How did hippopotamuses colonize islands if they couldn't swim?
Overview
Abstract
Hippopotamuses are typically thought to have swum to islands because of their aquatic lifestyle. However, other research indicates that because of their relatively high density, they are unable to swim. If so, it begs the question of how some islands would have been accessible to hippopotamuses. Because the British Isles, Sicily, Malta, Zanzibar, and Mafia are situated on continental shelves and were frequently connected to the mainland during the Pleistocene glacio-eustatic sea-level drop, their immigration into these regions can be explained. It would be more challenging to explain their presence in Madagascar, Cyprus, and Crete, however. The latter islands may have recently been connected to the closest mainland regions, according to the geological data now available. The purpose of this study is to examine potential routes by which hippopotamuses arrived on islands and to demonstrate the need for improved cooperation amongst experts in the fields of insular evolution, colonization, and speciation research.
Whether or whether Hippopotamus amphibius can swim is still up for debate. However, this disagreement has significant biogeographical and possibly geological ramifications. How did they go to and settle on islands if they couldn't swim?
Many scholars think that hippopotamuses are good swimmers and divers (e.g., Howell, Schule, Sundar & van der Geer, Marra, O'Regan et al., Ali & Huber). However, their aquatic lifestyle is the only basis for this notion. To the best of the author's knowledge, there isn't a single documented account that shows hippopotamuses swimming or floating at the water's surface. Modha's study on the basking behaviours of crocodiles on Central Island (Lake Turkana, Kenya) is the only one that appears to support their purported swimming abilities. Modha detailed a crocodile encounter with a lone hippopotamus in that study. The lake floor is −50 meters deep around Central Island, which is located around 9 kilometres from Lake Turkana's closest (western) coast. Therefore, the hippopotamus could only have swum there.
But the report is questionable. Modha provided detailed descriptions of his encounters with crocodiles interacting with birds, turtles, and other species, but he did not do the same for the hippopotamus, writing generically that the animal "had absolute right of way over basking crocodiles," except for brooding females that snarled at him as he went by. It is suspected that the author may have described a behaviour that he did not witness firsthand on the island.
Subsequent writers essentially overlook the Central Island case. According to van der Geer et al., hippopotamuses can only walk on the bottoms of rivers and lakes; they are unable to swim in freshwater. However, according to the authors, hippopotamuses can float in seawater. According to Hadjisterkotis et al., this is an untested belief based on Frädrich's theory that hippopotamuses must have swum to get to the islands of Mafia and Zanzibar. However, the continental shelf is where these two islands are located. These days, very narrow marine straits divide them from the African continent. Zanzibar and Mafia were a part of the African coast during previous glacial sea-level low stands.
Scientific studies on hippopotamuses' aquatic locomotion abilities were recently carried out by Coughlin and Fish. The findings suggest that current hippopotamuses are incapable of swimming or floating. Numerous studies that show the hippopotamus body is denser than water corroborate this conclusion. These findings support previous claims that they are incapable of swimming made by several. This presents several difficult issues. How were hippopotamuses able to travel to islands across tens or even hundreds of kilometres of open ocean? Furthermore, what compelled them to cross saltwater areas?
The underwater movement and lifestyle of hippopotamuses
According to Fish and Coughlin and Fish, hippopotamuses have non-buoyant barrel-like bodies and poor streamlining. According to Holwell, Fish, Eltringham, Coughlin, and Fish, their feet are not sufficiently altered for swimming. They prefer to walk underwater rather than swim, and they were rarely seen losing contact with the bottoms of rivers and lakes. They gallop short distances with brief unsupported intervals when they lose contact with the bottom (they "walk on the bottom as astronauts on the moon”. Hippopotamuses had to improve their particular bone density to combat buoyancy. To accomplish this, cancellous bone was firmly packed into the medullary cavity They can maintain their feet firmly on the bottom thanks to their great bone density and ability to adjust their specific gravity. A solution like the weighted boots used by deep-sea divers in the past. By moving the centre of mass beneath the centre of buoyancy, the counterbalance of the denser limb bones aids hippopotamuses in stabilizing their underwater locomotion. According to Fish and Coughlin and Fish, this stops their barrel-shaped body from rotating and causing unstable motions.
When hippopotamuses wander into saline water, where buoyancy is amplified, it becomes even more crucial to keep the feet firmly in contact with the bottom to control stability and prevent rolling over.
Juveniles must emerge to breathe every two to three minutes, whereas adults must do so every three to six minutes. When they sleep underwater, they also automatically surface again. Hippopotamuses tend to avoid deep water where hydrostatic pressure would prohibit them from resurfacing because they dislike being out of their depth if they are unable to swim.
Hippopotamuses can withstand fasting longer than other large mammals because they have lower metabolic energy needs.
However, hippopotamuses require water to prevent diurnal desiccation if they are not submerged in muddy wallows, which allows them to withstand periods of time without food, drink, or shade. They also require daily water consumption because they are evaporation-insensitive animals. According to a theoretical method based on the link between metabolic rate and body mass, hippopotamuses consume between 43 and 72 litres of water day (Calder).
With a mean calving interval of about two years and a fecundity of 0.5 to 0.55 (Laws & Clough), hippos also have a low birth rate. Therefore, enough people must be involved to colonize and settle on islands successfully and permanently.
The Pleistocene species Hippopotamus antiques, from which many endemic insular hippopotamuses descended (see below), was somewhat larger than the modern H. amphibs (usually 5–10% larger: Mazza & Bertini,), had a different skull shape, and had different limb bone proportions (Mazza; Kahlke,). Additionally, comparatively more graviportal limbs were present in H. antiques (Mazza; Kahlke).
It's important to note that, like H. amphibius, H. antiques showed higher limb bone density and a similar dense cancellous bone packing in the medullary cavity. Like (or possibly even more than) H. amphibius, H. antiques may have been a bottom walker rather than a swimmer due to its bigger size, barrel-like, poorly streamlined body, and dense, graviportality constructed limb bones.
Consequences of having an insular hippopotamus that cannot swim
Given the aforementioned factors, hippopotamuses' inability to swim may be a significant warning indicator if they are found on islands. Geologists may be able to better understand the dynamics of landmasses and sea-level excursions with the help of this information, but it also provides them with temporal limits. The existence of non-swimming hippopotamuses on islands can be utilized to evaluate the robustness and accuracy of existing geological reconstructions by exposing and potentially dating physical links with the closest mainland that have not yet been discovered. Given that hippopotamuses are incapable of swimming, the existing geological, palaeogeographical, and paleontological data may need to be re-examined. For instance, a thorough analysis of the seafloor surrounding islands like Madagascar and Cyprus reveals the existence of favourable structures covered under as-yet-undiscovered Quaternary sedimentary successions.
New opportunities for the study of insular colonization are presented by hippos.
Widely used models for the strategies, trends, and timing of the colonization and spread to other islands (such as Cyprus, Crete, and Madagascar) need to be re-examined if Coughlin & Fish's conclusion that hippopotamuses are incapable of swimming is accurate. They either "bottom walked" in waters that were shallow enough to allow them to surface every few minutes to breathe or walked on land bridges instead of swimming to the islands. Among the four techniques for dispersal
For homoiotherm animals, it is physically too taxing and stressful, and it is unable to ensure enough individuals and, thus, a sufficiently diverse genepool to prevent genetic drift and extinction. Our selection of possibilities must always be based on parsimony. This implies that land bridges or extremely shallow water that would permit the hippopotamuses to occasionally surface was probably what connected Cyprus, Crete, and Madagascar to the mainland. Although selective extinction and within-island speciation may have significantly changed the community's initial composition, associated insular faunal components can help us determine if the connection was a two-way or filtered corridor, or perhaps even a filtering stepping-stone connection.
Our understanding of the numerous intricate processes involved in insular systems, including colonization, insular evolution, and speciation, is far from comprehensive, as evidenced by the fact that hippopotamuses have colonized islands. Effective cooperation between biologists, paleontologists, geologists, and biogeographers will be necessary to solve such issues.