Sciency Thoughts: Walruses

Showing posts with label Walruses. Show all posts

Tuesday, 3 January 2023

Walrus sighted in English coastal town.

A Walrus, Odobenus rosmarus, spent a day resting on a harbour in the town of Scarborough in North Yorkshire, England. The Animal first appeared at about 11.30 on Friday 30 December 2022, returning to the sea at about 4.00 pm on Saturday 31 December, by which time the town had cancelled it's annual New Year Eve firework display to avoid distressing the Marine Mammal on the advice of the wildlife charity British Divers Marine Life Rescue; although as a way of attracting visitors to the town, Walruses proved to be rather more successful than fireworks, with thousands of people arriving to witness the event. The Walrus is believed to be the same Animal spotted on the coast of Hampshire earlier in December, where he was given the nickname 'Thor'. It was spotted again on a wooden pontoon at Blythe on the coast of Northumberland, about 160 km north of Scarborough, on Monday 2 January 2022.

Walrus resting on the harbour at Scarborough in North Yorkshire, England, on 31 December 2022. James Spencer/BBC.

Walruses are the third largest species of Seal, exceeded only by Northern and Southern Elephant Seals (Mirounga angustirostris and Mirounga leonina), and are armed with a large and impressive set of tusks. As such they have few natural enemies, giving them a degree of self confidence surprising compared to most wild Mammals in the UK. Both Polar Bears (Ursus maritimus) and Orca (Orcinas orca) will on occasion hunt Walrus, but they are the preferred prey of neither species, due to their large and potentially dangerous nature. Walruses are also hunted by Humans to some extent, although today this activity is tightly regulated and Walrus products are listed on Appendix 3 of the CITES convention.

Walrus resting on a pontoon dock at Blythe in Northumberland, England, on Monday 2 January 2022. The Guardian.

Walruses are, however still considered to be Vulnerable under the terms of the International Union for the Conservation of Nature's Red List of Threatened Species, due to the potential threat they face from a warming climate. The species is naturally found in the waters of the Arctic Ocean, around Greenland and Svalbard Island, and the northern coasts of Russia, Alaska, and Canada, where they feed on a wide variety of marine invertebrates, with a strong preference for Clams. These environments are considered to be particularly vulnerable to warming climates, which cause loss of the seasonal ice cover, lower the oxygen content of the water (cooler water can absorb and retain more dissolved oxygen than warmer water), and enable invasions by species from warmer climates. While occasional lone Walruses may venture southward (two Walruses where seen on the coasts of Europe in 2021, one in the Irish Sea and the other in the Baltic), it is doubtful that these environments produce enough food to feed a breeding population of these large Mammals, and doing so would almost certainly bring them into conflict with Human shellfish farmers and harvesters (notably, the Walrus seen in the Baltic in 2021 starved to death on the coast of Finland, despite efforts to rescue it).

Distribution map for Walrus, Odobenus rosmarus, populations. ICUN Red List.

Thursday, 13 August 2015

Archaeodobenus akamatsui: A fossil Walrus from the early Late Miocene of Hokkaido.

Walruses, Odobenidae, have a good fossil record from the middle Late Miocene (about 8 million years ago) onwards, and appear to have been a fairly diverse group of large Seals for much of that time, with the single surviving modern species, Odobenus rosmarus, being a rather modified and specialized member of the group. Fossil Walruses older than about 8 million years are rare, though the group was clearly present before this time, as the group was already quite diverse when it became abundant in the fossil record; older fossil Walruses are therefore key to understanding diversity within the group.

In a paper published in the journal PLoS One on 5 August 2015, Yoshihiro Tanaka of the Hokkaido University Museum, the Department of Geology at the University of Otago and the Numata Fossil Museum and Naoki Kohno of the Department of Geology and Paleontology at the National Museum of Nature and Science and the Graduate School of Life and Environmental Sciences at the University of Tsukuba describe a new species of fossil Walrus from a partial skull and skeleton from the 10.0-9.5 million-year-old Ichibangawa Formation of Tobetsu Town in Hokkaido.

The new species is named Archaeodobenus akamatsui, where ‘Archaeodobenus’means ‘ancient Walrus’ and ‘akamatsui’ honours Morio Akamatsu of the Hokkaido Museum for his longstanding contributions to the geology and palaeontology of Hokkaido. The species is described from a single specimen comprising a partial skull with most of the left side preserved, as well as the left mandible and several teeth, several vertebrae and ribs and an incomplete sternebra, a partial scapulae, and a nearly complete right humerus. The specimen is thought to have been a male animal between about 2.8 and 3 meters in length, and weighing about 390–473 kg.

Cranium of Archaeodobenus akamatsui, in (A) dorsal view, (B) left lateral view, (C) ventral view. Tanaka & Kohno (2015).

Archaeodobenus akamatsui clearly belongs to the Odobenidae, having a number of traits only found in Walruses, notably a laterally expanded pterygoid strut (supporting structure on the upper jaw which forms part of the palate), a bony tentorium that is closely appressed to the petrosal (part of the structure of the ear) and well developed first and second premolars. However it is otherwise distinct from other members of the group, suggesting that it represents a distinct lineage.

Reconstructed cranium of Archaeodobenus akamatsui, in (A) dorsal view, (B) left lateral view, (C) ventral view. Tanaka & Kohno (2015).

Archaeodobenus akamatsui was derived from a distinctive glauconitic (green sandstone, typically deposited in a marine environment with low oxygen levels) layer within the Ichibangawa Formation. It is the second Walrus species described from this layer, implying that two separate Walrus lineages were already present in the area at this time, and that therefore that the rapid diversification of the group had already begun. This glauconite layer is thought to represent a deposits laid down following a sudden transgression (rapid deepening of the sea) covering part of a delta environment following a period of receding sea levels over the previous million-and-a-half years.

Tanaka and Kohno reason that the falling sea levels between 12 and 10.5 million years ago would have reduced the number of coastal environments suitable for Seals on the East Asian coastline as the waters drew towards the continental shelf, leading to seas that increased in depth rapidly close to the shore. This would have caused reproductive isolation between groups of Early Walruses, with small isolated populations beginning the process of diversifying away from one-another. The sudden transgression after 10.5 million years ago would have flooded many coastal areas, creating many new shallow marine coastal environments, allowing these isolated Walrus species to greatly expand their ranges, bringing them into contact with other, now reproductively isolated, populations and triggering competition between these groups, pushing further morphological diversification within the group.

The role of eustasy in early late Miocene odobenid diversification in Hokkaido, Japan. Tanaka & Kohno (2015).

Such sea-level change events have been linked to events in Seal evolution elsewhere, most notably in South America where a sudden transgression in the Late Pliocene is thought to have triggered an extinction event among Phocids ('True' or Earless Seals) by replacing broad shallow seas covering much of what is now the Atacama Desert with deeper water environments less suitable for these animals.

Reconstruction of Archaeodobenus akamatsui. Tatsuya Shinmura/Ashoro Museum of Paleontology, in Tanaka & Kohno (2015).

Tuesday, 29 April 2014

An enigmatic Walrus from the Miocene of California.

Walruses (Odobenidae) are large Seals (Pinnipedia) related to Sea Lions and Fur Seals (Otariidae). The group first appears in the fossil record about 16 million years ago towards the end of the Early Miocene. The modern Walrus (Odobenus rosmarus) is one of the largest living Seals, and is a specialized shellfish eater, with well-developed tusks used to root out Clams, a thick moustache used to probe muddy sediments and a highly developed feeding system capable of sucking large Clams from their shells. Fossil Walruses, of which numerous species have been documented from the North Pacific region, apparently pursued a more diverse range of diets, with suction-feeding, specialist Fish eaters and generalist species all recorded.

Pelagiarctos thomasi is a large fossil Walrus described from partial remains from the Middle Miocene Sharktooth Hill Bonebed of Kern County, California in 1988. This has been identified as a large predatory Walrus, possibly a top-predator specializing in hunting other Marine Mammals, though it was initially described only from a partial lower jaw with most of the teeth missing.

In a paper published in the journal PLoS One on 16 January 2013, Robert Boessenecker of the Department of Geology at the University of Otago and the University of California Museum of Paleontology and Morgan Churchill of the Department of Geology and Geophysics and the Program in Ecology at the University of Wyoming, describe a second specimen of Pelagiarctos from the Middle Miocene ‘Topanga’ Formation of Orange County, California. The new specimen differs enough from the original that it is not placed within the species Pelagiarctos thomasi, but since modern Walruses are known to be somewhat morphologically variable, Boessenecker & Churchill refine from erecting a new species to describe it, instead referring to it as Pelagiarctos sp.

Mandibles of Pelagiarctos. Comparison of (A) Pelagiarctos sp. in dorsal aspect, and Pelagiarctos thomasi in (B) dorsal and (C) lateral aspect. Boessenecker & Churchill (2013).

The new specimen comprises the partial lower jaw of an animal similar to the first specimen, but this time with most of the teeth intact. Based upon comparison of these teeth to those of other large marine and predatory Mammals, Boessenecker & Churchill reject the idea that Pelagiarctos was a specialized predator hunting other large Marine Mammals. It was certainly a large animal, probably comparable to a Sea Lion in size, but its teeth were not exceptionally large and do not appear specialized for a subduing large prey, as its jaw structure, with robust strongly fused mandibles, while clearly strong, does not appear well suited to subduing large struggling prey or cracking the bones such animals, tasks which require a degree of flexibility apparently absent in the jaw of Pelagiarctos. Instead Boessenecker & Churchill suggest that Pelagiarctos was probably a more generalist feeder, which would have consumed the meat of large prey should this be available, but which principally ate fish or even foraged for shellfish.