In order to receive more reliable results, data should be
collected from wild, free-ranging dolphins over a long period of time in a
humane way, away from any form of human interaction. If this is not possible,
research should be conducted on dolphins that have died of natural causes,
focusing on their stress levels after death.
It has been shown from a variety of different studies that the
main limitation of collecting data on dolphins is the effect human interaction
has on their stress levels, making it particularly difficult to analyse a
control. Captive dolphins are the easiest to collect data from. However, it has
been proven that they are constantly under stress through tourist interactions
and limited social interactions with other dolphins (Morgan & Tromborg, 2006). Although wild dolphins are more difficult to
capture, monitor and collect data from, they are under less constant stress. However
they are recently becoming more and more exposed to stress due to fisheries.
This constant stress is so severe that it is beginning to have an adverse
impact on reproduction and survival on dolphin species (Curry & Edwards, 1997).
In conclusion, it has been proven that stress has an effect
on a variety of different hormones and neurotransmitters in many difference
species of dolphin. Cortisol is the hormone most affected by stress and plays
an important role in the survival of dolphins during stressful situations.
However, the other hormones (aldosterone, dopamine, norepinephrine and
epinephrine) all work together to enable the body to perform well under stress.
Without these hormones during stress, major organs can shut down and dolphins
can easily contract a life-threatening disease, resulting in death.
This study researched the importance of norepinephrine under
a stressful situation, but only under one specific stress. In order to develop
a better understanding of the effects of stress on norepinephrine, more
research should be conducted on a wider range of dolphin, perhaps by examining
whether young calves manage stress in the same way as mature dolphins or if the
depth of dive alters the levels of norepinephrine.
When diving, Bottlenose Dolphins (Tursiops aduncus) induce
a change in blood circulation to favour flow to the more important parts of the body, the brain and
heart. The vasoconstrictors in dolphins stimulate the contraction of muscles
in the wall of blood vessels, with noradrenaline being an important hormone in
vasoconstriction. Noradrenaline serum levels were monitored while dolphins
dived, showing significant
increases in noradrenaline levels and a significant decrease in heart rate (Tomoshique, et al., 2017). This showed that in
order to perform well under this stressful situation, norepinephrine was very
important. Norepinephrine showed to widen the air passages to the lungs and
decrease the heart rate of the dolphins, allowing the dolphins to dive
Captive dolphins are exposed to daily stresses, such as handling,
tourists and loud noises. When such captive dolphins are examined after death,
they show an increase in noradrenaline levels (Stanford, et al., 1984) .
is synthesised from dopamine. When dopamine is transported into vesicles, it is
then converted to norepinephrine by the enzyme dopamine ?-hydroxylase (DBH).
Norepinephrine can then be released from the adrenal medulla into the
bloodstream as a hormone (Systems, 2009).
Norepinephrine (NE), also known as noradrenaline (NA), is a catecholamine.
Norepinephrine has multiple
roles as a hormone and a neurotransmitter. Norepinephrine as a stress hormone plays a key role in
the fight-or-flight response as well as affecting parts of the brain where
attention and responding actions are controlled (Ortenholm,
As a hormone,
norepinephrine is also involved in vasoconstriction. Norepinephrine increases
or decreases heart rate, widens air passages in the lungs, increases blood
pressure and narrows the blood vessels in non-essential organs, enabling
the body to perform well in stressful situations (Goldstein,
Developing a capacity to be able to synthesis and store more stress
hormones, such as epinephrine, enables dolphins to increase their chances of
survival. It enables dolphins to become better equipped for stressful situations,
allowing them to perform better while under stress, and making them less likely
develop an infection and a long-term disease.
More than 60 Atlantic Bottlenose Dolphins (Tursiops truncates) were examined, after death, to see the effects of stress, both acute and
chronic, on the adrenal glands. It showed that the adrenal glands of
chronically stressed animals were significantly heavier than those of acutely
and that long-term stress lead to structural differences of the adrenal glands. Some changes
showed that over time, a bottlenose dolphin exposed to stressful situations
develops more numerous epinephrine-producing cells. This suggests that these differences might have
been caused by an individual’s ability to synthesize and store more stress
hormones (Clark, et al., 2006).
Epinephrine, also known as adrenaline, is a neurotransmitter and a hormone produced and secreted from the
adrenal glands. Epinephrine is released in response to stress, in order to
prepare the body for the fight-or-flight response, as it increases blood flow
to the muscles. A stressful event leads to the activation of nerves connected
to the adrenal glands, which in
turn trigger the secretion of epinephrine. When stress from a situation begins to subside,
the nerve impulses to the adrenal glands start to stop, resulting in epinephrine levels decreasing.
Dopamine helps to maintain a good mood and prevent pain.
Prolonged stress depletes dopamine levels in the body which instigates the
release of stress hormones, in this case mainly cortisol. This suggests why the
dopamine levels within the dolphins of this study decreased so significantly
when being caught for so long.
Free-ranging dolphins were examined based on their stress
response by measuring their physiologic stress response to capture and release. Over 150
bottlenose dolphins (Tursiops truncates) were
collected from two study sites (Charleston and the Indian River Lagoon) over 4 years. The stress
hormones were measured in the blood, which included dopamine, aldosterone,
cortisol, epinephrine and norepinephrine. Dopamine concentrations decreased
significantly as the time
being captured increased (Fair, et al., 2011). However, cortisol levels seemed to increased
significantly from the initial capture and continued to do so while
Studies have proved that when dolphins are exposed to stress,
dopamine levels rise. (Brock, 1999).
However, if the dopamine levels do not rise while under constant stress, over time
this can alter the behaviour of a dolphin. Behaviours can change so drastically
that a dolphin would lose their appetite, become increasingly agitated and
increasingly aggressive. This ultimately can lead to their death, suggesting
that a dolphin’s survival and reproductive success is ensured due to its
ability to withstand stress (Brock, 1999).
Dopamine (3,4-dihydroxyphenethylamine) is a catecholamine
produced in the dopaminergic neurons and in the hypothalamus. It has a role in
the body as a neurotransmitter that initiates adrenalin during the activation
of a stress response. Dopamine acts on G protein-coupled dopamine receptors
(GPCRs) (Kebabian & Calne, 1979).
There are numerous limitations to this study. For example, only
two dolphins were examined. For more reliable results, the same study should be
repeated on more individuals and possibly on different species of dolphin.
Researchers should also take into account that the individuals in this study
were captive dolphins, and that this could affect the dolphins’ levels of
A study conducted on two adult Bottlenose Dolphins (Tursiops truncates) exposed to stress showed the effects that stress has on the
hormone, aldosterone. These two dolphins were housed individually in captivity,
over a 10-day period, and were exposed to decreasing water temperatures. Voluntary
blood drawbacks were taken from each dolphin every 2-3 days, and the serum was
analysed via radioimmunoassay. As the water temperature decreased, the dolphins
showed an increase in serum aldosterone, with aldosterone levels being twice as
high at the coldest water temperature (Houser, et al., 2011).
Figure 3 shows the effects of psychosocial stress on hormone
release and cardiovascular disease. When in a stressful situation, increased
aldosterone levels activate mineralcorticoid receptors which in turn increases
oxidative stress and inflammation leading to cardiovascular and renovascular
Figure 3. A model diagram of aldosterone as a mediator of the
relationship between psychological stress and cardiovascular disease (Kubzansky
& Adler, 2009)
2 shows that stress causes the anterior pituitary to secrete ACTH, which in
turn enhances the secretion of aldosterone. Aldosterone then targets the
kidneys (particularly the kidney tubules), increasing the absorption of sodium
and water which increase blood volume and pressure. This ensures an
adequate delivery of oxygen and nutrients to the body while under stress.
Figure 2. The effects of stress on the body focusing on
Aldosterone (Chan, 2010)
studies have begun to support the evidence that exposure to stress (particularly chronic stress)
is now a risk factor
for cardiovascular disease (CVD) in many mammals. Aldosterone is released in
response to a stressful
situation, activating the hypothalamic-pituitary-adrenal (HPA) axis. Studies have shown that stress,
both inflammation and oxidative, plays a key role in the development of
aldosterone-induced cardiovascular problems (Yoshimoto & Hirata, 2007).
Aldosterone is also responsible for regulating sodium (Bollag, 2014), therefore has an important role in
the regulation of blood volume and blood pressure (Funder, 2004). Having too low aldosterone levels in
the body can lead to hypotension (low blood pressure) and circulatory shock,
whereas excessive aldosterone levels in the body can cause hypertension
(long-term high blood pressure) which can lead to congestive heart failure (Bollag, 2014).
is a mineralcorticoid, steroid hormone produced in the adrenal glands. When stress
arises, this causes a release of corticotropin releasing hormone (CRH) from the
hypothalamus of the brain. As these hormone levels rise, this stimulates the
secretion of the adrenocorticotropin (ACTH) hormone from the anterior pituitary.
These hormones act on the adrenal gland, which produce aldosterone (Chan, 2010).
This study only focuses on two hormones associated with
stress, cortisol and aldosterone. In order to gain more understanding of the
effects of stress on hormones, further research should be conducted to see the
effects of oil toxicity on the catecholamines associated with it.
Figure 1 shows that when the dolphins ingested the oil, it
lead to hypoadrenocorticism, decreasing the hormones associated with stress in
the body, such as cortisol and aldosterone. This lead to the deaths of the
dolphins affected by the oil spill. Similar effects occurred when the dolphins
inhaled the oil. Lung injury, hepatic injury and maternal transfer all occurred
which lead to chronic disease and death.
Figure 1. A diagram to show the effects of
ingesting and inhaling oil. Showing the health effects (yellow), and what these lead to (blue) (Schwacket, et al., 2013).
Based on a study investigated after the Deepwater Horizon
Oil Spill in 2010, it
showed that dolphins, particularly Common Bottlenose Dolphins (Tursiops truncates), suffered from
hypoadrenocorticism (Schwacket, et al., 2013).
Hypoadrenocorticism, also known as Addison’s disease, is caused by having too
little cortisol in the body. This was consistent with adrenal toxicity caused
by the oil in the water. This disease caused lower glucose levels in the blood
and increased inflammation, resulting in a poor body condition, increasing
disease susceptibility and lowering the chances of survival (Schwacket, et al., 2013).
Cortisol also plays an important role in pregnant mammals. Cortisol induces a variety of
enzymes before birth. During
birth, losing the placenta deprives the foetus of a source oxygen and natural
sugars, known as glucose, as well as heat. In preparation for this, in
the late stages of pregnancy, organs undergo maturational changes. The changes
are regulated by cortisol and include: the lungs maturing structurally and
functionally, and glycogen beginning to accumulate in the liver and
thermogenesis (Liggins, 1994).
When cortisol levels in the body get too high due to
continuous stresses, many species of dolphin can suffer from high blood
pressure, high blood sugar, depression (particularly in captive dolphins) and
even death (Marino & Lilienfeld, 2007). Having too high of
a cortisol level can also lead to the temporary shutdown of digestion and
reproduction (Ugaz, et al., 2012).
levels in the body should always be maintained. However, when cortisol levels
decrease significantly, the hypothalamus (area in the brain) releases the corticotrophin-releasing
hormone. This causes the pituitary gland to start secreting adrenocorticotropic
hormone into the bloodstream. As the adrenal gland begins to detect the higher
levels of adrenocorticotropic hormone within the bloodstream, the adrenal gland
begins the secretion of cortisol (Fair, et al., 2011).
Cortisol is one of the glucocorticoids, and is a steroid
hormone transported by the blood and controlled by the hypothalamus, pituitary
and adrenal glands. Nearly
every cell in the body have
receptors for cortisol. The
hormone, cortisol has many different effects within the body including: regulating blood sugar
levels, acting as an anti-inflammatory, regulating memory formation, controlling salt and
water balance, influencing blood pressure and helping the development of a
foetus. (Dickerson & Kemeny, 2004)
stressful situation arises, mammalian brains initiate a stress response. Stress activates the HPA-axis
(hypothalamic-pituitary-adrenal axis) which includes the hypothalamus,
pituitary and adrenal gland. The adrenal gland releases the catecholamines,
norepinephrine and epinephrine as well as steroid hormones, cortisol and aldosterone.
Captive dolphins frequently experience more stress than
free-ranging dolphins. This stress arises through noises emitted from the tanks
cleaning systems, people crowding around the pools, malnourishment, performing,
and people swimming with them. Wild, free-ranging dolphins also suffer from
stress. Acute stress can be caused by net entanglement, boat strike or acute infection.
Chronic stress can be caused by mothers being separated from their infant or through
long-term disease (Clark, et al., 2006).
is a result from demanding circumstances that increase an organism’s
susceptibility to disease. Stress is known as a state of mental or emotional
strain. The way in which an individual deals with stress through a physiological
stress response, is often the only way in which they survive stressful
situations. (Clark, et al., 2006)
What is Stress?
There are numerous hormones and neurotransmitters in mammals
that are involved with stress, such as steroid hormones and catecholamines. The
steroid hormones involved in stressful situations are cortisol and aldosterone
released from the adrenal glands. The catecholamines involved in stress are dopamine
(which is a neurotransmitter), norepinephrine (also known as noradrenaline,
which is both a hormone and neurotransmitter) and epinephrine (adrenaline).
Stress can be seen in dolphins in numerous ways, mainly
through vocalisation and their behaviour. Whistles increase in both volume and
frequency when under stress. Changes in their behaviour can be seen through
pacing, abstaining from eating, becoming aggressive and becoming ill. Stress is
measured in dolphins through hormone levels within their blood and blubber.
Dolphins are thought to be one of the most intelligent
mammals in the world as they form complex bonds, show emotion and can
communicate (using whistle-type noises) with individuals of not only the same
species, but other species (Lusseau, 2003). However, as they
are such complex mammals, they are prone to stress, particularly in captivity,
which can have such a negative effect, it can lead to death.
Dolphins are marine mammals that live in groups known as pods, of up to a
dozen individuals (Bazua-Duran & Au, 2004). They are highly
sociable mammals that establish close links with other individuals in their pod
and seem to show empathic, cooperative and altruistic behaviours (Wiszniewski,
et al., 2008).
Dolphins rely on acoustic
signals. The acoustic signals determine and negotiate their environment, both
physical and social (Smolker, et al., 1992). They also rely on
echolocation to manoeuvre.