Decisions at Depth

In the study of marine mammals, scientists often describe an animal as behaving or performing in an ‘optimal’ manner. In truth, life in the open ocean is rarely optimal; survival is only achieved through a continuous series of compromises. When deciding when or how deep to dive, or which prey to pursue, an animal must weigh the potential benefits of that decision against the costs. The reward for a good decision might be a hefty meal; a poor decision could lead to starvation or death.

Foraging is a particular challenge for marine mammals because their need to breathe limits the depth and length of each dive. Thus, scientists studying foraging behavior have, to date, been chiefly concerned with the ways in which the storage and use of oxygen limits foraging.

A team of scientists recently reviewed a wide range of studies of marine mammal physiology, with a view to expanding the range of commonly studied constraints to foraging. Drs. David Rosen (University of British Columbia), Arliss Winship (University of St. Andrew’s, UK) and Lisa Hoopes (Texas A&M University) present a novel framework of interconnecting processes that together form the physiological constraints to foraging behavior. Their results were published in a special edition of the Philosophical Transactions, Royal Society of London titled “Environmental constraints upon the locomotion and energetics of aquatic organisms” published by Springer-Verlag.

Competing Priorities
A marine mammal’s ability to find enough food to meet its energy requirements – and hence stay warm and survive – is determined by three key factors, the authors write. First is the ability to find and acquire prey, which is limited by the amount of time and energy spent foraging, as well as its diving capabilities.

The second constraint to foraging is the ability to digest prey and generate metabolic energy from the meal. This is limited by the size of the stomach and the amount of time required for digestion. “As an obvious example,” the authors write, “time spent ashore (to nurse, mate, or for thermoregulatory considerations) directly decreases potential foraging time.”

The third factor, a common thread between the cost of acquiring food and the cost of processing food, is thermoregulation, or the cost of generating body heat to guard against the deadly cold water. As the above example illustrates, hauling out on land can help regulate body temperature but thermoregulation is chiefly fuelled by the energy extracted from prey.

Even the decision to go foraging has important repercussions, given the high cost of foraging must be balanced with the chances of actually catching prey. If the gamble is successful and the animal catches its prey, all is well. But if no prey is caught, the animal incurs an energy deficit, having spent more energy than it gained.

Downward Spiral
“Energy deficits will be met through catabolism of tissues, principally the hypodermal lipid layer,” the authors write. “Depletion of this blubber layer can affect both buoyancy and gait, increasing the costs and decreasing the efficiency of subsequent foraging attempts. More importantly, decreased blubber will also potentially increase the costs of thermoregulation (although the heat generated through digestion and foraging activity may help to offset thermoregulatory costs). A downward spiral of increased tissue catabolism to pay for increased energy costs may result.

In other words, an animal that has begun to break down its own tissues to meet its energy requirements is colder, less agile and less likely to catch prey. Its chances of survival decrease as it spends more energy on each trip, with less likelihood of recouping that energy.

The blubber layer’s dual role as insulator and energy source is especially significant in younger animals, who have a higher surface-area-to-volume ratio than adults, and who would be expected to lose heat more quickly in water when the blubber layer is depleted.

The authors also examine the role of the circulatory system in constraining foraging among marine mammals. The automatic slowing of the metabolism at depth (bradycardia) and the constriction of blood vessels in the extremities (vasoconstriction) each serve to increase dive time and minimize heat loss, thus improving foraging success. But thermoregulation and digestion also make their own demands on the circulatory system, which may be incompatible with the circulatory demands of diving.

Marine mammals must constantly compromise between foraging, digesting, and staying warm. By examining the roles of prey acquisition, prey processing and thermoregulation in limiting foraging, the study offers scientists the tools to refine predictive models of foraging behavior. In addition, the paper highlights areas where research is currently lacking, in the hope of defining and stimulating future research.

Life in the open ocean may not be ‘optimal’ for marine mammals, but by understanding the physiological limits they face – and the compromises they make in the daily struggle for survival – scientists are better equipped to advise on the conservation of populations and species.

November 29, 2007

Publication:
Thermal and digestive constraints to foraging behavior in marine mammals. Rosen, D.A.S., A.J. Winship, and L.A. Hoopes. 2007. Philosophical Transactions, Royal Society of London B 362:2151-2168. abstract While foraging models of terrestrial mammals are concerned primarily with optimizing time/energy budgets, models of foraging behavior in marine mammals have been primarily concerned with physiological constraints. This has historically centered on calculations of aerobic dive limits. However, other physiological limits are key to forming foraging behavior, including digestive limitations to food intake and thermoregulation. The ability of an animal to consume sufficient prey to meet its energy requirements is partly determined by its ability to acquire prey (limited by available foraging time, diving capabilities and thermoregulatory costs) and to process that prey (limited by maximum digestion capacity and the time devoted to digestion). Failure to consume sufficient prey will have feedback effects on foraging, thermoregulation, and digestive capacity through several interacting avenues. Energy deficits will be met through catabolism of tissues, principally the hypodermal lipid layer. Depletion of this blubber layer can affect both buoyancy and gait, increasing the costs and decreasing the efficiency of subsequent foraging attempts. Depletion of the insulative blubber layer may also increase thermoregulatory costs, which will decrease foraging abilities through higher metabolic overheads. Thus, an energy deficit may lead to a downward spiral of increased tissue catabolism to pay for increased energy costs. Conversely, the heat generated through digestion and foraging activity may help to offset thermoregulatory costs. Finally, the circulatory demands of diving, thermoregulation, and digestion may be mutually incompatible. This may force animals to alter time budgets to balance these exclusive demands. Analysis of these interacting processes will lead to a greater understanding of the physiological constraints within which foraging behavior must operate.