Thermoregulation (Cold defense)

The Bowhead whale has a combination of thermoregulatory mechanisms that makes cold stress from the Arctic environment unlikely (Ford et al., 2013). The Bowhead thermoregulatory mechanisms are;  increase in the body mass (reducing the surface area to volume ratio), maximisation of insulation (blubber layer), circulatory system adaptation, metabolic rate and an organ cooling mechanism.

Surface area to Volume Ratio

Bowheads  have a large body mass and rotund body forms which reduces surface area at which metabolic heat can be lost to the surrounding water (Illustrated Figure 6) (Blix, 2005, Ford et al., 2013). A large body size also increases the distance Bowheads can travel over a period of time (Laidre et al., 2007). Bowheads have an extended longevity due to their low metabolic rate, slow growth, delayed maturity and take several years for them to achieve sufficient body mass and skeletal development, which are selective adaptations due to increased survival rate in the challenging Arctic habitat (Corkeron and Connor, 1999, Ford et al., 2013, George et al., 1999, Seim et al., 2014). The Bowhead also has gene and protein adaptations to cope with its long life-span (159yrs old recorded) such as: series of genes that protect the kidney tissue against age-related decline; low levels of Perp which increase stress tolerance in liver; and Grb14 expression improves the energy homeostasis in lipid-rich diets and protects bowhead against chronic dietary diseases (Seim et al., 2014).

4.3 Effect of cell size on s

Figure 6: Illustration of surface area to volume ratio, reducing the surface area with increase mass.

Insulation Layer

Majority of the Bowheads body is effectively insulated with a blubber layer approximately 20-35cm thick (maximum reported 50cm)(Ford et al., 2013, George, 2009). Blubber is a sub-dermal layer of adipose tissue (fat) used for lipid storage and is directly under the skin (Ford et al., 2013). The blubber layer (Figure 7) acts as a barrier retaining metabolic heat and maintaining constant core temperature, thus is effective thermoregulation mechanism (Seim et al., 2014). The blubber layer insulation properties are improved by the stratification of fatty acids in blubber (Budge et al., 2008).


Figure 7: Blubber layer diagram

Vascular system

The flippers and flukes of Bowheads are not insulated with blubber as it restricts movement (Heyning, 2001) so to prevent heat loss from these areas cetacean have adapted circulatory architecture to regulate blood temperature (Elsner et al., 2004). Bowheads use two circulatory systems techniques for thermoregulation: the countercurrent exchange and the arteriovenous anastomoses (AVAs) (Elsner et al., 2004). These are located in and near the skin surface of the flippers and flukes (Blix, 2005). The countercurrent heat exchange functions by conducting heat from warm arterial blood coming from the body core and transferring that heat to the adjacent cold venous blood flowing from body surface, therefore the ventricular blood that re-circulates back to the body core is warmed up (figure 8) (Elsner et al., 2004, Heyning, 2001, Blix, 2005). The Arteriovenous anastomoses (AVAs) is alternate circuit which provides heat dissipation (Elsner et al., 2004). However the Bowhead has uniquely adapted thick-walled AVAs that’s also larger in diameter and length (2-4mm lumen diameter) (minimises heat loss) (Elsner et al., 2004). The control of heat conservation or expulsion is regulated by the sympathetic vasoconstriction or sympathetic activation of the AVA. When AVA is constricted blood flow is directed to the countercurrent (heat retention) and when AVA relaxes there is an increase blood flow through the AVA (heat loss to external environment) (Elsner et al., 2004). Therefore thermoregulation occurs at the flippers and flukes of the Bowhead regulated by current system mechanisms.

circulatory system

Figure 8: Thermoregulation Circulatory systems. Left is the countercurrent heat exchange system  with thin vascular walls (heat retained) . Right is the arteriovenous anastomosis (AVAs) system (heat lost). (Source: Seaworld park and entertainment)



Heat retention during filter feeding

An area of suspected high heat loss is thought to be the tongue during continuous filter feeding, as the mouth stays open and fast flowing currents move over this highly vascular and relatively uninsulated organ. However, Bowheads have adapted a tongue (Figure 9) with a diffusion layer of fatty tissue (2cm thick) and a countercurrent circularly system throughout the mouth (Heyning, 2001). The gums and palate of the bowhead mouth have small countercurrent systems aiding in the heat retention (Heyning, 2001). The tongue has a complex vascular system that has slow flowing blood in each blood vessel increasing efficiency of the heat transfer. The tongue has posterior and ventral network of numerous individual countercurrent heat exchanges (0.7cm in diameter) around the tongue that converge at the base of the tongue to form bilateral pair of lingual retia composing of approximately 30 countercurrent organised into a close-knit vascular bundle(Heyning, 2001). With the thin insulation layer on the tongue the mouth is able to retain heat, thus heat is not lost during feeding.

Figure 9: Bowhead tongue (source: Hannah Foss (Project Lead Animator))

Figure 9: Bowhead tongue (source: Hannah Foss (Project Lead Animator))

Prevention of Hyperthermia

Due to the effectiveness of the Bowhead heat retention mechanisms and that they are covered in blubber or avascular keratinous baleen (Ford et al., 2013). Bowheads are at risk of over-heating (hyperthermia) especially when their metabolism increases due to muscle movement and little surface area where heat can be lost to the environment (Ford et al., 2013). To combat this problem, Bowheads have adapted a special organ called corpus cavernosum maxillaris (CCM) (Figure 10). The CCM lies on the oral surface of the maxilla and is uninsulated. It has an extensive venous network, large cavernous space and sponge-like trabeculae, which contracts and expands (Ford et al., 2013). The location at the center of the mouth, is the ideal location for CCM (0.48m2) to radiate excess body heat. The CCM is 6-8°C hotter then other external body surfaces and heat loss from this organ is regulated by the shape and position of the tongue and gape angle of the mouth. Blood flow can also be restricted to the CCM resulting in organ contraction and heat conservation. However when the mouth is closed no heat is lost as blubber around the head provides sufficient insulation (Ford et al., 2013, Heyning, 2001). Therefore during periods of high energy exertion, like feeding dives (high fluking rate and significant drag) and long migrations, Bowheads swim with the mouths agape allowing water to flow over the CCM to shed the excess heat generated (Ford et al., 2013).


Figure 10: The corpus cavernosum maxillaris (CCM) and the location within the Bowhead mouth. (source: Ford et al., 2013)

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