More on Warm-Bodied Sharks
Shortfin Mako (Isurus oxyrinchus)
A question came across SHARK-L concerning makos being considered "warm-blooded" and various replies touch upon aspects of the topic. I responded to the original query and went onto expand upon or correct some of those responses
There are currently two species of mako recognized, the Longfin (Isurus paucus) and the Shortfin (I. oxyrinchus). Although the Longfin Mako possesses a regionally specialized circulatory system essentially like that of other lamnids, it does not appear to significantly warm any portion of its body thereby. In contrast, parts of the Shortfin Mako's body are regionally warmer than others, most notably the intestinal valve and stomach, the lateral swimming muscles, and the brain. But, because its heart and gills fluctuate with changes in environmental temperature, the Shortfin is not truly "warm-blooded" (that is, maintaining a uniform blood temperature through physiological mechanisms), but is instead regionally "warm-bodied".
Within strict limitations, muscle tissue operates more efficiently at higher temperatures than lower. This is largely due to the fact that the complex suite of chemical reactions involved in muscle contraction occur at a faster rate at higher temperatures than they do at lower. It has been demonstrated that a 10 degree Celsius (18 degree Fahrenheit) increase in temperature can increase the rapidity and strength of muscle contraction up to three times. In the Shortfin Mako and other warm-bodied sharks, this may translate to greater tail-beat frequency and a concomitant increase in swimming speed. But this is not the only benefit of warm-bodiedness, nor is it likely to be the primary factor driving the evolution of the facilitating circulatory modifications. Recent work by Barbara Block and her co-workers suggests that the primary selection pressure favoring the evolution of heat-retaining mechanisms in tunas, billfishes, and lamnid sharks is niche expansion — the ability to exploit cooler environments (water masses that are deeper or occur at higher latitudes) with greater efficiency than "cold-blooded" fishes.
The apparently cold-blooded Longfin Mako may have secondarily lost the ability to regionally warm its body. Because the Longfin typically inhabits relatively deep water (maximum recorded depth: 748 metres or 2454 feet), which tends to be relatively depauperate in living and non-living organic matter, and seems to employ a relatively low-energy lifestyle, the increased metabolic demands of warm-bodiedness may have been too high an energetic price to pay.
Lastly, the Shortfin Mako has been reported to leap some 6 metres (20 feet) above the surface of the sea, a feat which has been calculated to require a starting speed of at least 22 miles per hour. In an attempt to verify or refute some of the most commonly (and uncritically) cited estimates of the Shortfin's top speed, I have recently re-done the relevant calculations and found the 22 mph figure to be an underestimate by a considerable margin.
One writer speculated on the relevance of gigantothermy.
While it is true that large organisms have a relatively high volume of heat-generating tissue and a relatively low surface area from which to radiate metabolic heat (a consequence of the familiar cube-square law), the Shortfin Mako is not simply a case of gigantothermy (in which loss of metabolic heat is slowed by virtue of large size alone). If large size were the only factor that mattered in retention of metabolic heat, the Basking Shark (Cetorhinus maximus) would be among the very warmest-bodied of sharks; measurements reveal that this is not the case at all. In short: in addition to relatively large body size, the regionally modified portions of the Shortfin Mako's circulatory system plays a significant role in further reducing its rate of metabolic heat loss at the cost of significantly increased metabolic rate and overall caloric energy requirement.
The best available evidence strongly suggests that neither the Shortfin Mako nor any other lamnid actively regulates its body temperature. The combination of large body size and regionally modified circulatory system go a long way toward slowing the rate of metabolic heat loss to the environment. But the end result is merely an increased time delay between heat production and loss. This is not the same thing as thermoregulation, in which body temperature is maintained within a narrow range by physiological mechanisms. When a Shortfin Mako or other lamnid enters cooler or warmer water, its body temperature slowly reflects changes in the ambient water temperature.
Another thought it might be attributed to the mako's need for speed, noting he have considered the warm-bloodedness as tied closely to the red muscle/high energy burst phenomenon rather than with deep diving thermoregulation characteristics.
As explained above, increased speed is probably a side-benefit of warm-bodiedness in the Shortfin Mako, but unlikely to be the major selective pressure favoring its evolution.
You've got it backward: red muscle is relatively slow contracting but has high endurance; white muscle is fast-contracting but has low-endurance. These differences are due to the aerobic and anaerobic mechanisms, respectively, by which these two muscle types convert chemical potential energy into kinetic and thermal energy. However, you are correct in that the mammal-like blood characteristics of the Shortfin Mako and other lamnids is an important adaptation — among others — to fueling these sharks' increased metabolic needs.
As explained above, niche expansion — including into colder, deeper water — seems to have been the primary selective pressure favoring the evolution of the regionally modified circulatory system of the Shortfin and its lamnid relatives.
One major source of metabolic heat in warm-bodied and other sharks that seems to have been largely overlooked by shark biologists is the huge liver, a remarkable internal chemical processing plant that generates prodigious quantities of waste heat — probably far more than that produced by the relatively small quantity of relatively cool red muscle along the flanks.
It is worth mentioning that red muscle is probably the major source of heat warming the eye and brain of certain lamnid and alopiid (thresher) sharks.
A final participant noted that makos have a number of counter-current heat exchangers (the rete mirabile—a parallel network of arteries and veins in this case) whereby warm vein blood traveling back to the gills "gives" its heat to the cold blood coming from the gills in the arteries. This retains heat and allows makos, white sharks and other lamnids to be much more active predators in cool or cold waters.
All this is essentially correct, as far as it goes.
It is again worth stressing that range expansion into cool or cold waters is particularly important in the evolution of the heat-retaining mechanisms of the Shortfin Mako and most of its lamnid relatives. Cool temperate and boreal waters are relatively productive compared with nutrient-poor tropical seas. Since an animal's reproductive success is strongly dependent upon its feeding success, the ability to exploit the rich feeding in these chilly waters probably enhanced the ability of warm-bodied sharks to perpetuate their genes, including those for the development of regionally modified circulation. In this way, the marvelous heat-retaining mechanisms of these sharks may have become part of their evolutionary legacy.
I hope that this post answers your questions and clarifies confusing aspects of the warm-bodiedness of the Shortfin Mako. I have avoided introducing peripheral issues and endeavored to be as accurate, precise, clear and terminology-free as possible.
— R. Aidan Martin