|Dwafing on Malta and Sicily|
One hypothesis describes two elephant migrations from the mainland to Sicily. The first wave may have taken place at the early/middle Pleistocene boundary (0.4 MA) when low sea level related to a cold phase would have reduced the distance between the island and mainland coastlines (Bonfiglio et al., 1992). During the late Pleistocene (0.23 Ma) there was another period of low sea level, which is thought to have enabled additional migration across to the island.
Belonging to the middle to late Pleistocene of continental Europe, E. antiquus of the paleoloxodontine line (subgenus of the genus Elephas) stood 3.0-3.5m at the shoulder and weighed as much as 10 tonnes, living amongst forest or woodland habitats. Molar fragments have been found as far north as the Netherlands, Holland, the North Sea, with a complete skeletons found in Aveley, Essex (Mol et al., 2007). A complete skeleton has also been found in the Upper Rhine area of Germany and remains excavated as far as southern Italy. It is thought that all elephants of the Mediterranean Islands, with the exception of the species belonging to Sardinia, originated from E. antiquus. Groups of individuals migrated over to each island, thereafter evolving into different species on each island. Having survived most of the Pleistocene interglacial cycles, Stuart (2005), hypothesises that E. antiquus went extinct in the last cold stage (LCS), with some surviving as late as 33-34ka in Iberia. However Mol et al. (2007) believe that the species survived a lot later than this, basing evidence on carbon dating of skeletal remains. This extinction is thought to be the consequence of the declination of woodland habitats due to the last cold stage. Nevertheless, reasons why they did not become extinct in previous interglacial cycles still remain unknown. The only significant difference between the last cycle and those preceding it is the arrival of humans. What effect humans had on the population is still undecided.
On Malta and in the Spinagallo Caves, Eastern Sicily, Southern Italy, the smallest elephant that ever lived was discovered: Elephas falconeri. In the early 1960s, a total of 2493 remains were found with a minimum of 104 individuals identified. Dating to the early to middle Pleistocene found inter-bedded between lower to early middle Pleistocene marine deposits (Bonfiglio et al., 1992), this species only measured approximately 0.9 m to the withers and weighed 100 kg (Raia et al., 2003). Its ancestor on the mainland is believed to be the straight-tusked elephant, Elephas antiquus. The accumulation of these elephant remains within a cave is interesting. Elephants wouldn’t usually inhabit caves although they may have visited them to access mineral salts, a behaviour modern elephant’s exhibit, or for shelter. Consistent with the matriarch-led herd structure of modern elephants, the remains found in the cave consist of adult females accompanied by their young and only a few adult males. Raia et al. (2003) composed comparisons of these specimens with Loxodonta Africana, the African elephant. This is justifiable as although they are not closely related, the environment they experience is very similar: open environments with sparse tree cover. It was assumed that the Spinagallo population represent a good approximation of the living elephant population and so Raia et al. (2003) could construct a survivorship curve for the population. The data was assigned to dimensional classes by the length of the longest bone: the tibia.
These elephants are thought to have exploited an r-selected strategy. Species that adopt this lifestyle tend to exploit less-crowded environments and produce many offspring. Juveniles constituted 58% of the population found in the caves, recognisable by non-fused epiphyses (rounded end of a long bone). The high calf mortality can be related to reproductive factors; for example, smaller calves are less resistant than larger calves. It is thought that dwarfism of this species arose from the lack of predators on the island and other large herbivores to compete with. This reduced the competitive advantage of a large size. Small size allowed enhanced reproduction rates at younger ages. This hypothesis has sufficient support. The Spinagallo adults were paedomorphic and so they retained their juvenile characteristics. This was established in this case by progenesis: achievement of sexual maturity early. Additionally sexual dimorphism was high in this species as competition for females was high. Furthermore, ‘paedomorphosis indicates fast growth and generally fast-growing organisms have high calf mortality, mainly because of physiological factors’ (Raia et al., 2003). Benton et al. (2010) suggested that the calf numbers may have been depleted during seasonal droughts as it is assumed that E. falconeri would have bred at discrete times of the year unlike modern elephants who breed all year round.
Material overlying the Spinagallo calcarenite sediments that E. falconeri was found in, were fossil remains of E. mnaidriensis. Remains found date back to the middle to late Pleistocene and thus E. mnaidriensis is younger than E. falconeri. Standing 1.6-1.8 m at the shoulder, it was first thought that as E. mnaidriensis, a direct descendant of E. antiquus and standing taller than E. falconeri, it was the intermediate form between E. antiquus and E. falconeri in terms of the evolution of dwarfism. However as E. mnaidriensis was found in younger sediments, this is not the correct scenario.
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