The effect of distance and time on turning alternation behaviour in woodlice (Oniscus asellus)

Introduction

Orientation behavior is important in providing organisms with the highest chance of finding favorable environmental conditions and thus ensuring a higher probability of growth, safety and consequentially successfully mating and producing offspring (Hughes, 1989). Oniscus asellus, the common woodlouse, is a crustacean that requires humid habitats, commonly being found in rotting logs and under stones in Europe and the Americas. Consequently, orientation behaviours such as hygro-kinesis are to be expected (Hughes, 1989). Kinesis enables efficient relocation to more favorable conditions using simple procedural rules resulting in woodlice maintaining a straight line of movement rather than turning, even if obstacles block their path. As a result, it is hypothesised that woodlice, if forced to turn left, are more likely to immediately turn right afterwards, and vice-versa. This would increase the efficiency of the woodlouse moving away from an unfavorable area and thus decrease the average time until it finds a more favorable habitat (Hughes, 1967, Shafer 1972, Sutton 1972). Furthermore, the longer the time between turns, the lower the turning bias effect.

This experiment demonstrates this turning behaviour and tendency to move in a straight line as well as investigating the duration of this turning bias in an attempt to aid our understanding of how organisms maximize their probability of remaining in a suitable microhabitat.

Methods

Two experiments were carried out. One to test whether a turning bias was observed (Experiment 1 visible in Fig.1), and a second to investigate how time (Experiment 2a visible in Fig.2) and distance travelled (Experiment 2b visible in Fig.3) between turns affects the presence of a turning bias. All experiments used the same base maze system and the same pool of woodlice were used across all experiments although woodlice were not used multiple times for each experiment. Woodlice which took over 120 seconds to make the first turn or repeatedly turned around were discounted.

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Experiment 1 saw woodlice placed in one of the starting points (A or A’ in Figures 1-3). An alley block was placed to prevent it moving forwards at intersection O creating a forced turn. Another was placed to prevent progression past intersection P. Whether the woodlouse’s turning direction at intersection P was the same as its initial forced turn direction was noted.

Experiment 2a was much the same as Experiment 1 but was simply repeated at each of the four intersections with alley blocks being moved and placed accordingly.

Experiment 2b again followed the same process as Experiment 1 except for the addition of a hold phase in the channel between intersection O and P. This phase involved the woodlouse being held in place by a paintbrush for 5, 10, 15 and 20 seconds on separate repeats.

Results

A Chi square test of independence was calculated, with Yate’s correction applied, comparing the frequency of woodlice turning the same direction at P as at O versus those turning the opposite at P as at O. A significant interaction was found (X2 (1) = 153.372, p < 0.0001). Woodlice were significantly more likely to turn the opposite direction at P than the forced turn direction made at O in Experiment 1.

As Fig.4 shows, the turning bias effect demonstrated in experiment 1 decreases with increasing distance travelled. Initially 87% of woodlice followed the expected bias and turned the opposite way after 23mm (1 intersection in the maze). However, by intersection S, after 92mm, the turning bias was removed. In fact, 53% actually turned the same way as the forced initial turn.

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Fig.5 shows a similar trend with turning bias reducing over time. Initially 82& of woodlice turned the opposite direction to their forced initial turn after a 5 second delay. However, as the delay time increased the turning bias reduced and by 15 seconds the only 53% were turning the opposite way to the initial turn direction. This was maintained after 20 second delays too.

Discussion

Hughes (1967, 1989) argues that kinesis behaviours are designed to help individuals spend as much time in favourable locations and as little in unfavourable locations. Experiment 1 provided an extremely statistically significant effect of turning bias. The turn alteration effect demonstrated by experiment 1 is an example of such behaviour and principally aims to keep the woodlouse moving in as efficiently away from negative stimuli and toward positive conditions as possible. (Carbines & Dennis, 1992). There is wide support in the literature that this kinesis behaviour is key in avoiding and fleeing predators (Carbines & Dennis, 1992, Shafer, 1982 & 1972). This enables the rapid escape and relocation of an individual indicating that such behaviour has likely been selected for as it increases survival chance and thus subsequent reproductive success (Sutton, 1972).

Interestingly, experiments 1 and 2 demonstrated that this turning alteration behaviour decays with time and distance. Although it should be noted that these may go hand in hand; as distance to walk increases, so does time. This is congruent with the literature on woodlice and other invertebrates such as mealworms and boxelder bugs (Dingle, 1961 & 1963, Shafer, 1982). Further testing looking at how faster individuals which covered the longer distances in shorter times displayed turn alteration behaviours could help indicate which factor is more important in the decaying of the turning bias effect. Dingle argues that distance is more important in mealworm turn alteration behaviour based on such an experiment (Dingle, 1964) leading to his rejection of the ‘reactive inhibition hypothesis’. This hypothesis entails individuals responding to a stimulus less over time. Conversely, Grosslight (1953) argues for the hypothesis following similar experiments to experiment 2a and 2b. Grosslight states that reactive inhibition is displayed and that it ‘dissipates as a function of time’. Much of the literature on proximal reasons for woodlouse turning alteration behaviour focuses on biomechanics (Hughes, 1985 & 1987, Beale & Webster, 1971). The literature on mealworms is in disagreement and literature on woodlice lacking, thus similar tests should be done on woodlice to expand the scope of debate and look for behavioural similarities.

Our investigation produced clear support for turning alternation behaviour in woodlice via Experiment 1. However, it offers less to the debate regarding the effect of distance and time on turning alternation behaviour. Experiments 2a and 2b do show general trends of reduction in the turning bias as distance and time increase, but more intermediate and extreme times and distances would certainly help in analysing the trends. For example, experiment 2b produces a graph which appears to indicate exponential decay of turning alternation behaviour with time but due to the limited number of times sampled it is difficult to state this with any certainty. Furthermore, we used delay times of 5, 10, 15 and 20 seconds rather than medians of the time taken for woodlice to travel between intersections O and P, Q, R and S. This was due to time constraints and although relatively accurate for P, Q and R, the delay time for S was 20 seconds whereas our median time for woodlice to reach S was 16 seconds. This again means testing a greater number of intermediate values would be a good place to start further research and retesting when looking at the effect of distance and time on turning behaviour.

Furthermore, our experiment occurred inside a large lab with different groups spread out. This means the environmental conditions may have varied throughout. For example, groups near the windows would have had woodlice exposed to more light. This may act as a negative stimulus, causing individuals to move faster and express greater levels of turn alternation behaviour (Sutton, 1972). Additionally, the handling of individual woodlice may change their behaviour in ways we cannot tell; some may be less active in an attempt to hide in the maze and others may have run faster and displayed greater turn alternation behaviour than they would usually show. All of the literature on turning behaviour in mazes details placing woodlice, mealworms, boxelder bugs etc. inside the mazes so it is possible that turn alternation behaviour is due to the stimulus of being picked up and moved, an escape response, rather than in response to environmental conditions. Experiments in petri dishes and woodlouse congregation behaviour (Broly, 2012, Morris, 1999) should therefore be considered/adapted in future experiments.

Ultimately, we can reject the null hypothesis that ‘the direction a woodlouse turns is not influenced by the direction of the previous turn, regardless of the direction of the previous turn, or the distance, or the time between turns’.

Bibliography

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