Chemical signals in the form o f odours and tastes are used throughout the animal kingdom, their function being to communicate information between individuals in a wide variety o f circumstances. The study o f chemical signalling between animals is called semiochemistry, semeion being the Greek word fo r signal or a sign. A semiochemical is therefore "a substance with which organism interacts with organism in the shared natural environment" (Albone, 1984). Due to the complexity o f the numbers and types o f these interactions, some attempt has been made to classify compounds, although there is often overlap between groups. The terminology currently in use is described below (Nordlund and Lewis, 1976).
Chemical signals which act between individuals o f the same species are known as pheromones, and can be further divided into two categories. Releaser pheromones invoke responses in the receiving animal which are usually behavioural and immediate. When the response is physiological and the e ffe c ts longer term the signal is termed a primer pheromone. Primer pheromones are known to mediate reproductive suppression by causing endocrine changes in the recipient animal. This is known to occur in a
variety o f species, in particular amongst rodents and will be discussed fully in section 1.2.2.2.
Other chemical signals which act between animals o f d ifferen t species are termed allelochemics and sub-divided into two groups, according to whether the transm itter or the receiver benefits from the signal. Allelochemics which mediate e ffe c ts beneficial to the emitting species are known as allomones. An example o f this is the defensive secretion emitted from the anal gland o f the striped skunk Mephitis mephitis which is highly noxious and effectively repels predators (Albone, 1984). Conversely, allelochemics which favour the receiving species are known as kariomones, and these may often act as allomones or pheromones in other species. A good example o f this are American bolas spiders, who a ttr a c t their prey (male moths), by emitting a chemical signal which is the same as the female moth sex a ttra c ta n t (Albone, 1984).
Semiochemicals are produced and released into the environment by the emitting species in a variety o f ways, but perhaps one o f the most important sources is the skin. Sebaceous, apocrine and eccrine glands under neural and hormonal control secrete a diverse array o f chemical compounds including steroids and lipids. These impart odours o f their own, but are also broken down by micro-organisms to produce a complex mixture o f odorous materials. Because some o f these secretions are under hormonal control, they may re fle c t and communicate some aspects o f the physiological state o f the emitting animal. In many mammals, groups o f specialised skin glands form discrete scent glands, secretions from which are often used in chemical signalling. Examples include the tarsal and m etatarsal glands o f black-tailed deer (Quay & Muller-Schwarze, 1970); the temporal gland o f the A frican elephant (Wheller et al, 1982) and the Harderian gland o f the Mongolian gerbil (Thiessen et al, 1969).
The most important sources o f pheromones which a ffe c t reproduction in rodents are urine, and secretions from the reproductive tr a c t. In particular, the preputial glands o f rodents are a rich source o f semiochemicals. These are large groups o f specialised sebaceous glands which lie between the pubic skin and body wall or between the prepuce and the penis, depending on the species. Homologous structures are found in females and are sometimes known as clitoral glands. In the male r a t, the preputial glands may be up to 2 cm long and 10-20 mg per lOOg body weight (Albone 1984). Androgens e ffe c t the size o f the preputial glands and
their ra te o f secretion, therefore in mature dominant male ra ts the size o f these glands is maximal. If intact male and female ra ts are given exogenous testosterone, their preputial glands become enlarged, provided th at they are not already maximal in size (Ebling, 1977). The naked m ole-rat is known to possess preputial glands (Tucker, 1981), and it is possible they could be important in conveying information related to dominance or reproductive status within the colony. The presence o f a communal to ilet chamber in naked m ole-rat colonies provides a fo cal point where exchange o f urinary (or other) chemosignals could occur. Observations o f grooming behaviour when animals enter the toilet chamber suggest th at during this process, scent may be transfered from the soiled litte r to the animal (see section
1.1.4, pp 30).
In many hystricomorph rodents, including the guinea pig Cavia aperea (Wellington et al, 1979) and the capybara Hydrochoerus hydrochaeris (Macdonald et al, 1984), there are well developed perineal glands. The gland lies between the genitalia and the anus and is composed o f sebaceous tissue. Its secretion is deposited, particularly in dominant animals, by a behaviour known as perineal dragging, where the ano-genital region is rubbed along the substrate leaving a scent mark. This type o f behaviour has also been observed in captive colonies o f naked m ole-rats, and occurs as the animal leaves the toilet chamber (Jarvis & Bennett, 1990). However, it is unknown whether or not the naked mole-rat possesses a perineal gland.