What is Iceland's smallest animal?

Answer

I presume this means mammal. In Iceland there was only one terrestrial mammal when the country was settled, 1100 years ago. This was the fox (Alopex lagopus). It is assumed that two species of rodent arrived on ships coming from Norway or from the Scottish Islands. These were field mice (Apodemus sylvaticus) and house mice (Mus musculus). The brown rat (Rattus norvegicus) came later, possible in the 18th century, though sources are not exact and they may have been referring to the black rat (Rattus rattus). But it is known that the brown rat was established in Iceland in 1820. Reindeer (Rangifer tarandus) were introduced from Northern Norway in the late 18th century and finally the American mink (Mustela vison) was brought here about 70 years ago. More species of wild animals have been introduced but none of them have survived.

The house mouse is the smallest of these animals. Fully grown house mice are on average 12-25 grammes, with females usually less than 20 grammes and males often over 20 grammes. House mice can reach sexual maturity at the age of a few weeks, weighing only 10-15 grammes. House mice have a gestation period of 19-21 days. The mice have an average litter of 6, but there is a wide variation, ranging from 3 to 13.

The house mouse habitat is normally close to humans and they live in and close to human dwellings, e.g. in outbuildings or stores. In Iceland the species is normally found in towns and cities, but they can be found on some farms and even far from human habitation in South Iceland, where the climate is mildest. House mice that live in outhouses and in stores can breed all year round. In optimal conditions a female can have 5-6 litters a year.

Field mice are much larger than house mice, with fully-grown adults weighing 25-40 grammes. This mouse is much less dependent on man than the house mouse, though in some years in the autumn they will be frequently seen in outbuildings. They only breed in summer and autumn, so they are most plentiful in early winter, which explains why people see them most often at that time of the year.

Translated by Paul Richardson.