How large are the extinct giant insular rodents? New body mass estimations from teeth and bones

Abstract

The island rule entails a modification of the body size of insular mammals, a character related with numerous biological and ecological variables. From the Miocene to human colonization (Holocene), Mediterranean and Canary Islands were unaltered natural ecosystems, with paleofaunas formed with endemic giant rodents among other mammals. Our aim is to create methods to estimate the body masses of fossil island rodents and address the nature of ecological pressures driving the island rule. We created regression equations based on extant rodent data and used these to estimate the body masses of the extinct species. Our results show strong correlations between teeth, cranial and postcranial measurements and body mass, except for the length of the long bones, the transversal diameter of the distal tibia and the anteroposterior diameter of the proximal tibia, where the equations were less reliable. The use of equations obtained from a more homogeneous group (suborder and family) is preferable when analyzing the area of the first molar. The new regressions were applied to estimate the body masses of some Mediterranean and Canarian fossil rodents (Canariomys, C. bravoi 1.5 kg and C. tamarani 1 kg; Hypnomys, H. morpheus 230 g and H. onicensis 200 g; and Muscardinus cyclopeus 100 g). Our results indicate that under absence of predation, resource availability (island area) is the key factor that determines the size of the Canariomys sp. However, under presence of specialized predators (birds of prey), body size evolution is less pronounced (Hypnomys sp.).

Genetics of Rapid and Extreme Size Evolution in Island Mice

“…Evidence for adaptive change is often observed in island populations, where the limited geographic scope, sharp boundaries, and simplified biotas of islands facilitate the interpretation of evolutionary patterns (Losos and Ricklefs 2009). Insular mammals show elevated rates of morphological evolution (Pergams and Ashley 2001;Millien 2006) and include several examples of gigantism and dwarfism (Stock 1935;Freudenthal 1972;Roth 1992;Moncunill-Solé et al 2014). Populations that colonize islands often experience substantial changes in predation risk, competition, and resource availability that together generate strong selection for shifts in body size (Sondaar 1977;Case 1978;Heaney 1978;Lawlor 1982;Lomolino 1985;Lomolino et al 2012).…”

Estudio preliminar del esqueleto postcraneal del <em>Muscardinus cyclopeus</em> Agustí, Moyà-Solà & Pons-Moyà, 1982 (Mammalia, Rodentia, Myoxidae)

“…cyclopeus es, sin duda, uno de los mamíferos más desconocidos del Neógeno balear. Tan solo muy recientemente, M. cyclopeus ha sido incluido en un estudio en el que se relaciona la masa corporal de algunos roedores insulares con la superficie de las islas (Moncunill-Solé et al, 2014).…”

“…Si bien es cierto que en M. cyclopeus existe un importante incremento del peso corporal en relación a M. avellanarius, comprendido entre un 132% y un 335% (100 g de M. cyclopeus frente a los 23-43 g de M. avellanarius) (Moncunill-Solé et al, 2014;Papillon et al, 2000), es probable que tal incremento no suponga, para un mamífero de talla relativamente pequeña, unos problemas biomecánicos equiparables a los existentes en otros mamíferos más grandes, como el conejo gigante de Menorca (N. rex) o el bóvido endémico de las Baleares (M. balearicus). Además, la existencia de aves rapaces en los yacimientos de Punta Nati (Seguí, 1998), debió suponer, para M. cyclopeus, un estrés que impidió, hasta cierto punto, cambios alométricos significativos relacionados con la longitud funcional y diámetro sagital de los huesos largos de las extremidades y al tipo de locomoción, de manera parecida a las observaciones realizadas en Hypnomys (Quintana & Moncunill-Solé, 2014b).…”

Paleontology and evolution–part I

“…Moncunill-Solé et al (2014) used regression equations devised for extant rodents to estimate the body masses of extinct species from Mediterranean and Canary Islands. The results of this study reveal the nature of the ecological pressures that drive the evolution of body size in insular rodents.…”

“…The results of this study reveal the nature of the ecological pressures that drive the evolution of body size in insular rodents. Moncunill-Solé et al (2014) suggest that resource availability, i.e. island area, is a key factor in determining rodent size in the absence of predators (e.g.…”