Habituation towards the devices

Results from the behaviour recordings showed a decreasing amount of behaviours performed towards the CPODs during the first three weeks after first introduction, indicating that the novelty of the CPODs decreased. The results showed that there was a high interest in the CPODs after they were first introduced on day one. This interest was especially high in the morning, where the devices were touched and investigated. This interest declined throughout the day, and two weeks later play and investigative behaviours are only occassionally recorded. This habituation to novel objects is well known and documented in many species, and is one of the biggest challenges when it comes to improving the animals welfare over a longer period of time by the introduction of enrichments. It is known that habituation occurs very quickly after repeated presentation of a novel object. And in this case the objects were placed in the pool continuously from mid-July till mid-October, and only taken out of the pool for a short period on a weekly basis to extract the data. The process of habituation can be reduced by lowering the frequency and the duration of the presentation of the object and novelty responses can be regained after removal or replacement of the object.

The PCLs were added to the pool several weeks later. When analysing the PCL recordings, a decline in recorded echolocation activity throughout a period of seven weeks as well as whitin the first week of introduction was found. During the first recording day an average of 20.000 clicks per hour was recorded, this quickly declined over one week and leveled at an average of 5000 clicks recorded per hour. This again indicates a process of habituation. Interestingly the dolphins were already habituated to the CPODs for several weeks before the introduction of the PCLs. The PCLs have a different colour, and had 2 small buoys attached to the top, which probably had enough novelty to regain their interest. This finding suggests that changing the look and acoustic target profile of an enrichment will recover habituated behaviours.

Diel patterns

Clear diel patterns have been recorded in many other dolphin species, both in the wild and under human care. The diel patterns recorded in wild dolphin species often correlate with the behaviour of their prey. Risso’s dolphin and harbour porpoises are found to forage mainly at night, and in these species more clicks are recorded during dusk and night time. But also wild bottlenose dolphins in the Bermuda Pedestal and off the coast of San Diego county show more diving, foraging and social activity during dusk and night time, indicating that also this species is more active at night. However, two studies recording the activity of bottlenose dolphins under human care found contrasting results. These studies both found that activity was highest during the afternoon. Whistle and click production, as well as swim speed and respiration rate peaked around the time the trainers were present during working day hours. However, during the present study, echolocation clicks were recorded continuously throughout the day for a duration of three months, and found a clear diel pattern pointing towards a significantly higher click production during night time and early morning (00.00 to 6.00). Average recorded clicks per hour decreased throughout the day, with lowest click activity recorded during evening hours (18.00 to 00.00). The dolphin group seemed to go into a resting mode after the trainers leave (>18.00), and rested until after 00.00 – 03.00AM (around 8 hours). The diel pattern of this dolphin group relates more to the diel pattern seen in wild dolphin species than those recorded in other captive bottlenose dolphins, which is interesting since this nocturnal activity is not related to foraging, as it is in their wild counterparts. The reason that higher click activity was recorded during night time might be due to the reduced visibility at night in an otherwise clear and light pool. After rest the dolphins are active and use their echolocation, but not for foraging activities as in wild species. Possibly some of the click trains were part of social communication and part of navigating in the pool whilst avoiding collision with the walls or the click loggers.

Movement patterns

During undisturbed moments, the dolphins were observed to swim in a typical circular pattern, mainly clockwise. During these typical circular patterns, the dolphins often were seen swimming around the whole pool, but were also frequently seen circling in the deep area. It was observed that when the dolphins rested they often were in close contact with one or more individuals. These results match with previous studies on captive dolphin movement patterns. During rest the deep areas were most visited by the dolphins, and a longer time was spent in these areas, suggesting a preference for the deeper part in the pool whilst swimming undisturbed or resting. In relation to this finding, significantly more echolocation clicks were recorded in the deeper area of the pool. When the gate between the laguna and the holding pool was closed, dolphins were observed to visit the area around this gate very often. Through the gate the dolphins can have visual as well as acoustic contact with the dolphins in the other pool. But most importantly, they can also keep an eye on the trainers and the fish kitchen from this position.

Landmark use

Seventeen click-trains were classified as decreasing ICI type click trains, with ICI’s averaging from 40ms at the beginning of the approach phase to 10ms at the terminal phase. The terminal phase is often recognized as a buzz. These findings relate to previous findings  where range locking click trains, with decreasing ICIs from approach to terminal phase, were recorded in harbour porpoises during navigational tasks, indicating the use of acoustic landmarks. Also in bottlenose dolphins similar type of range locking click trains have been recorded, with a mean ICI of 20ms. The behaviours displayed during the recording of these click-trains was mainly exploratory, where the dolphin approached the click logger, and quickly investigated it (pointing snout towards it, and often emitting a constant or decreasing like click-train). A total of 13 constant like click trains were observed where two were classified as a buzz. These constant like click-trains had an average ICI of around 20ms and were mainly recorded during play, but also when dolphins swam closely past the click logger, which might suggest that they did not scan the environment for a target to lock onto, but that the target was already detected visually and inspected acoustically whilst swimming past it.

During analysis of 34 minutes where the dolphins were in recording range of the devices, 15 click trains were recorded. The ICIs were typically between 300 and 4 ms. In one occasion, after a dolphin had been playing with one of the PCLs for a short amount of time at 17.15, no clicks were recorded thereafter for 62 min., even though the dolphins swam past the devices repeatedly during their resting period. They were often seen to swim in close groups and might have relied on visual cues and each other instead. As mentioned before, the echolocation activity during the evening period is very low, and only in 33 to 50% of the time there is a possibility that the dolphins use landmarks).
This finding suggests that during rest, the dolphins are fairly quiet, and on some occasions use echolocation for orientation. It could very well be possible that they are able to orientate with visual cues, even at night time when the lights in the dolphinarium are turned off. However, a total of five illuminating exit signs are still on and surround the Laguna and can be visual from within the water through the glass. The eyes of cetaceans have large ganglia which makes them able to compensate for low light conditions. This means that the light originating from the exit signs can be perceived by the dolphins underwater and might well be enough for them to orientate themselves with.