Last time, I mentioned in my article on the size of Amphicoelias fragillimus (rather briefly) that whales "win" in terms of size when compared to sauropods. This statement generated some controversy in the comments.
First off, what do biologists and paleontologists normally mean when they say one species is larger or bigger than another?
What is "big"?
In my discussion of Amphicoelias fragillimus I concentrated on mass as a proxy for size (note that mass is not the same as weight). Other measures of size can be length, height, width or volume. Each measures something different, and so when talking about which "thing" is larger than another "thing" one should be careful to specify what is meant. This generated some confusion in my last article, where I implied that "bigger" meant "having more mass", but did not specify this. I will now be more explicit: I equate "bigger" or "larger" with being more massive, i.e., the mass of animal.
I use mass as the method of size comparison based on a couple of reasons. The main reason is that linear measures such as length, height and width can be misleading. For instance, the average adult sized giraffe is taller than the average adult sized elephant, but the elephant is much larger in terms in mass. One would never say that giraffes are larger than elephants, but one would say they are taller. The same can be said for birds, where one species might be wider (in terms of wingspan), but smaller in terms of mass (compare, for instance the average sizes in terms of both mass and wingspan of wandering albatross to the kori bustard).
Volume is also misleading, because an animal that is far more pneumatic, such as a bird or sauropod, can be more voluminous than an animal of the same or higher mass. Volume can also be quite variable, which brings up whales.
Image by Jeremy A. Goldbogen & Nicholas D. Pyenson
The image above is of a fin whale feeding sequence. According to Goldbogen et al. (2007), "we suggest that the majority of fin whale lunges result in a volume of water that ranges from 120 to 160% body volume." For instance, an average sized adult fin whale at 20 m long, massing 50 tonnes would engulf, on average, 71 m^3. This means while feeding, at least some whales more than double their volume. Wow. So volume is highly variable and is not necessarily very informative, especially when comparing animals that are not closely related (like sauropods and whales, for instance). So I also choose not to use volume when comparing sizes. I was criticized for not comparing volumes, as this is supposedly somehow "misleading". In fact, as far as I can tell, volume is rarely used when comparing animal sizes, if ever, and is certainly not the standard*. As noted, volume is actually considerably more misleading when comparing size, since if fails to consider density. Mass is generally the gold standard, but, of course there are problems with it too.
Mass can vary in individuals, too, but it is not as prone to wide changes in short periods of time like volume can be. In fact, in individual blue whales, masses are known to vary by a significant margin during the year, depending if it is feeding season or migrating season. According to Lockyer (1981), "For blue whales lean and fat ratios are 4.66 +/- 0.10 and 6.94 +/- 0.43 respectively...[t]he fattening is thus significant and average 49% of lean body weight in blue whales." This means a "small" blue whale individual in lean condition massing 70 tonnes could mass upwards of 100 tonnes by the time it has fattened up at the end of the feeding season. An medium-sized individual of 100 tonnes in lean condition during "winter" (March - Sept.) could be nearly 150 tonnes by the end of the feeding season ("summer": Oct. - Feb.). A large individual that in lean condition massed 140 tonnes, could be over 200 tonnes by the end of the season. So even comparing mass cannot be that straight forward.
The size of blue whales
Considering the above, how big are blue whales in terms of mass? Sounds like a simple question, doesn't it? Unfortunately, it is not so simple. When we ask, 'how big are blue whales?', what do we mean? Maximum size? Minimum size? Average size? Which average size? Lean condition? Fat condition?
These are not easy answers, and the data that is published does not necessarily get as specific as we need to get a particular average size.
Table 10. in Lockyer (1981) gives masses for blue whales of different lengths:
16 m long...23,749 kg
20 m long...49,045 kg
24 m long...88,702 kg
28 m long...146,390 kg
Table 5. in Lockyer (1981) gives mean (average) proportions of tissues (by % of total body weight):
Table 11. in Lockyer (1981) gives a average mass of the blue whale based on the sample studied of 90,000 kg.
Depending on the source, however, one can get different average masses for blue whales. For instance, Trites and Pauly (1998) estimate the mean mass for female blue whales to be 110,126 kg and for male blue whales to be 95,347 kg, for a gender-neutral mean of 102,736.5 kg. This difference could be down to different populations (there are different subspecies of blue whale that vary significantly in size, for instance), and could also be related whaling. Blue whales were hunted to near extinction and by the end of the 1960s their population was possibly less than 1,000 individuals.
This is speculation on my part, but it is possible that the masses in Lockyer (1981), which are actually cited from Lockyer (1976), were based on whale populations just starting to recover from whaling, and therefore a higher proportion of younger (and therefore, smaller) whales. And therefore the masses in Trites and Pauly (1998) are based off of recovering populations that may be healthier and have more mature whales and therefore have higher masses.
Branch et al. (2007) don't give mass estimates, but do give average lengths for the populations they studied between 83.4 and 86.3 ft (25.42 m - 26.31 m) (all based on sexually mature females). Scaling again from Lockyer (1981), one gets estimates masses of ~105 tonnes to ~116 tonnes for averages.
An average mass of about 100-110 tonnes for blue whales seems a reasonable educated guess, although it is possible whales were much larger before industrial-scale whaling. One individual is listed by noted whaler Sigurd Risting as being 107 Norwegian fot (Risting, 1922)*. A Norwegian fot is about 313.74 mm, which means it would have been about 33.57 meters long. Scaling from the weight and lengths listed by Lockyer (1981)(see above), this whale may have massed about 250 tonnes (!). The longest blue whale scientifically validated, however, is "only" 29.9 m, such a whale however could be expected to be about 177 tonnes, again, scaling from Lockyer (1981) (Sears and Calambokidis, 2002). Blue whale sizes can also be estimated off of oil yield, due to a close mathematical relationship between oil yield and total mass, with some estimates based on oil yield giving a mass of at least 200 tonnes. While on the subject of oil yields, I should point out that the second largest whale (in terms of mass), the bowhead, is estimated to mass up to 164 tonnes, again based on oil yields (George et al., 2007). So we have at least two living whales species for which individuals are known to get well over 150 tonnes. So yes, the whales do win in terms of mass (and even volume), when comparing largest known specimens. Unfortunately, we cannot compare average sizes of whales and sauropods since we lack samples that are large enough to make meaningful statistical comparisons for large sauropods. Of course, sauropods probably win when it comes to length, with the longest sauropod known from good material, Supersaurus, being at least 34 m long. It is quite likely that even a rebbachisaur-like Amphicoelias fragillimus reached at least 35 meters. More on that soon.
 To oblige one of the commenters who protested that I did not compare volumes, I'll note that the density of blubber is 900 kg/m^3, the density of muscle is 1060 kg/m^3, the density of viscera is about 920 kg/m^3, and the density of bone is 1,856 kg/m^3. This means we can find the rough volume of the 90,000 kg whale:
Blubber: 27%*90,000/900=27 m^3
Muscle: 39%*90,000/1,060=33.11 m^3
Bone: 17%*90,000/1,856=8.24 m^3
Blood (estimated): 5%*90,000/1,057=4.26 m^3
Total Volume without lungs=84.35 m^3
The mean lung capacity is estimated to be 2.5% of mass for blue whales according to Lockyer (1981). Which would add another 2.25 m^3 to the volume, for a total volume of 86.6 m ^3 (average density=1,039 kg/m^3).
 Google translates the relevant passage as, "The largest whale which has been measured by the Argentine station at South Georgia in the 16 years the catch has been driven there, had length av 107 fot." (p. 3)
Branch, T. A., Abubaker, E. M. N., Mkango, S. and Butterworth, D. S. (2007), SEPARATING SOUTHERN BLUE WHALE SUBSPECIES BASED ON LENGTH FREQUENCIES OF SEXUALLY MATURE FEMALES. Marine Mammal Science, 23: 803–833.
Goldbogen,Jeremy A., Pyenson, Nicholas D., Shadwick, Robert E. (2007) Big gulps require high drag for fin whale lunge feeding. MARINE ECOLOGY PROGRESS SERIES. Vol. 349: 289–301
Lockyer, C. (1981) Growth and energy budgets of large baleen whales from the Southern Hemisphere. FAO Fish. Ser. (5) [Mammals in the Seas] 3:379-487.
Risting, Sigurd. (1922) Av hvalfangstens historie. J.W. Cappelen. 658 pp.
Sears, R. and J. Calambokidis. 2002. Update COSEWIC status report on the Blue Whale Balaenoptera musculus in Canada, in COSEWIC assessment and update status report on the Blue Whale Balaenoptera musculus in Canada. Committee on the Status of Endangered Wildlife in Canada. Ottawa. 1-32 pp.
Trites, A.W. and Pauly, D. (1998) Estimating mean body masses of marine mammals from maximum body
length. Can. J. Zool. 76:886-96.