Thermal Adaptation - Behavioural Thermoregulation

An ectotherm is an organism with internal temperature that is similar to its external environment. Fish, reptiles and invertebrates are most prominent examples of ectotherms. The issue with ectothermy is that the increasing environment increases internal temperature, affecting physiology such as metabolism, circulation and reproduction (Speed et al. 2012). A change in body temperature by 1°C alters the rate of many physiological processes by 6–10% (Johnston & Bennett, 1996). For ectotherms to maintain optimal or preferred body temperature, they rely significantly on behavioural adaptations. This blog post will discuss the behavioural thermoregulation of ectothermic marine and freshwater fishes, and the trade-offs involved with this process.

   

The movement of fish species is dependent on many environmental variables. These include resource availability (prey), shelter from predation, oxygen availability, water currents, tidal movements etc. (Ward et al. 2010 & Speed et al. 2012). It is important when studying behavioural thermoregulation to ensure movement is directly caused by a need to alter environmental temperature, rather than a combination of other variables. This is difficult to achieve with fish species, considering that movement to warmer areas of water generally increases risk of predation. This presents a trade-off situation; growth, reproductive or survival related benefits from thermal refuge use must outweigh the risks and cost of refuge use for the strategy to make biological sense (Westhoff et al. 2014).

A 2012 study was conducted on the behavioural thermoregulation of reef sharks in areas of the Ningaloo Reef in Western Australia. It was found that on average, body temperatures of female blacktip reef sharks (Carcharhinus melanopterus) were consistently warmer than average (±SE) water temperature by 1.3 ± 0.57°C, as a result of behavioural thermoregulation (Speed et al. 2012). In 1978 a study was conducted on the sand goby (Pomatoschistus minutus) in Norway, an example of an extremely eurythermal fish (adapted to survive between 5 and 22°C.) Common for almost all the fish tested was a period of about 10 min immediately after transfer to the gradient trough when they hovered along the sides (Hesthagen, 1979). This period of time is inferred to be for the fish to determine its thermal preferendum, when provided with a range of different temperatures to select.

Behavioural thermoregulation is vital to the optimal growth and survival of aquatic and marine ectotherms. Studies of fish such as sharks (large scale predator) through to sand goby have shown that the majority of fish have a thermal preferendum, where growth and survival rates are at their best. However, the energy expended and risks involved with achieving thermal preferendum are varied greatly among species. The common ecological situation of risk vs reward is evident in behavioural thermoregulation.

References:

Hesthagen, I. H, 1979, ‘Temperature selection and avoidance in the sand goby, Pomatoschistus minutus (Pallas), collected at different seasons’,  Environmental Biology of Fish, Vol. 4, No. 4, pg 369-377

Johnston, I. A. & Bennett, A. F, 1996, ‘Animals and Temperature : Phenotypic and Evolutionary Adaptation’. Cambridge: Cambridge University Press.

Speed, C. W. Meekan, M. G. Field, I. C. McMahon, C. R. Bradshaw, C. J. A, 2012, ‘Heat-seeking sharks: support for behavioural thermoregulation in reef sharks’, Marine Ecology Progress Series, Vol. 463: pg 231–244

Ward, A. J. W. Hensor, E. M. A. Webster, M. M. Hart, P. J. B, 2010, ‘Behavioural thermoregulation in two freshwater fish species’, Journal of Fish Biology (2010) Vol. 76, pg 2287–2298  

Westhoff, J. T. Paukert, C. Ettinger-Dietzel, S. Dodd, H. Siepker, M, 2014, ‘Behavioural thermoregulation and bioenergetics of riverine smallmouth bass associated with ambient cold-period thermal refuge’, Ecology of Freshwater Fish 2016: Vol. 25, pg 72–85

Thermal Adaptation - Antifreeze Glycoprotein

The Antarctic Icefish (Dissostichus Mawsoni) is a cryophilic (cold-loving) fish species endemic to the Southern Ocean. They are stenothermic, meaning that they can only survive in limited environmental temperatures. The Southern Ocean varies between -1.9 and 3°C (Deacon 1984; Eastman 1993). Antarctic Icefish can grow to over 2 metres in length and reach 40 years of age (Brooks et al. 2010). How does a fish that lives in sub-zero arctic temperatures survive for 40 years? This blog post will address the adaptations of the Antarctic Icefish and how it thrives in extreme cold.

Fig. 1: Antarctic Icefish (Dissostichus Mawsoni) next to a Bald Notothen (Pagothenia Borchgrevinki)

(Fields 2012)

The Antarctic Icefish belongs to a suborder of ‘white-blooded’ fishes called Notothenioids. The evolution of Notothenioids has resulted in several biological adaptations allowing them to maintain circulatory and cellular function (Beers & Jayasundara 2015). Notothenioids possess ‘antifreeze’, a glycoprotein which prevents the growth of ice crystals by binding water molecules to them in the blood (Russo et al. 2010). This process is known as adsorption inhibition, likened to by DeVries as “putting a silk-stocking around the ice crystal” (Goodman 1998). Notothenioids also have membranes and specifically cold-adapted proteins to maintain metabolic rate.

The Antarctic Icefish has evolved so specifically to cold waters, that it is foreseeable that climate change will present significant threats to not only Notothenioid species, but other cryophilic marine organisms. The heat tolerance of Antarctic fishes was first studied by Somero and DeVries (1967) who determined the upper incipient lethal temperature (UILT) in three species of Notothenioids; the Striped Rockcod (Trematomus hansoni), Emerald Rockcod (Trematomus bernacchii) fishes and the Bald Notothen (Pagothenia Borchgrevinki) (Bilyk 2011). These fishes shared UILT values of only 5-7°C after being acclimatised to habitual arctic temperatures of -1.9°C. The heat intolerance of marine invertebrates such as a species of Brittle Starfish (Ophionotus victoriae) is more concerning, with fatalities recorded when subjected to temperatures as low as 2°C (Bilyk 2011).

The Antarctic Icefish has evolved with specialised biochemical adaptations which allow it to thrive in its sub-zero habitat. However, the icefish along with other species in its ecosystem will be threatened with extinction as global warming increases the temperature of the Southern Ocean.

References:

Beers, J. M. and Jayasundara N, 2015, ‘Antarctic notothenioid fish: what are the future consequences of ‘losses’ and ‘gains’ acquired during long-term evolution at cold and stable temperatures?’, The Journal of Experimental Biology (2015), pages 1834-1845

Bilyk, K. T. DeVries, A. L. 2011, ‘Heat tolerance and its plasticity in Antarctic fishes’, Comparative Biochemistry and Physiology, Part A 158 (2011) pages 382–390

Brooks, C. M, Andrews, A. H, Ashford, J. R, Ramanna, N, Jones, C. D, Lundstrom, C. C, Cailliet, G. M, 2010. ‘Age estimation and lead–radium dating of Antarctic toothfish (Dissostichus mawsoni) in the Ross Sea’, Polar Biology

Deacon, G., 1984. ‘The Antarctic Circumpolar Current’. Cambridge University Press, Cambridge.

Eastman, J.T., 1993. ‘Antarctic Fish Biology: Evolution in a Unique Environment’ Academic Press, San Diego.

Goodman, B. 1998, ‘Where Ice Isn't Nice’, BioScience, Vol. 48, No. 8 (Aug., 1998), pages 586-590

Russo, R. Riccio, A. Di Prisco, G. Verde, C. Giordano, D. 2010, ‘Molecular adaptations in Antarctic fish and bacteria’, Polar Science 4 (2010), pages 245-256

Figures: 

Fields, L. G. 2012, ‘“Mortimer Bob” with a borch for scale’, photograph viewed 19/03/2016

Thermal Adaptation - Gular Fluttering 

Birds, like mammals, are endothermic organisms meaning they have to change physically or behaviourally in order to maintain an optimal internal temperature. This is important in maintaining homeostasis for circulatory and osmoregulatory function in the body. In the previous post, the mammalian thermoregulatory adaptation of sweating was discussed; which is an evolved physical adaptation. Birds however have evolved without sweat glands, and depend on techniques such as gular fluttering and behavioural techniques such as shading and wing-drooping.

Gular fluttering is both a behavioural and physical adaptation for reducing internal heat in birds. The bird will stretch its mouth up and open, expanding the gular skin sac whilst pulsing the hyoid apparatus (Lasiewski R.C et al. 1966). Stretching the throat increases the surface area of the gular skin, which contains four pairs of blood vessels (McSweeney T. 1988). The interior of the gular sac and buccal cavity is cooled by increasing the speed of air flow in and out of the mouth. Heat moves from the blood vessels close to the skin surface, and through moist membranes into the fast moving air where it is pushed out of the birds’ mouth (Fowler M.E et al. 2003).

Fig. 1 - Diagram of Gular Flutter Cooling Mechanism
(Harrington 2012)

Lasiewski R. C. and G. A. Bartholomew were among the first to study gular fluttering and found that in poor-wills (nightjar) the birds opened their mouths and commenced gular fluttering upon reaching a temperature of 39°C and higher. Cooling of the gular skin occurred instantly after commencement of gular fluttering. The temperature of the gular skin was found to be 1.5 to 3°C cooler than body temperature, and up to 9°C cooler than external air temperatures. This suggests that gular fluttering is a very successful thermoregulatory adaptation for birds (Lasiewski R.C et al. 1966).

Gular fluttering is crucial to the prevention of heat exhaustion, hyperthermia and dehydration in many bird species. The temperature of gular skin as a result of fluttering is reduced greatly and in conjunction with increased blood flow, reduces overall body temperature (Lasiewski R.C et al. 1966). Gular fluttering in birds is an effective way to reduce internal body heat with minimal water loss. 

References:

Lasiewski, Robert C and Bartholomew, George A. 1966, 'Evaporative Cooling in the Poor-Will and the Tawny Frogmouth’, The Condor, Volume 68, no. 3

McSweeney, Terese and Stoskopf, Michael K. 1988, Selected Anatomical Features of the Neck and Gular Sac of the Brown Pelican', The Journal of Zoo Animal Medicine, Volume 19, no. 3, pp. 116-121

Fowler, Murray E. and Miller, Eric R. 2003, ‘Zoo and Wild Animal Medicine: Current Therapy’, Volume 6.

Figures:

Harrington, Emily. 2012, ‘Gular Flutter Cooling Mechanism’, Diagram, asknature.org, viewed 12/3/16, URL: http://www.asknature.org/media/image/17403

Thermal Adaptation - Sweating

Mammals are endothermic organisms, meaning that they have to physically or behaviourally change in order to maintain optimal body temperature. This is crucial in maximising the operation of bodily functions such as metabolism, osmoregulation and heart function. In excessive heat, some mammals have evolved with sweating mechanisms allowing thermal regulation. Humans possess three types of sweat glands (apocrine, eccrine, and apoeccrine) which are located on various parts of the body and serve different functions (Asahina et al 2014).

Fig. 1 Diagram of sweat glands present in some mammals

(Asahina et al 2014)

Apes, horses, monkeys and goats have evolved with similiar types of sweat glands that humans possess (apocrine, eccrine and apoeccrine) (Baker 1989). Sweat glands are merocrine glands, a class of exocrine glands. All types of sweat glands are tubular epithelial structures, apocrine being more prevalent in hair follicles, and apoeccrine located on soles/palms and eccrine in normal skin (Asahina et al 2014). Apocrine and eccrine sweating works by coating the outer skin with moisture which is then evaporated, expending heat energy from the body to the atmosphere. It is theorised that apocrine and eccrine sweating in early hominids allowed easier hunting of prey without thermoregulatory systems. The prey would tire faster than the hominids making them easier to ambush and kill (Noakes 2006). This proves the value of sweating as a thermoregulatory structure throughout evolution.

Fig. 2 Hominids ambush exhausted deer
(The Natural History Museum)

Sweating that occurs on the palm/sole of many mammals is referred to as ‘emotional sweating’. This process increases friction while grasping or performing delicate tasks (Asahina et al. 2014). In cats and dogs it is beneficial in increasing ground traction when running. Humans possess apoeccrine sweat glands likely due to our relation to primates, where it was advantageous in gripping tree branches. The sweat is produced as a result of anxiety or stress, so when primates were attacked or predated, their palms sweat, increasing friction and allowing a swift escape (Asahina et al. 2014).

Sweating is an important thermal adaptation to combat heat exhaustion caused by physical exertion or the environment. It has aided the evolution of humans in hunting prey, and allowed for primates and other mammals to escape quickly in times of stress. Sweating continues to be a relevant and important thermal adaptation for humans and other mammals.

References:

Asahina, Masato et al 2014, ‘Sweating on the palm and sole: physiological and clinical relevance’, Clin Auton Res, Chapter 25 pp 153-158

Noakes, Timothy 2006, ‘Exercise in the heat: Old ideas, new dogmas’, International SportMed Journal; Vol. 7 Issue 1, p58

Baker, M.A 1989, ‘EFFECTS OF DEHYDRATION AND REHYDRATION ON THERMOREGULATORY SWEATING IN GOATS’, Journal of Physiology 417, pp. 421-435. 

Fig, 1:
Asahina, Masato et al 2014, ‘Sweating on the palm and sole: physiological and clinical relevance’, Clin Auton Res, Chapter 25 page 154

Fig. 2: 

The Natural History Museum, date unknown, 'Paleolithic'. 

Welcome to Thermal Adaptations

(Fig. 1 Thorny Devil in Kalbarri National Park)

(Duncan, 2014)

The external environment of all species is constantly changing and varies greatly throughout areas of the world. A significant environmental change affecting all organisms is climate change, specifically temperature. The survival of a species in a changing environment is dependent on their ability to either adapt or relocate. 

Fig. 2: The relationship between thermoregulators and thermoconformers with increasing external temperature.
(Angilletta 2009)

Each species has an optimal range for temperature, dependent on their evolutionary origin and physiology. When environmental temperature is outside optimal range, the organism must either thermoconform or thermoregulate. The methods of doing so are varied between endothermic and ectothermic species as will be explained in this blog.



Fig 1:
Duncan, Paul 2014, Thorny Devil in Kalbarri National Park, viewed 8/03/16. 

Fig 2: 
Angilletta Jr. M.J 2009, The relationship between air temperature and organismal temperature might indicate some form of thermoregulation, viewed 10/3/16