| Summary: | Black rats, Rattus rattus, from Chafarinas Archipelago constitute one of the westernmost insular populations of the species in the Mediterranean region. To evaluate the influence of insular conditions on body and skull dimensions, the biometric characteristics of a sample from Congreso island were determined and compared with nearby mainland populations (Málaga -southeastern Iberia- and Morocco). Results show that Congreso specimens are similar in size to Iberian specimens and in skull shape to Morocco animals. Considering this later fact and the geographical proximity between Chafarinas and the Morocco coastline, it seems likely that Congreso black rats had their source in North-African populations. The size similarity between Iberian and Congreso rats does not appear to be the expression of a genetic relationship but a biometric convergence. Actually, the specimens from Málaga correspond to one of the largest mainland forms of R. rattus from western Europe, and the size of Congreso rats might constitute another case of gigantism that appears as an adaptive response to insularity in black rat populations from small western Mediterranean islands. Lack of interspecific competition and low predation pressure in Congreso could have contributed to the black rat size increase. La rata negra, Rattus rattus, de las islas Chafarinas constituye una de las poblaciones más occidentales de la especie en la región mediterránea. Para evaluar la influencia de la insularidad en las dimensiones corporales y craneanas, se determinan las características biométricas de una muestra procedente de la isla del Congreso y se comparan con las correspondientes a poblaciones continentales próximas (Málaga -sudeste ibérico- y Marruecos). Los resultados indican que los ejemplares de Congreso presentan un tamaño similar a los ibéricos y una forma semejante a los de Marruecos. Esta última circunstancia, y la proximidad geográfica existente entre Chafarinas y la costa marroquí, sugiere que los animales de Congreso proceden de poblaciones ...
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