Encyclopedia of Marine Mammals

Measurement of energy acquisition and allocation provides a quantitative assessment of how animals organize their daily or seasonal activities, and how they prioritize their behaviors to maximize fitness. What goes in as food and comes out as growth, reproduction, repair, waste, or metabolic work can be described by energy flow models (Fig. 1). At a minimum, survival requires that the individual breaks even in terms of energy costs and benefits, balancing the costs of self-maintenance versus energy acquired. To grow and reproduce, animals must obtain more energy than is needed to survive, so that they are in positive energy balance. Many marine mammals undergo variations in energy balance, gaining energy while feeding in highly productive environments, followed by fasting during migration or reproduction. This balance between energy acquisition and expenditure, as well as the temporal and spatial scales across which it is achieved, differs markedly between species and environments. For some species, including sea otters, sea lions, and fur seals, high rates of energy expenditure are met by high rates of energy acquisition (Costa and Williams, 2000). These animals preferentially live in nearshore environments or upwelling regions where food is abundant (Costa, 1993), finding energy balance on smaller (10s to 100s of kilometers), more immediate (daily to weekly) scales (Williams and Maresh, 2016). Other species, including seals and baleen whales, have low rates of energy expenditure, allowing them to alternate between high rates of energy acquisition and fasting. These animals achieve energy balance over larger spatial (100s to 1000s of kilometers) and time (months to years) scales. This strategy takes advantage of seasonally productive environments, building energy reserves that can be used when food is not available. Although maintenance costs may be elevated in polar regions, the seasonally high productivity associated with sea ice during the polar summer supports high-energy acquisition rates that more than compensate. As time passes and whales are confronted with the high costs of reproduction during a polar winter, they opt for the more thermally benign but energy-poor tropics. Spending time in the tropics also provides an opportunity to repair and “refurbish” their skin (Durban and Pitman, 2012). As obligate herbivores, manatees and dugongs have adapted to a diet of seagrasses and other aquatic plants that are in high abundance but of low caloric value. Sirenians are able to offset low energy intake rates by living in tropical environment that keeps maintenance costs low while consuming a herbivorous diet that takes little energy to acquire. However, the thermal constraints of extant sirenians make one wonder how the Steller sea cow managed to survive in the cold waters of the Bering Sea on a diet of kelp (Estes et al., 2016). While the best situation is to have access to abundant, high-energy prey, in some scenarios low-quality prey that is more abundant and nearby may be more optimal than searching for high-quality prey that is difficult to find. As prey becomes less available, the cost of finding it increases and the animal spends a greater proportion of its time and therefore energy searching for it. Eventually, there is a threshold beyond which more energy is spent searching for prey than is obtained and the animal goes into negative energy balance (Villegas-Amtmann et al., 2015).