Monachinae
Monachinae is a subfamily of the family Phocidae (true seals) within the order Carnivora, commonly referred to as the "southern seals," and includes the elephant seals (genus Mirounga), monk seals (genera Monachus and Neomonachus), and the Antarctic seals of the tribe Lobodontini (genera Hydrurga, Lobodon, Leptonychotes, and Ommatophoca).[1] This subfamily is characterized by features such as eight incisors (I 2/2 formula), a long concavity on the maxilla, indistinctly marked anterior palatine foramina, and adaptations for terrestrial locomotion including an archlike internal crest on the humeral trochlea and absence of an intertrochanteric crest on the femur.[2] Members exhibit a karyotype of 2n = 34 chromosomes, distinguishing them from the northern seals of the subfamily Phocinae.[3] The taxonomy of Monachinae is divided into three main tribes: Monachini (monk seals), Miroungini (elephant seals), and Lobodontini (Antarctic seals), with additional extinct basal genera such as Homiphoca, Pliophoca, and Callophoca contributing to its fossil diversity.[1] Extant species number eight, including the southern elephant seal (Mirounga leonina), northern elephant seal (Mirounga angustirostris), Mediterranean monk seal (Monachus monachus), Hawaiian monk seal (Neomonachus schauinslandi), leopard seal (Hydrurga leptonyx), crabeater seal (Lobodon carcinophaga), Weddell seal (Leptonychotes weddellii), and Ross seal (Ommatophoca rossii).[4] While the subfamily's distribution spans tropical, temperate, and polar regions predominantly in the Southern Hemisphere—such as Antarctic waters for Lobodontini and subtropical areas for monk seals—some species like the northern elephant seal and Hawaiian monk seal occur in the Northern Hemisphere.[2] Monachinae originated in the Southern Hemisphere approximately 15 million years ago during the Miocene, with early diversification in regions like Australasia, as evidenced by fossils such as the newly described Pliocene species Eomonachus from New Zealand, marking the first Southern Hemisphere monk seal record.[1] The subfamily's evolutionary history involves multiple dispersals across the equator, at least eight times over the past 15 million years, leading to its current global presence despite a southern-centric phylogeny.[1] Many species face conservation challenges; as of 2025, both monk seal species are classified as Vulnerable by the IUCN due to habitat loss and human impacts, while Antarctic species are more abundant but vulnerable to climate change.[5][6][7]Taxonomy and classification
Etymology and naming
The name Monachinae derives from the genus Monachus, established by John Fleming in 1822 for the Mediterranean monk seal (Monachus monachus), the type genus of the subfamily. The term Monachus originates from the Greek monachos, meaning "solitary" or "monk," a reference suggested by naturalist Johann Hermann in 1779 to describe the seal's reclusive habits or the monk-like folds of skin around its head and neck that resemble a hood.[8][9] The subfamily Monachinae was formally named by Édouard-Louis Trouessart in 1897 as part of his revision of mammalian taxonomy within the earless seal family Phocidae.[10] Trouessart's classification grouped southern hemisphere phocids together under Monachinae, reflecting an early understanding of their shared southern distribution, though this encompassed a broader assemblage than the modern definition. A historical synonym is Mesotaria Trouessart, 1898, proposed for similar southern forms but later subsumed under Monachinae.[11] All extant species in Monachinae share a diploid chromosome number of 2n=34, a conserved cytogenetic marker that distinguishes the subfamily from many northern phocids and was first documented in karyotype studies during the 1970s.[3][12]Phylogenetic position
Monachinae represents one of the two primary subfamilies within the family Phocidae, the true seals (earless seals), alongside Phocinae, which includes the northern seals. This division is well-established in pinniped systematics, with Phocidae forming a monophyletic group distinct from the eared seals (Otariidae) and walruses (Odobenidae) in the order Pinnipedia. The monophyly of Monachinae is robustly supported by molecular evidence from complete mitochondrial DNA coding regions and multi-locus nuclear datasets, which recover it as a strongly supported clade (100% bootstrap and posterior probability values) sister to Phocinae. These analyses, encompassing mtDNA protein-coding genes and nuclear loci, indicate that the divergence between Monachinae and Phocinae occurred approximately 15 million years ago in the early to middle Miocene, likely in the North Atlantic region, based on Bayesian time-calibrated phylogenies using fossil constraints. Morphological synapomorphies further corroborate the monophyly of Monachinae, including greatly reduced claws on the hind flippers compared to the more prominent claws in Phocinae, a derived dental formula of I²/² C¹/¹ P⁴/⁴ M¹/¹ (with two upper incisors per side, differing from the typical phocid condition), and specialized postcanine teeth featuring simple, conical crowns adapted for grasping and puncturing soft-bodied prey rather than crushing. Additional cranial features include the absence of an entepicondylar foramen on the humerus and a characteristic isosceles triangular tympanic bulla with an anteriorly turned lateral lip, reflecting adaptations in the ear structure for underwater hearing without external pinnae.[12][13] Within the pinniped phylogeny, Phocidae branches as the sister group to the Otariidae-Odobenidae clade, with the Monachinae-Phocinae split marking the basal dichotomy in Phocidae; subsequent diversification within Monachinae yields internal clades corresponding to its tribes, excluding any otariid affinities. This structure is consistently depicted in maximum likelihood and Bayesian trees, emphasizing Monachinae's southern hemisphere dominance post-divergence.Tribes and genera
The subfamily Monachinae is divided into three tribes: Monachini (monk seals), Miroungini (elephant seals), and Lobodontini (Antarctic seals). This tribal structure reflects distinct evolutionary lineages within the southern true seals, supported by mitochondrial and nuclear DNA analyses that affirm the monophyly of Monachinae and its internal divisions.[14] The tribe Monachini includes the monk seals, which are now classified into two genera based on molecular and morphological evidence from ancient DNA and skull analyses.[15] Genus Monachus contains a single extant species, the Mediterranean monk seal (M. monachus), which inhabits coastal waters of the Mediterranean Sea and eastern Atlantic.[15] Genus Neomonachus, erected in 2014 for the New World clade, comprises the Hawaiian monk seal (N. schauinslandi), endemic to the Hawaiian Islands, and the extinct Caribbean monk seal (N. tropicalis), last reliably sighted in 1952 and formally declared extinct in 2008.[15]| Tribe | Genus | Extant Species | Notes on Extinct/Status |
|---|---|---|---|
| Monachini | Monachus | M. monachus (Mediterranean monk seal) | Critically endangered[15] |
| Monachini | Neomonachus | N. schauinslandi (Hawaiian monk seal) | Critically endangered; N. tropicalis (Caribbean monk seal) extinct (last seen 1952)[15] |
| Tribe | Genus | Species |
|---|---|---|
| Miroungini | Mirounga | M. angustirostris (northern elephant seal) M. leonina (southern elephant seal) |
| Lobodontini | Leptonychotes | L. weddellii (Weddell seal) |
| Lobodontini | Lobodon | L. carcinophaga (crabeater seal) |
| Lobodontini | Hydrurga | H. leptonyx (leopard seal) |
| Lobodontini | Ommatophoca | O. rossii (Ross seal)[14] |