Some aspects of the biology of the grey squirrel (Sciurus carolinensis) in Great Britain.



  • 1The question of sub-specific rank for the American grey squirrel in Britain is discussed. In America the two forms concerned show intergradation over a large area, and the definitions do not appear to be consistent. Our present animal is probably intermediate between Sciurus c. carolinensis Gmelin and S. c. leucotis Gapper.
  • 2Seasonal variations in the coat are described. Ear-tufts are present in winter. Juveniles and adults in summer coat are brownish in colour and may be confused with the red squirrel, S. vulgaris leucourus Kerr. Erythristic mutants have been observed in the New Forest area.
  • 3Albino, melanie and partially-melanie forms have been recorded in America and in Britain. The genetics of the melanic form are still not understood. The result of a cross between a female melanic and a normal grey male is described. In Britain melanics are to be found in Bedfordshire and Hertfordshire, and albinos occur in Kent, Surrey and Sussex.
  • 4Sex-ratio tables for nestlings, juveniles and adults are given. Although methods of collection may possibly influence this ratio, a slight but steady preponderance of females is recorded in all classes except 1946 nestlings. A total sex ratio for males; females of 86: 100 is given. American records show a preponderance of males over females.
  • 5The grey squirrel has two breeding seasons in Britain: one starts in early January and continues into March, and lactating females are found until late May. The second season starts in early June and continues until the end of July. No breeding females were recorded in October, November and December of any year. Males do not have a definite breeding season. First-year females do not appear to have more than one litter, but adults may have two per year. Breeding seasons in America and South Africa are compared with those for Britain.
  • 6Breeding rates for four years in Britain are compared.
  • 7Tables for litter size in America and in Britain are given. There is a suggestion that autumn litters are larger than those in spring. One hundred and forty-eight litters gave an average of 2.50 young per litter, compared with 3.23 for 55 autumn litters (Britain).
  • 8A group of squirrels kept in captivity was observed to exhibit mating “play”, but there were no pregnancies. Artificial insemination was attempted without effect. Injections of a gonadotrophic preparation were given to five animals, but no pregnancies were recorded. The provision of extra light proved impracticable as the squirrels took cover in their nest-boxes.
  • 9In 1948 breeding started among the captive stock. In this year two litters were born, one to each of the elder females. The conditions under which this occurred are described. The young were reared successfully by the parents.
  • 10The growth of young squirrels was studied. The weight at birth is estimated to be between 13–17 g. Records for growth of hair, opening of eyes, eruption of teeth and weaning are correlated with body-weights. A growth curve from young born in captivity is used to calculate age from body weight. The coat is moulted when weaning is completed. Post-weaning growth curves were plotted from captive animals. Weight records of juvenile squirrels coming in from the field were plotted month by month for comparison with the rising weight of laboratory animals, and an agreement was found.
  • 11Notes were kept over an 18-month period on the nesting activities of grey squirrels in a 34-acre patch of mixed woodland. Records showed that there is a preference for certain trees as nest sites, and showed the relation between summer and winter dreys. They also showed the danger of carrying out a census of squirrels based only on drey-counts without inspection of each drey.