The Proboscidea of the Past

[Ngala]

The effect of area and isolation on insular dwarf proboscideans Van der Geer et al., 2016

*This image is copyright of its original author

Figure 1.  Reconstruction of four insular dwarf proboscideans with their respective mainland ancestors. Mainland proboscideans: 1, Palaeoloxodon antiquus; 2, Mammuthus columbi; 3, Stegodon zdanskyi. Insular proboscideans: 4, Palaeoloxodon ‘mnaidriensis’; 5, Palaeoloxodon falconeri; 6, Mammuthus exilis; 7, Stegodon aurorae. 
Based on skeletons at Museo di Paleontología, University of Rome, Italy (1), American Museum of Natural History, New York (2), Taylor Made Fossils, U.S. (3), Museo di Paleontología e Geología G.G. Gemmellaro, Palermo, Italy (4), Forschungsinstitut und Naturmuseum Senckenberg, Frankfurt, Germany (5), Santa Barbara Museum of Natural History, Santa Barbara, U.S. (6), Taga Town Museum, Honshu, Japan (7).
Photo and information credits: Van der Geer et al.

Abstract
Aim: We investigated the hypothesis that insular body size of fossil elephants is directly related to isolation and surface area of the focal islands.
Location: Palaeo-islands worldwide.
Methods: We assembled data on the geographical characteristics (area and isolation) of islands and body size evolution of palaeo-insular species for 22 insular species of fossil elephants across 17 islands.
Results: Our results support the generality of the island rule in the sense that all but one of the elephants experienced dwarfism on islands. The smallest islands generally harbour the smallest elephants. We found no support for the hypothesis that body size of elephants declines with island isolation. Body size is weakly and positively correlated with island area for proboscideans as a whole, but more strongly correlated for Stegodontidae when considered separately. Average body size decrease is much higher when competitors are present.

Main conclusions
Body size in insular elephants is not significantly correlated with the isolation of an island. Surface area, however, is a significant predictor of body size. The correlation is positive but relatively weak; c. 23% of the variation is explained by surface area. Body size variation seems most strongly influenced by ecological interactions with competitors, possibly followed by time in isolation. Elephants exhibited far more extreme cases of dwarfism than extant insular mammals, which is consistent with the substantially more extended period of deep geological time that the selective pressures could act on these insular populations.

Conclusions
This study provides further support for the applicability of the island rule to the largest terrestrial mammalian herbivorous taxa (see Benton et al., 2010 and Stein et al., 2010 for intriguing indications that very large herbivorous dinosaurs such as titanosaurian sauropods may have also followed the predicted pattern).
The patterns discussed here for body size variation in proboscideans over space and time do not support a significant relationship between isolation and body size decrease for proboscideans. Islands relatively close to the mainland may harbour extremely size-reduced proboscideans as well as normal-sized species. Islands further away than 48 km (the maximum reported distance an Asian elephant can swim) do not harbour the smallest species.
The correlation between area and body size is positive but weak with much scatter around the trend for all proboscideans, but more robust for Stegodon from low latitude islands. Our observations support an ecological hypothesis of body size evolution, inferred from the significant influence of competition on body size evolution (see also Palombo, 2009). On the mainland and on relatively balanced and ecologically rich palaeo-islands such as Sulawesi, interaction with ecologically relevant species resulted in a (relatively) large body size. On islands with more depauperate assemblages, however, release from these ecological interactions appears to have resulted in a smaller body size. In the absence of competitors, body size tends to trend towards a size positively correlated with island area, provided that deep geological time was available to allow the dwarfing process to proceed beyond the initial phases.

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