CHEMICAL INFORMATION, SCENT TRAILS AND SPATIAL BEHAVIOUR OF SMALL FOREST RODENTS. A REVIEW

Michał Kozakiewicz, Anna Kozakiewicz

ABSTRACT

Numerous authors indicate a significant role of chemical information (scent) left in the environment in communication between individuals in natural mammal populations. The basic functions of olfactory signals in small forest rodents are believed to include information related to reproduction: identification of sex, age, sexual status and social position of prospective sexual partners. It seems that scent-marking of movement routes thus creating a network of scent trails in the environment, channelling animal movements and making finding the way easier (particularly in case of long-distance movements) is another very important function of scent marking. Some examples of field experiments that confirmed the concept of scent trails are presented and discussed. However, some experiments suggest that besides the use of olfactory information animals have also other possibilities of finding the way in unknown space.

Key words: odour, scent marking, small mammals..

CONVEYING INFORMATION BETWEEN INDIVIDUALS

Conveying information between individuals is one of the basic conditions for population existence and it takes place in all group of organisms, irrespective of their taxonomic status and habitat. Interactions through concentration of metabolites in the environment might be recognised as one of the most primitive forms of conveying information [1]. In animals – particularly in mammals – many special forms of conveying information between individuals based on chemical changes made in the environment (e.g., odour) or conveying information in other ways (e.g., images, sounds) have evolved.

Conveying information between individuals may be of direct or indirect character. Direct communication takes place when there is direct contact between information donor and its recipient – for instance individuals see and/or hear one another. Information between individuals may be conveyed directly to a specific addressee, that is, may be of individual character. The direct contact between an information donor and a recipient enables also an instantaneous effect of the information conveyed. Thus, addressee's reaction becomes in turn a piece of information to a partner and "a dialogue" between individuals is possible in which one partner receives direct signals from another partner and reacts appropriately. Mating behaviour or aggressive behaviour in many animal species are typical examples of such a "dialogue". Direct contact between individuals conveying information one to another simultaneously makes it also possible to use a variety of communication forms (e.g., acoustic signals, appe arance, behaviour) and reach considerable precision of reading them. It is also possible to modify information communication depending on addressee's reactions. However, conveying information directly requires two partners to meet in a specific location at the same time. Thus, a number of individuals taking part in such information exchange is usually small, often limited to two. Another "weak point" of such a communication method is also the fact that information conveyed is short-lived as its reception is possible only at the time of sending it. (Table 1).

Conveying information between individuals in an indirect way does not require contact between an information donor and recipient, they do not have to meet. Such communication takes place through leaving information in the environment (e.g., in the form of faeces, urine, traces of activity – e.g., tracks, feeding signs, burrows, treaded tracks) what makes it possible for many possible recipients to read it (information left in the environment becomes commonly available). Duration of information in the environment is much longer in relation to direct communication so it is available for a greater number of recipients also for that reason. The weak point of such a way of information conveying is a possibility of reading it by undesired recipients (e.g., by predators) and also a possibility that out-of-date information stays in the environment (Table 2).

THE ROLE OF ODOUR INFORMATION IN SMALL RODENTS

Chemical information is a special type of information that might be conveyed directly between individuals or indirectly by placing it in the environment. Conveying chemical information received in the form of odours is particularly common in mammals, including populations of many small rodent species. Scent signals might be conveyed in faeces, urine, saliva and through excretions of special glands [8]. According to Mech et al. [17], olfactory signals in small mammals first of all play the role upon choosing partners for reproduction. According to Kruczek [13, 14, 15], particularly male odour might evidence its social status and sexual attractiveness. Olfactory information in small rodents serves also the purpose of identification of individual animals and making it possible to recognise them as an alien or familiar individual. [20]. In laboratory and field studies it was found that odour may convey information on the species, sex, sexual condition and social status of an individual. (e.g. [3, 16, 19]). Information recorded in odour may lead to various short- and long-term changes in animal behaviour of animals that read it. They may include aggression, sexual behaviour, escape, and others [11, 21 and others].

Dębowska [6] investigated the influence of odour left in live-traps by captured rodents on likelihood of trapping other individuals. When comparing successive captures of individuals of two forest rodent species, the yellow-necked mouse (Apodemus flavicollis) and bank vole (Clethrionomys glareolus) in the very same traps, the author found mutual negative responses of individuals of one species to signals left in traps by individuals of another species. Frequency of successive catches of individuals of the same species (both mice and voles) was significantly higher than that assumed for random sequence of captures (allowing for proportions of numbers of the two species in the environment). On the other hand, catching individuals of different species in the same trap one after another (a mouse after bank vole, a bank vole after mouse) was significantly less frequent than in the situation when random order of catches was assumed. (Table 3).

When analysing the order of catches in live-traps of bank vole individuals, the same author investigated reactions to olfactory signals originating from adult individuals of the opposite sex. She found that sexually active males were captured in traps where previously adult females had been captured more often than randomly, however, such regularity was not found for the opposite situation (that is, captures of adult females in traps in which sexually active males had been caught previously) (Table 4) [6]. The results of the experiment described clearly show the important role of olfactory signals as information on the species and also – within the species – on sex and sexual status of individuals. Finding clear reactions of males to odour of active females and a lack of opposite reaction seems to confirm the hypothesis by Ims [9] that in the bank vole males are individuals which actively search for partners for reproduction and just like distribution of females in sp ace corresponds to distribution of resources in the environment, distribution of males depends on distribution of females.

The above examples show that in addition to species identification, the basic function of olfactory signals in small rodent populations include first of all conveying information related to reproduction: identification of sex, age, sexual status and social status of possible sexual partners.

SCENT TRAILS

According to Naumov [18], habitat of each population is filled with various information left by individuals so that a population lives in "the environment of information", created by that population itself. Information contained in the environment has specific potential for affecting individual animals and their fate (e.g., their behaviour, mating, etc.), thus population itself may shape its history. It seems that the presented concept, called by Naumov [18] "the concept of informational fields" well explains the role of the population ability to make informational modifications in the environment. Marking movement routes and creating a network of so called scent trails channelling animal movements and making it easier for animals to find a way in the environment seem to exemplify such a function of olfactory signals, particularly in case of long-distance movements. Existence of such scent trails was suggested by Andrzejewski and Babińska-Werka [2] and Jamon [10].

If one assumed correctness of the suggestion of scent trail occurrence, one should expect that animal activity should not be necessarily concentrated around burrows (so far believed to be centres of animal activity) but rather in locations where saturation of the environment with scent trails is high [4], and thus – perhaps, in better microhabitats. Therefore, experimental homogenous saturation of the environment with animal odour should lead to "homogenisation" of animal spatial activity.

The above hypotheses have been tested in the field experiment by Gortat et al. (paper submitted for publication). The experiment was carried out in two forest habitats: an alder-ash forest – a rich habitat with dense albeit mosaic plant layer covering the forest floor and in a pine forest – a poorer habitat, with scarce undergrowth, of more homogeneous character. In both habitats lines of rodent localisation stations were set. Each line was made up by 260 stations arranged at 0.5 m intervals. At each point one PCV pipe equipped with a strip of paper was placed. On paper strips animal tracks were left [5]. At the first stage of the experiment, bank vole visits to pipes were analysed with no bait provided, at the second stage pieces of sponge soaked with scent of alien individuals were placed in pipes. It was found that in the pine forest the distribution of numbers of animal visits to pipes was of aggregated character both before and after providing odour of alien individuals (dispersion c oefficient I values over one; I = V/x, where V is variance and x – mean value), however, the aggregation index declined after providing scent. (Figure 1). In the alder-ash forest the aggregation level that had been initially lower than that in the pine forest increased after providing scent of alien individuals (Figure 2). As a consequence, placing scent resulted in smaller differences in animal spatial distribution in both habitats under analysis. After providing odour also differences in a number of stations visited by bank voles where trails were collected and frequency of visits to them (average visit number/ day) diminished. (Table 5).

Thus, the results of the above experiment completely confirmed assumptions resulting from adoption of the hypothesis of scent trail occurrence in free-living populations of forest rodents. It was experimentally shown that the scent left in the environment by individuals is of informational character and its uniform placing leads to disturbance of the existing situation (differences in spatial activity of animals in different habitats diminish).

Thus, it also seems that the hypothesis of scent trail occurrence that may channel animal movements in the populations and also help animals find their way during long-distance movements is correct. That hypothesis was tested in the field experiment in which bank voles living at the forest edge were displaced at various distances (100, 350, 750 meters) out of the area where they lived and then their returns were analysed [12]. Animal returns from considerable distances might evidence the use of scent trails for finding the way. There were three variants on animal displacement in the experiment: further into the forest, out of the forest to an uncultivated field and out of the forest to just ploughed arable field. Considerable numbers of returns were expected in the first two variants, according to the assumption of scent trail occurrence in the forest environment and in uncultivated field and no returns of animals from the ploughed field due to destruction of possible trails as a result of ploughing. The results obtained have not fully confirmed the above presumptions – successful returns from considerable distances were observed in all the three experiment variants (Table 6). It suggests that there are other possibilities of animal orientation in space in addition to making use of olfactory information.

Thus, the research results described above seem to confirm the hypothesis of the significant role of olfactory information in natural populations of small forest rodents. In addition to information related to reproduction such as identification of sex, sexual status and social status of a prospective sexual partner, odours could also be used for intra- and interspecific identification of individuals. The results described also seem to conform to the Naumov's theory of informational fields (1977) and yield the sound evidence confirming correctness of the concept of scent trails by Andrzejewski and Babińska-Werka [2] following from Naumov's hypothesis. On the other hand, the fact of finding the way by animals in the environment devoid of olfactory information seems to evidence that olfactory information is not the only one clue making it possible for animals to orientated themselves in space during long-distance movements. Thus, the problem needs further investigations.

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