Stress is
wildly used as an indicator of negative welfare. There are two main types of
stress: acute and chronic stress. Acute stress is often a short-term event
which can have negative or positive physiological effects, whereas chronic
stress has long-term, negative effects
1. Animals can either visibly
display their stress, such as behavioural changes, or physiological parameters
can indicate their levels of stress, such as changes in cortisol. Where it is
not possible to gain physiological readings of animals, particularly wild
animals, behavioural observations are the main source used. Therefore the
behavioural indicators for recognising stress in dolphins are described as
follows:
Respiration rate and chuffing
An increase
in breathing rate is commonly used as an indicator of stress in farm animals,
and as dolphins are mammals, a higher respiration rate could also mean that the
dolphin is experiencing stress. However, Serres and Delfour (2019)
2
concluded that a change in breathing frequency does not indicate whether the
stress is negative or positive, such as excitement, and that other
physiological parameters should be measured. Although, as physiological
parameters are not always accessible, noticing a change in a dolphins breathing
rate is a good place to start. Furthermore, dolphins may exhale sharply and
loudly known as “chuffing”, which is used to infer that the dolphin is bothered
by something
3.
Whistles
The dolphin’s
whistle rate has been studied as a potential indicator of stress. During
captive dolphin swim programs, an increased in whistle rate was found which
Akiyama and Ohta (2007)
4 suggested was due to stress. In a capture
and release study, Esch et al (2009) also found that the whistle rate and
number of loops were greater during brief capture-release events when compared
to undisturbed conditions.
Stereotypies
For most
captive wild animals, the appearance of abnormal, repetitive behaviours cause a
welfare concern. This has also been observed in captive dolphins, in the form
of vomiting, head bobbing, circular swimming and biting on gates and bars
5.
However it is not fully understood how a stereotypical behaviour reflects an
animals’ internal state. The behaviour may have arisen out of feeling a state
of chronic stress, however the behaviour may also be a coping mechanism, and
therefore performing the behaviour can lower the animals’ stress, as has been
found in rhesus macaques
6.
Dive duration
An increase in
the length of time that a dolphin dives for has been used to indicate stress in
dolphins. Dolphins in areas of high vessel traffic were found to dive for
longer
7, likely as an avoidance tactic. Therefore the longer a dolphin
is under the water, the more stress it is likely experiencing. A study on a solitary
common dolphin in Scotland found that the dolphin dove for longer as the vessel
passed by, compared to when there were no passing vessels in the area (Hurdle
unpublished). Ocean noise can also be a stressor to dolphins, causing longer
dives.
Increased swimming speed
Often seen
in the wild, an abrupt change in a dolphin’s swimming speed and direction
caused by vessels can be an indicator of stress. If the dolphin feels disturbed
it will flee the area, whereas a dolphin that feels undisturbed will remain.
Therefore it is essential that vessel users are mindful of how their boat behaviour
affects dolphins.
Body slaps and aerial behaviours
Dolphins may
slap the water, with either their head or tail as a sign of aggression. Dolphins
have been observed tail slapping when drones are flown too close to the group
8,
therefore it is assumed the behaviour is delivered as a warning. This has also
occurred when vessels are too close in proximity. An increased occurrence in
breaches have also been observed, which may also indicate stress.
Slide-outs and strandings
Within captivity,
dolphins may be seen “sliding out” onto dry land areas outside of shows and
training sessions. This behaviour is often interpreted as play by the trainers,
however it could be seen as a sign of stress that the dolphin is trying to get
out of the water. In a video of a facility in Japan, a false killer whale slid
over the tank glass and onto the concrete ground during a show
9.
This was obviously not a planned behaviour, and the trainers had to quickly act
to get the animal back into the water. These captive instances could be the
equivalent of wild cetaceans stranding on beaches. Cetaceans may strand for a
variety of reasons, however the stress of intense ocean noise caused by naval
activity has been known to cause fatal strandings of cetaceans
10.
References
1. Esch HC,
Sayigh LS, Blum JE and Wells RS 2009 Whistles as potential indicators of stress
in bottlenose dolphins (
Tursiops
truncatus).
Journal of Mammalogy
900(3): 638-650
2. Serres A and
Delfour F 2019 Social behaviors modulate bottlenose dolphins’ (
Tursiops truncatus) breathing rate.
Animal Behaviour and Cognition 6(2):
127-140
3.
https://www.bornfree.org.uk/storage/media/content/files/Publications/EU%20DOLPHINARIA%20REPORT.pdf
4. Akiyama J
and Ohta M 2007 Increased number of whistles of bottlenose dolphins,
Tursiops truncatus, arising from
interaction with people.
Journal of
Veterinary Medical Science 69: 165-170
5.
https://cetaceaninspiration.wordpress.com/2011/12/22/stereotypical-behavior-in-captive-whales-and-dolphins/#:~:text=Like%20many%20animals%20in%20captivity,%2C%20or%20abnormal%2C%20stereotypical%20behavior.&text=Some%20examples%20of%20stereotypy%20include,and%20bars%2C%20and%20tongue%20playing.
6. Pomerantz
O, Paukner A and Terkel J 2012 Some stereotypic behaviors in rhesus macaques (
Macaca mulatta) are correlated with both
perseveration and the ability to cope with acute stressors.
Behaviour Brain Research 230(1): 274-280
7. Ng SL and
Leung S 2003 Behavioural response of Indo-Pacific humpback dolphin (
Sousa chinensis) to vessel traffic.
Marine Environmental Research 56:
555-567
8. Fettermann
T, Fiori L, Bader M, Doshi A, Breen D, Stockin K and Bollard B 2019 Behaviour
reactions of bottlenose dolphins (
Tursiops
truncatus) to multirotor Unmanned Aerial Vehicles (UAVs).
Scientific Reports 9: 8558
9.
https://www.youtube.com/watch?v=wxzTyQillMQ
10. Parsons
ECM 2017 Impacts of navy sonar on whales and dolphins: now beyond a smoking
gun?
Frontiers in Marine Science 13
