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Ice Breaking Adaptation

Bowheads have adapted to live in an ice-dominated habitat through morphological, physiological and behavioural adaptions (George et al., 1989).  Bowheads are able to break through ice in order to breathe (blow) by using their unique bow-shaped head (Figure 11) where the skin layer is thicker and the large body size (advantage for lifting force) allows the Bowheads to break through the thick sea ice (George et al., 1999, George et al., 1989, Seim et al., 2014). Bowheads have been observed breaking ice approximately 60cm thick by Eskimo hunters and can easily breaking 20cm thick ice (Corkeron and Connor, 1999, George et al., 1989, Krutzikowsky and Mate, 2000). This bow-shape head adaptation also allows the Bowhead to breath without exposing any of their body to the freezing air (only blow hole exposed during breathing) (Figure 11).  Bowheads not only break through ice but also utilise polynyas and leads for breathing holes (Carrol and Smithhisler, 1980, George et al., 1989, Krutzikowsky and Mate, 2000, Stirling, 1980). To distinguish between areas of no ice or thin ice from dense ice cover they use a combination of acoustic and visual cues, which are also used in under-ice navigation (George et al., 1989).

Figure 5:  A bowhead breaking through the ice. Illustration by Craig George.

Figure 11: A bowhead breaking through the ice. Illustration by Craig George.

 

Bowheads use acoustic adaptation as simple call sequences which echoes off the keels of thick ice, giving an indication of ice thickness and allowing them to avoid large multi-year floes and navigate underwater (George et al., 1989, Stafford et al., 2012).

 

Bowheads have also adapted eyes that have a tapetum (Figure 12), which allows them to see in low-light conditions thus they can identify areas of thin ice from ambient light cues (more light, thinner ice) (George et al., 1989).

combined eye

Figure 12: Eye of the Bowhead whale, with the inner eye of the tapetum on the top left. (source: www.arkive.org)

 

These are vital adaptations for surviving in dark, turbid waters and utilised for detecting cracks in the ice-covered seas (George et al., 1989, Stafford et al., 2012). The distance at which cracks or suitable breathing areas can be detect by Bowheads remains unknown (George et al., 1989).

 

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