsh conditions of the Arctic, which had (1) a large body size and, apparently, a large body weight, which is typical for northern species and subspecies, and confirms Bergman’s ecological rule; (2) short ears and feet, which are smaller than those of the extant mountain hares from Yakutia, which can be explained by the fact that the Don hare lived under the extremely cold conditions of the Arctic steppe of the cryotic interval of the Karginian Time, the climate of which was colder than in modern Yakutia [9, 10], and corresponds to Allen’s ecological rule; (3) a large tibia, as evidence of the good locomotor abilities of these hares, which are necessary for them to survive in the open areas of the Arctic steppe of the Late Pleistocene; (4) the coat, which has excellent heat-shielding properties due to the topographic features of the length and thickness of the hairs and their specific microstructure, which also contributed to the survival of the hares in the Arctic conditions.