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Redundant sources of spatial information in animals






 

Animals refer to different sources of information and use different sensory modalities in order to navigate successfully, and similar mechanisms may be involved in different taxa. Although comparative studies have clarified many problems of how animals find their paths in the world, many questions are still discussable after more than a century of investigations. As Wehner (1998) notes, in spite of the impressive body of literature that studies animal migration and homing have produced in the last decades, we seem only to have touched the surface of the navigation system at work.

There are several lovely objects for students of navigation such as Pigeon, Honeybee, Desert fortis -ant, Loggerhead Sea Turtle and some others with whom we have already met in the previous section and whose secrets of multiple-ways navigation have come to light little by little but still remain rather far from the crystal clarity.

For example, the so-called multi-coordinate grid maps were first proposed 1882 by Viguier to explain long-distance navigation in birds. This question was reopened in the mid-twentieth century by the proposals that the grid map is derived from Coriolis force and geomagnetic fields and that birds use the gradient of sun’s movement (Yeagley, 1947). In the 1950s and 1960s, the debate on birds homing was dominated by Matthews’ sun navigation hypothesis (Matthews 1955). More recent studies essentially corrected this hypothesis although it remained obvious that the sun is a key element of the homing processes in birds. Over 50 years, Kramer’s “map-and-compass concept” remained an accepted constant (Kramer, 1957). Further experiments added the use of olfactory gradients in birds as well as using visual signals from environment such as visual landmarks. Additional methods of navigation in birds are still being intensively investigated and discussed such as using ultrasounds, polarised and ultraviolet skylight, and different sorts of landmarks (for reviews see: Papi, 1986; Wiltschko and Wiltschko, 2003; Wallraff, 2004).

Another set of examples concerns studying mapping behaviour in hymenopterans that dates back to classic experiments of Fabre (1879) and Romanes (1885) who displaced individually marked bees, wasps and ants to unfamiliar sites and tried to clarify details of their homing. More than a century of studies mainly based on different variants of displacement experiments have revealed that in bees and ants path integration employing a skylight compass is the predominant mechanism of navigation, but landmark-based information is used as well (Lewtschenko, 1959; Lindauer, 1963; Dyer, 1987, 1996; Wehner, 1998). Indeed, as Wehner and Menzel (1990) note, unexpected displacements in dark boxes carried by human experimenters have obviously not been an evolutionary force that has shaped the insect’s navigation system. Nevertheless, plenty of displacement experiments have allowed showing that the flexible use of vectors, snapshots and landmark-based routes enables insect’s navigation system to cope with many different ecological situations such as obscured sun, overcast sky, apparent movement of the sun and drift by wind. Hymenopterans demonstrate a great deal of flexibility switching between different sources of spatial information. Experiments by Chittka et al. (1999) showed bumblebees Bombus impatiens as being able of foraging in complete darkness by walking instead of flying. Using infrared video, the researchers mapped walked trails. They found that bumblebees laid odour marks. When such odour cues were eliminated bees maintained correct directionally, suggesting a magnetic compass. They were also able to access travel distance correctly, using an internal, non-visual, measure of path length. The ability to switch between different orienting cues conformable to changing circumstances (including complete darkness) were demonstrated on several ant species by Mazokhin-Porshnyakov and Murzin (1977), Dlussky et al. (1978) and Reznikova (1983). Indeed, ants amaze researchers by the diversity of ways they use orienting cues. For instance, Camponotus japonicus aterrimus Em. marks stems in a purely dogs fashion standing on tiptoe and moving the abdomen along the stem (Reznikova, 1981). Ants of different species were discovered to use different orienting cues when searching for food, homing, and patrolling boundaries of defended territories (Reznikova, 1974).

It is interesting to note that hymenopterans switch between different ways of spatial orientation as their experience increases, in a fashion analogous to vertebrates that switch navigational tactics with experience. For instance, naive giant tropical ants Paraponera clavata initially use chemical trails to find food sources and to return to the colony but switch to visual cues as they gain experience. Ants using visual cues run twice as quickly between sites as those following chemical trails (Harrison et al., 1989). Laboratory experiments with the use of artificial flowers have demonstrated that naive bumblebees use relatively simple tactics of foraging; as their experience increased, however, they increasingly depend on their spatial memory for locating rewarded flower locations (Dukas and Real, 1993; Burns and Thomson, 2006).

Applying imagination about redundancy of information sources to water dwellers, we can consider fishes for which hydrostatic pressure acts as a global landmark or reference. Similar to compass direction or distal visual information, it provides information about the location of a goal, especially to an organism that must orient and navigate in three dimensions (Healy, 1998). Fishes also use landmarks to orient and navigate. They can detect changes in landmark size and will modify their locomotor behaviour to integrate the change into an internal representation. If the water level changes, increasing hydrostatic pressure, the fish orient to a landmark, if present. If no landmark is present, the fish rely on an internal representation oriented to hydrostatic pressure (Cain and Malwal, 2002). Braithwaite and Girvan (2003) tested experimentally whether river three-spined sticklebacks are more adept at using water-flow as a spatial cue than fish from ponds. Fishes from two ponds and two rivers had to learn the location of a food patch in a channel where water flow direction was the only reliable indicator of the goal position. All fish were able to use water flow as a spatial cue but one of the two river groups was significantly faster at learning the patch location. When the task was reversed so that fish that had formerly been trained to swim downstream now had to learn to swim upstream and vice-versa both river groups learned reversal task faster than two pond groups. In a second experiment, fish were given a choice between two different types of spatial cue: flow direction or visual landmarks (rocks, plants). A test trial in which these two cues were put into conflict revealed that the river population showed a strong preference for flow direction whilst the pond population preferred visual landmarks. Thus learning and memory are fine-tuned to the fish’s local environment, and fish sampled from contrasting environments use different types of spatial information.

Surprisingly little is known how mammals navigate under natural conditions and cope with given environmental constrains. Thus, field experiments in which positions of several feeding platforms as well as several artificial landmarks were manipulated revealed that free ranging Columbian ground squirrels (Spermophilus columbianus) rely on multiple types of cues. Local landmarks were used only as a secondary mechanism of navigation, and were not attended to when a familiar route and known global landmarks (such as forest edge, or the outline of the mountains) were present (Vlasak, 2005).

Yet a fundamental principle of spatial orientation is that navigators utilize multiple and redundant sources of spatial information (Keeton, 1974; Cheng and Spetch, 1998; Jacobs, 2003). Compasses based on different cues interact in intricate ways; they are calibrated against each other, replace each other, and do so differently during successive stages of development (Wehner, 1998).

In this issue we consider examples from two series that illustrate the principle of redundancy. The first series includes specimens being placed sequentially under different circumstances and thus forced to use different ways of navigation. The second series includes species as wholes which are characterized by using multiple navigation systems owing to peculiarities of their natural history.

But one example stands beyond the sequences mentioned above. In ants’ society perhaps the most original way of realization of the principle of redundancy could be observed: the use of different cues is implemented in different family members. It is known that threading their way through stems of grass, these insects use multiple sources for navigation, referring to celestial cues, local landmarks of different sizes, and also pheromone trails (Rosengren, 1971). It have revealed that in steppe ants Formica pratensis individual members of each group that carry out their tasks on a local plot of a common feeding territory possess different preferences when choosing ways of navigation. Some ants use small local landmarks (such as pebbles) and experimental removal of small artificial beacons would be sufficient to confuse them. Some family members prefer bigger landmarks, from size of a bottle to size of a bush or, say, a man. And for other ants shape of the wood against skyline is significant to find their plots, so global changes of a scene would be needed to confuse their orientation. At last, there are some ants that are guided by odour cues only and they do not pay attention to any visual cues when looking for their destination (Reznikova, 1983, 2005).

More individualistic organisms than ants could consider different sources of information reliable for mapping depending on context of their life. For example, experiments on laboratory rats indicate that they apply different systems of orientation depending on circumstances (Diez-Chamzio et al., 1985). Rats were trained in a radial maze in a rather unusual way. They were placed at the end of one arm and then had to go to the end of another arm in order to receive food. The correct arm was distinguished by a sandpaper floor. Throughout the training, the landmarks provided by various objects in the room, and the shape of the room itself, were made irrelevant by rotating the maze from one trial to the next. In a second stage of the experiment, food was placed again at the end of sandpaper arm, but the maze was not rotated so that all the cues associated with the room were no longer irrelevant to the solution of the problem. Despite this change in training, subsequent testing revealed that rats had learned very little about the cues that lay outside the maze. In contrast, a control group, which received just the second stage, learned a great deal about the significance of the extramaze cues. One way of summarising these results is to say that pre-training with the local landmark of sandpaper blocked the development of a cognitive map based on extramaze cues in the second stage of the experiments (Pearce, 2000).

This example gives a flavour of the argument that the same navigator being placed in different situations is able to be or not to be a constructor of the cognitive map. What system of navigation would be chosen is likely to be determined by such important factors as inherited predisposition for the use of definite ways to orient as well as reliability of these ways in current context of life. Tied by wired program of preferred ways of navigation, animals nevertheless demonstrate high level of freedom in combining them. Recent studies have revealed a great deal of flexibility in the use of navigation systems in many species.

In order to judge whether a subject really prefers any particular system of orientation a so-called disorientation procedure (reorientation task) has been applied. When searching for a hidden food, the subject was disoriented between acquisition phase and the test itself.

For instance, in Cheng’s (1986) experiments rats searched for food previously hidden in one of the four corners of a rectangular apparatus. After animals were familiarized with the experimental environment, they were removed from the apparatus, disoriented within a closed box, and returned to the apparatus to search for food. In this reorientation task, the rat had to re-establish its position and heading before it engaged itself in goal-directed behaviour. To be oriented again, the rat could rely on the shape of the apparatus, on the patterns, on the odours, or on the brightness of the walls. In the cited experiments rats showed a high rate of searching both at the correct (reward) corner and at the rotationally equivalent opposite corner, and these two corners on the same diagonal are defined by the same geometric relation within the apparatus (length and width). This search pattern was constant, despite the availability of many other cues including strong distinctive odours and large differences in contrast and luminosity. This and other findings (Gallistel, 1990) suggest that geometric features of environment are spontaneously taken into account by rats and dominate over other landmarks.

Gouteux et al. (1999) have applied reorientation test to rhesus monkeys and baboons and revealed more flexible behavioural tactics. Although primates prefer to rely on the large scale geometric cues of the experimental rooms in order to reorient, they also used nongeometric information (coloured wall). Surprisingly flexible ability to use both geometric and nongeometric spatial information to reorient has been found in pigeons and chickens (Vallortigara et al., 1990; Kelly et al., 1998).

Influence of ageing is significant to our knowledge about flexibility of navigation. Hermer and Spelke (1994, 1996), in a series of studies conducted with human children, have examined the use of a geometric module by toddlers. Children saw a desired toy that was hidden in one of the corners of a rectangular homogeneous experimental chamber. In one of the experiments, the chamber contained no distinctive landmarks. It turned out that when no information other than the shape of the environment was available, children searched equally often in the correct and in the rotationally equivalent corner. When nongeometric information (a blue wall or a pair of toys placed in the room) was added, children still divided their searches between two diagonally situated corners and seemed to ignore the added cues. Unlike children, human adults in the same experiments were able to use both geometric and non geometric information to optimise their search.

 


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