Circulatory and Respiratory Systems
Both species of squirrel have systems typical of small mammals and are not reported to have any particular specialisation. Although it cannot hibernate, the Grey Squirrel may reduce its activity during short periods of adverse weather during the Northern Hemisphere winter.
Excretory systems in both species are typical of small mammals. Grey Squirrels produce a few faecal pellets at a time and drops them haphazardly within their home range. The animals do not foul their nest with droppings which are typically tapered cylinders with a conical projection at one end and measure about 10 x 6 mm. The colour of the faeces depends on diet.
The Grey Squirrel partially marks territory with urine containing secretions from Cowper’s glands (Barkalow & Shorten 1973).
Sense Organs and Nervous System
Grey Squirrels have large, mobile nostrils, large nasal cavities and appear to have a well-developed olfactory system, which they use to locate food and to follow scent trails during breeding or range-patrolling. Large ears aid in the reception of community auditory signals. Vibrissae are well developed around the head, especially the mouth, and also on wrists, anus and nipples. Large, dark eyes on the sides of the head give a very large field of vision. Stereoscopic vision is well developed, as is upward vision as a protection from aerial predators. The retina has cones only and visual acuity is, therefore, outstanding. The species has poor visual sensitivity, as is typical of a diurnal organism and is most active when light intensities are moderate. The eye has no tapetum and, hence, no eyeshine. The presence of cones only in the retina and the striking colours and coat patterns of both Palm and Grey Squirrels suggest that colour vision is well developed, but the Grey Squirrel is not thought to have regular trichromatic vision (Barkalow & Shorten 1973).
Endocrine and Exocrine Systems
The endocrine glands of both species seem characteristic of small mammals. Prasad et al. (1966) have described changes in the cytology of the pituitary gland of Palm Squirrels and observed that adrenalectomy of females resulted in the development of adrenal-like tissue in the ovary.
The squirrels’ exocrine glands are characteristic of the small-mammal type. The Grey Squirrel has four pairs of functional mammae serving the mammary glands, Palm Squirrels have but two pairs. Differences in the male reproductive glands are recognised. Grey Squirrels have large, well-developed Cowper’s glands which secrete wax that ‘fixes’ reproductive odours in urine, whereas Palm Squirrels have minute Cowper’s glands. In both species, the weights of testis and accessory gland show cyclic changes. Both species also have sweat glands on palms and soles and also may have apocrine glands in the mouth to mark food or food sites. The sweat glands possibly are used in range marking.
The male reproductive tract of the Palm Squirrel has been described as a non- penile duct type, but the Grey Squirrel, like most squirrels, has a penile duct type tract and thus is quite different (Prasad et al. 1966). Female reproductive tracts in both species are typical of rodents.