Some humans struggle to balance their energy budget in the affluent west, but imagine the complexity of balancing energy intake against expenditure when you gorge for half of the year before embarking on a 6 month fasting migration to your barren equatorial breeding grounds. This is the dilemma faced by minke whales feeding in the abundant waters near Iceland. Fredrik Christiansen from the University of Aberdeen, UK, explains that binging whales store large amounts of excess energy in specialised fat tissue, known as blubber, and scientists are keen to know more about how, when and where mature and immature whales use and store energy. He adds that youngsters are more likely to invest energy in growth, while older (reproducing) whales are more likely to store energy for the lean months ahead, especially if they're pregnant females. Curious to discover more about the whales' energy storage strategy, Christiansen and his colleagues, Gísli Víkingsson, David Lusseau and Marianne Rasmussen decided to analyse the blubber distributions of minke whales in their Icelandic feeding grounds to learn more (p. 427).
Fortunately, Víkingsson, from the Marine Research Institute, Iceland, has more than 20 years of experience of working with whales; he captured over 150 minke whales over a 4 year period, measuring their girth and the thickness of their blubber at 18 different locations across their bodies. Using these measurements, Christiansen was able to calculate the total amount of blubber carried by each animal: ‘Measuring absolute blubber volume, rather than the relative blubber thickness, made sense since we wanted to quantify the actual amount of blubber that the whales accumulated, so we could convert this to energy’.
Equipped with his blubber volume estimates, Christiansen was able to calculate the rate of blubber accumulation and the total amount gained throughout the feeding season in whales ranging from fast-growing juveniles to pregnant females and mature bulls. He was then able to ask what specific blubber-gaining strategy different whales used during the summer. Did they pile on the pounds at the very end of their feeding season or did they squirrel energy away gradually? The team revealed that the youngsters didn't gain much blubber – instead they dedicated their energy to growth. However, as expected, pregnant females, who need to stock up for both the migratory journey and motherhood, almost doubled their blubber content to 449 kg, gaining a steady 0.0024 m3 of blubber each day. Mature males also used the same strategy, steadily piling on an astonishing average of 532 kg, which equates to an impressive 15.5 GJ of energy, enough to keep a 60 W light bulb burning for almost 8 years.
So why do males stock up just as much as females? Do they just enjoy eating or do they really need all this energy? Pondering this question, Christiansen suggests, ‘Maybe we underestimate the energetic costs of male reproduction, or reproductive behaviour’. He adds that males may be promiscuous and therefore devote considerable time and energy to searching for mates, while females who expend a lot of energy nursing new-borns may ‘benefit from conserving energy by being quite inactive, and instead let the males do the work of locating them’.
It is clear that we still have a lot to learn about these mysterious creatures. However, Christiansen's study will hopefully help us develop non-invasive ways of measuring whale blubber thickness so that we can gain more insight into how they acquire, store and use energy ready for their epic migration south.