Congreso Island Rat

Isla de Congreso (or Congreso Island) measures just 0.256 square kilometers and is part of the small Spanish archipellago of the Islas Chafarinas, a group of three small islets located off the coast of Morocco. Obviously, the ownership of the islands is disputed by Morocco.

The island is uninhabited, but it is home to some rabbits and a pigeon colony. This rocky islet offers just some grass to herbivores. However, the young rabbits and the eggs of the pigeons can be a meal for the indigenous population of black rats (Rattus rattus).

These black rats constitute one of the westernmost insular populations of the species in the Mediterranean region. 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.

To evaluate the influence of insular conditions on body and skull dimensions, scientists measured the biometric characteristics of a Congreso Island Rat and compared these with specimens from nearby mainland populations from Malaga (southeastern Iberia) and Morocco[1].

Results show that Congreso Island Rat was similar in size to Iberian specimens, but the skull shape resembled Moroccan rats. Actually, the specimens from Malaga correspond to one of the largest mainland forms of rat from western Europe. The size of Congreso Island Rat 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 on Congreso Island could have contributed to the black rat size increase.

Queija, López Fuster: Morphometric analysis of the black rat: Rattus rattus, from Congreso Island (Chafarinas Archipiélago, Spain) in Geography – 2000