Anglesey's insular nature along with its long history of agricultural activity have significant implications for the mammal fauna of the island.
The most numerous mammal on the island is probably the sheep - over 400,000 in 1991, and more in winter when the hill farmers from the mainland move their flocks onto the lowland pasture - followed by humans (66,829 in 2001) and circa 40,000 cattle!
The largest native mammal generally found on Anglesey is the Atlantic grey seal. Though there are no large breeding colonies to compare with the Pembrokeshire islands or even Ynys Enlli (Bardsey), seals can be seen regularly at many locations around the island. Puffin Island regularly hosts over 100, hauled out at the eastern end, while most rocky headlands around the north and west coasts have their attendant seals. Pups are born in the autumn in scattered locations, particularly in sea caves and gullies on the Skerries and at North Stack near Holyhead.
Recent work by Marine Awareness North Wales has revealed a significant population of harbour porpoise off the north coast of the island. Porpoise, bottlenose dolphin and common dolphin are commonly seen around the coast. Risso's dolphin is occasionally recorded. Good observation points include Ynys Llanddwyn, Point Lynas and Fedw Fawr / Penmon. Whale sightings are not uncommon. Strandings in recent years have included pilot whales and finn whale.
On land, large predators and native grazing animals were probably exterminated at an early stage in prehistory and replaced by domestic stock. Indeed, foxes were absent from the island until very recent times, in the late 1960s (although dog-foxes were reputedly brought to the island as hunters' quarry). Badgers also appear to have been exterminated until their (re)introduction on the Penrhos Coastal Park near Holyhead in the early 1970s.
The narrow waters of the Menai Strait have long prevented the ingress of species and (until very recently) was a barrier to the invasion of American mink. As a result, water vole thrives on many sites in contrast to its dramatic decline on the British mainland (where habitat loss and predation by mink have combined to decimate populations). Brown rats arrived in Anglesey in the 1740s, supplanting the earlier black rat. The rat population on Puffin Island appears to have been successfully eliminated in 1998 in an attempt to improve conditions for nesting seabirds.
Otter has re-occupied many areas on the island after an apparent virtual absence during the 1970s and 1980s due to agricultural pesticides and over-zealous river management and are now thought to be breeding. Otter are known to compete successfully with mink and in the long term a healthy otter population may be the best defence against mink. See Otter Research Anglesey website.
Red squirrel have persisted on Anglesey despite its low woodland cover (4% in total, half the national average) and the fragmentation of the habitat. American grey squirrel, which competes with and eventually eliminates the native red squirrel, invaded the island in the 1970s and rapidly became widespread. By the late 1990s reds were restricted to Pentraeth forest in the east of the island. Grey squirrel control, since 1997, has reduced the pressure and enabled the population of reds to bounce back, assisted by a trial re-introduction project at Newborough forest. However, greys remain a threat and the long term prospect for reds remains in question.
Native polecat have also appeared in the island for the first time in recorded history in the late 1990. Their status remains uncertain but a number of roadkills appears to confirm a rapid spread.
The absence of many ground predators led to high breeding success for ground nesting birds such as terns, lapwing, curlew, redshank, snipe, etc and Anglesey became a stronghold for these species until recently.
ANGLESEY MAMMAL FAUNA:
hedgehog Erinaceus europeaus
mole Talpa europea
common shrew Sorex araneus
pygmy Shrew Sorex minutus
water shrew Neomys fodiens
lesser horseshoe bat Rhinolphus hipposideros
long-eared bat Plecotus aurituus
whiskered bat Myotis mystacinus
Daubenton's bat Myotis daubentoni
noctule Nyctalus noctula
rabbit Oryctolagus cuniculus
brown hare Lepus capensis
bank vole Clethrionomys glareolus
field vole Microtus agrestis
water vole Arvicola terrestris
wood mouse Apodemus sylvaticus
house mouse Mus musculus
harvest mouse Micromys minutus
brown rat Rattus norvegicus
red squirrel Sciurus vulgaris
grey squirrel Sciurus carolinensis
porpoise Phocoena phocoena
pilot whale Globicephala melaena
Risso's dolphin Grampus griseus
bottle-nosed dolphin Tursiops truncatus
common dolphin Delphinus delphis
fox Vulpes vulpes
badger Meles meles
otter Lutra lutra
weasel Mustela nivalis
stoat Mustela erminea
polecat Mustela putorius
grey seal Halichoerus grypus
fallow deer Dama dama