I read with interest Vaclav Smil's Millennium Essay1, in which he describes the general importance to bioenergetics of Max Kleiber's studies on the scaling relationship of metabolic rate with body mass in animals. Across 18 orders of magnitude from unicellular organisms to whales, it seems metabolic rate is proportional to body mass raised to the power of 0.75; the so-called three-quarters rule, exemplified by the well-known mouse-to-elephant curve.

Smil gives the example of marine mammals as species whose metabolic rates lie outside this relationship because, apparently, seals and whales have basal metabolic rates (BMR) about twice as high as those of other animals their size, which illustrates their environmental specialization for thermoregulation in cold water. However, there is evidence that is inconsistent with this view2.

The perception that pinnipeds (seals, sea lions, fur seals and walrus) and cetaceans (whales, dolphins and porpoises) have BMRs twice as high as similar sized animals is an idea that has been widely accepted for decades. It primarily arises from comparisons of marine with terrestrial mammal data.

In his studies, Kleiber was very specific about the conditions under which BMR measurements should be made, in a bid to reduce variance between comparisons of basal rates in animals of different size and from different taxonomic groups. (Measurements should be made on mature animals in a post-absorptive state, non-reproductive, at thermoneutral ambient temperatures, and without abnormal activity2.) For these reasons, Kleiber rejected the use of the two determinations for marine mammal BMR available to him when preparing his original paper.

Nevertheless, published BMR data for marine mammals often have not conformed to these criteria, but have been included in comparative analyses with data that do. This has led to the widely held view that marine mammals have higher BMRs and that they are therefore not 'normal' mammals.

In the analysis by Lavigne et al.2, where data from studies on seals and whales were excluded when determinations did not fulfil Kleiber's criteria, it appears that metabolic rates of marine mammals were indistinguishable from those predicted for other mammals under similar conditions. In support of this conclusion, the best available data on minke whale ( Baleanoptera acutorostrata) metabolic rates, determined from field tracking studies and heat loss determinations3 , give a value at zero swimming speed only 17 per cent greater than the BMR value predicted from Kleiber's general equation.

Clearly, these studies suggest that Kleiber's relationship applies just as well to marine mammals as to terrestrial species.