In order to receive more reliable results, data should becollected from wild, free-ranging dolphins over a long period of time in ahumane 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 themain limitation of collecting data on dolphins is the effect human interactionhas on their stress levels, making it particularly difficult to analyse acontrol. Captive dolphins are the easiest to collect data from. However, it hasbeen proven that they are constantly under stress through tourist interactionsand limited social interactions with other dolphins (Morgan & Tromborg, 2006). Although wild dolphins are more difficult tocapture, monitor and collect data from, they are under less constant stress.
Howeverthey 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 adverseimpact on reproduction and survival on dolphin species (Curry & Edwards, 1997).In conclusion, it has been proven that stress has an effecton a variety of different hormones and neurotransmitters in many differencespecies of dolphin. Cortisol is the hormone most affected by stress and playsan important role in the survival of dolphins during stressful situations.However, the other hormones (aldosterone, dopamine, norepinephrine andepinephrine) all work together to enable the body to perform well under stress.Without these hormones during stress, major organs can shut down and dolphinscan easily contract a life-threatening disease, resulting in death.Conclusion This study researched the importance of norepinephrine undera stressful situation, but only under one specific stress.
In order to developa better understanding of the effects of stress on norepinephrine, moreresearch should be conducted on a wider range of dolphin, perhaps by examiningwhether young calves manage stress in the same way as mature dolphins or if thedepth of dive alters the levels of norepinephrine.When diving, Bottlenose Dolphins (Tursiops aduncus) inducea change in blood circulation to favour flow to the more important parts of the body, the brain andheart. The vasoconstrictors in dolphins stimulate the contraction of musclesin the wall of blood vessels, with noradrenaline being an important hormone invasoconstriction. Noradrenaline serum levels were monitored while dolphinsdived, showing significantincreases in noradrenaline levels and a significant decrease in heart rate (Tomoshique, et al., 2017). This showed that inorder to perform well under this stressful situation, norepinephrine was veryimportant. Norepinephrine showed to widen the air passages to the lungs anddecrease the heart rate of the dolphins, allowing the dolphins to diveeffectively.
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) .Norepinephrineis synthesised from dopamine. When dopamine is transported into vesicles, it isthen converted to norepinephrine by the enzyme dopamine ?-hydroxylase (DBH).Norepinephrine can then be released from the adrenal medulla into thebloodstream as a hormone (Systems, 2009). Norepinephrine (NE), also known as noradrenaline (NA), is a catecholamine.Norepinephrine has multipleroles as a hormone and a neurotransmitter.
Norepinephrine as a stress hormone plays a key role inthe fight-or-flight response as well as affecting parts of the brain whereattention and responding actions are controlled (Ortenholm, 2004).As a hormone,norepinephrine is also involved in vasoconstriction. Norepinephrine increasesor decreases heart rate, widens air passages in the lungs, increases bloodpressure and narrows the blood vessels in non-essential organs, enablingthe body to perform well in stressful situations (Goldstein, 2010).Norepinephrine Developing a capacity to be able to synthesis and store more stresshormones, such as epinephrine, enables dolphins to increase their chances ofsurvival.
It enables dolphins to become better equipped for stressful situations,allowing them to perform better while under stress, and making them less likelydevelop 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 andchronic, on the adrenal glands. It showed that the adrenal glands ofchronically stressed animals were significantly heavier than those of acutelystressed dolphinsand that long-term stress lead to structural differences of the adrenal glands. Some changesshowed that over time, a bottlenose dolphin exposed to stressful situationsdevelops more numerous epinephrine-producing cells. This suggests that these differences might havebeen caused by an individual’s ability to synthesize and store more stresshormones (Clark, et al.
, 2006).Epinephrine, also known as adrenaline, is a neurotransmitter and a hormone produced and secreted from theadrenal glands. Epinephrine is released in response to stress, in order toprepare the body for the fight-or-flight response, as it increases blood flowto the muscles. A stressful event leads to the activation of nerves connectedto the adrenal glands, which inturn 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.Epinephrine Dopamine helps to maintain a good mood and prevent pain.Prolonged stress depletes dopamine levels in the body which instigates therelease of stress hormones, in this case mainly cortisol. This suggests why thedopamine levels within the dolphins of this study decreased so significantlywhen being caught for so long.
Free-ranging dolphins were examined based on their stressresponse by measuring their physiologic stress response to capture and release. Over 150bottlenose dolphins (Tursiops truncates) werecollected from two study sites (Charleston and the Indian River Lagoon) over 4 years. The stresshormones were measured in the blood, which included dopamine, aldosterone,cortisol, epinephrine and norepinephrine. Dopamine concentrations decreasedsignificantly as the timebeing captured increased (Fair, et al.
, 2011). However, cortisol levels seemed to increasedsignificantly from the initial capture and continued to do so whilebeing caught.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 timethis can alter the behaviour of a dolphin.
Behaviours can change so drasticallythat a dolphin would lose their appetite, become increasingly agitated andincreasingly aggressive. This ultimately can lead to their death, suggestingthat a dolphin’s survival and reproductive success is ensured due to itsability to withstand stress (Brock, 1999).Dopamine (3,4-dihydroxyphenethylamine) is a catecholamineproduced in the dopaminergic neurons and in the hypothalamus.
It has a role inthe body as a neurotransmitter that initiates adrenalin during the activationof a stress response. Dopamine acts on G protein-coupled dopamine receptors(GPCRs) (Kebabian & Calne, 1979). Dopamine There are numerous limitations to this study. For example, onlytwo dolphins were examined. For more reliable results, the same study should berepeated on more individuals and possibly on different species of dolphin.
Researchers should also take into account that the individuals in this studywere captive dolphins, and that this could affect the dolphins’ levels ofstress. A study conducted on two adult Bottlenose Dolphins (Tursiops truncates) exposed to stress showed the effects that stress has on thehormone, aldosterone. These two dolphins were housed individually in captivity,over a 10-day period, and were exposed to decreasing water temperatures. Voluntaryblood drawbacks were taken from each dolphin every 2-3 days, and the serum wasanalysed via radioimmunoassay. As the water temperature decreased, the dolphinsshowed an increase in serum aldosterone, with aldosterone levels being twice ashigh at the coldest water temperature (Houser, et al., 2011).Figure 3 shows the effects of psychosocial stress on hormonerelease and cardiovascular disease. When in a stressful situation, increasedaldosterone levels activate mineralcorticoid receptors which in turn increasesoxidative stress and inflammation leading to cardiovascular and renovasculardisease.
Figure 3. A model diagram of aldosterone as a mediator of therelationship between psychological stress and cardiovascular disease (Kubzansky & Adler, 2009)Figure2 shows that stress causes the anterior pituitary to secrete ACTH, which inturn enhances the secretion of aldosterone. Aldosterone then targets thekidneys (particularly the kidney tubules), increasing the absorption of sodiumand water which increase blood volume and pressure. This ensures anadequate delivery of oxygen and nutrients to the body while under stress.Figure 2. The effects of stress on the body focusing onAldosterone (Chan, 2010) Numerousstudies have begun to support the evidence that exposure to stress (particularly chronic stress)is now a risk factorfor cardiovascular disease (CVD) in many mammals.
Aldosterone is released inresponse to a stressfulsituation, activating the hypothalamic-pituitary-adrenal (HPA) axis. Studies have shown that stress,both inflammation and oxidative, plays a key role in the development ofaldosterone-induced cardiovascular problems (Yoshimoto & Hirata, 2007). Aldosterone is also responsible for regulating sodium (Bollag, 2014), therefore has an important role inthe regulation of blood volume and blood pressure (Funder, 2004).
Having too low aldosterone levels inthe 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).Aldosteroneis a mineralcorticoid, steroid hormone produced in the adrenal glands. When stressarises, this causes a release of corticotropin releasing hormone (CRH) from thehypothalamus of the brain.
As these hormone levels rise, this stimulates thesecretion of the adrenocorticotropin (ACTH) hormone from the anterior pituitary.These hormones act on the adrenal gland, which produce aldosterone (Chan, 2010). Aldosterone This study only focuses on two hormones associated withstress, cortisol and aldosterone. In order to gain more understanding of theeffects of stress on hormones, further research should be conducted to see theeffects of oil toxicity on the catecholamines associated with it.Figure 1 shows that when the dolphins ingested the oil, itlead to hypoadrenocorticism, decreasing the hormones associated with stress inthe body, such as cortisol and aldosterone.
This lead to the deaths of thedolphins affected by the oil spill. Similar effects occurred when the dolphinsinhaled the oil. Lung injury, hepatic injury and maternal transfer all occurredwhich lead to chronic disease and death.Figure 1. A diagram to show the effects ofingesting 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 HorizonOil Spill in 2010, itshowed that dolphins, particularly Common Bottlenose Dolphins (Tursiops truncates), suffered fromhypoadrenocorticism (Schwacket, et al., 2013).Hypoadrenocorticism, also known as Addison’s disease, is caused by having toolittle cortisol in the body.
This was consistent with adrenal toxicity causedby the oil in the water. This disease caused lower glucose levels in the bloodand increased inflammation, resulting in a poor body condition, increasingdisease susceptibility and lowering the chances of survival (Schwacket, et al., 2013).
Cortisol also plays an important role in pregnant mammals. Cortisol induces a variety ofenzymes before birth. Duringbirth, losing the placenta deprives the foetus of a source oxygen and naturalsugars, known as glucose, as well as heat. In preparation for this, inthe late stages of pregnancy, organs undergo maturational changes.
The changesare regulated by cortisol and include: the lungs maturing structurally andfunctionally, and glycogen beginning to accumulate in the liver andthermogenesis (Liggins, 1994).When cortisol levels in the body get too high due tocontinuous stresses, many species of dolphin can suffer from high bloodpressure, high blood sugar, depression (particularly in captive dolphins) andeven death (Marino & Lilienfeld, 2007). Having too high ofa cortisol level can also lead to the temporary shutdown of digestion andreproduction (Ugaz, et al., 2012).Cortisollevels in the body should always be maintained.
However, when cortisol levelsdecrease significantly, the hypothalamus (area in the brain) releases the corticotrophin-releasinghormone. This causes the pituitary gland to start secreting adrenocorticotropichormone into the bloodstream. As the adrenal gland begins to detect the higherlevels of adrenocorticotropic hormone within the bloodstream, the adrenal glandbegins the secretion of cortisol (Fair, et al., 2011).Cortisol is one of the glucocorticoids, and is a steroidhormone transported by the blood and controlled by the hypothalamus, pituitaryand adrenal glands. Nearlyevery cell in the body havereceptors for cortisol. Thehormone, cortisol has many different effects within the body including: regulating blood sugarlevels, acting as an anti-inflammatory, regulating memory formation, controlling salt andwater balance, influencing blood pressure and helping the development of afoetus. (Dickerson & Kemeny, 2004)Cortisol When astressful 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 thanfree-ranging dolphins.
This stress arises through noises emitted from the tankscleaning systems, people crowding around the pools, malnourishment, performing,and people swimming with them. Wild, free-ranging dolphins also suffer fromstress. 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 throughlong-term disease (Clark, et al., 2006). Stressis a result from demanding circumstances that increase an organism’ssusceptibility to disease. Stress is known as a state of mental or emotionalstrain.
The way in which an individual deals with stress through a physiologicalstress response, is often the only way in which they survive stressfulsituations. (Clark, et al., 2006)What is Stress? There are numerous hormones and neurotransmitters in mammalsthat are involved with stress, such as steroid hormones and catecholamines. Thesteroid hormones involved in stressful situations are cortisol and aldosteronereleased 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, mainlythrough vocalisation and their behaviour. Whistles increase in both volume andfrequency when under stress. Changes in their behaviour can be seen throughpacing, abstaining from eating, becoming aggressive and becoming ill. Stress ismeasured in dolphins through hormone levels within their blood and blubber.Dolphins are thought to be one of the most intelligentmammals in the world as they form complex bonds, show emotion and cancommunicate (using whistle-type noises) with individuals of not only the samespecies, but other species (Lusseau, 2003). However, as theyare 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 adozen individuals (Bazua-Duran & Au, 2004). They are highlysociable mammals that establish close links with other individuals in their podand seem to show empathic, cooperative and altruistic behaviours (Wiszniewski, et al., 2008).Dolphins rely on acousticsignals. The acoustic signals determine and negotiate their environment, bothphysical and social (Smolker, et al.
, 1992). They also rely onecholocation to manoeuvre. Introduction