Energetics of Killer Whales and Their Diet

The Cost of Being a Top Predator

Living animals require energy for movement, growth, reproduction, digestion, and for all metabolic processes that occur in the body. The basal metabolic rate (BMR) is a measure of how much energy an animal requires to maintain homeostasis. Field metabolic rate (FMR) is a similar measurement of how much energy is required, although includes essential functions such as digestion and physical activity. The metabolic rates of animals vary greatly depending on factors such as sex, size, lifestyle, and if they are growing or lactating. At the bottom of the food chain, small animals can get enough energy by consuming plants, although the requirements of a large killer whale are a little more robust. As the top predator, killer whales consume various different animals to meet their needs, but what are these needs?

In the following section, I will calculate and describe the maintenance BMR and FMR, the FMR during growth, and the FMR of lactating killer whales. I will also conduct an analysis of the energy content of their prey, chinook salmon (Oncorhynchus tshawytscha) and steller sea lions (Eumetopias jubatus), to determine how much prey is required to satisfy their energy needs.

Daily Metabolic Rates

BMR for Maintenance

Maintenace BMR describes the energy required for processes needed to maintain homeostasis (respiration, circulation, kidney function).

The BMR of an adult male killer whale (5318 kg) was measured to be around 46,968 kcal/day*. This works out to about 8.8 kcal/kg/day. (Worthy et al., 2013)

*This measurement was taken of a captive killer whale, although their measurement was consistent with previous field studies and accurately followed the BMR relationship of marine mammals predicted by Klieber (1975). (Worthy et al., 2013)

FMR for Maintenace

Maintenace FMR describes the energy required for all physiological body functions, including BMR along with metabolic heat and physical activity.

The FMR of an adult male killer whale was measured to be between 43-51 kcal/kg/day. (Noren, 2010)

FMR for an adult female was measured to be slightly higher, between 46-55 kcal/kg/day. (Noren, 2010)

These FMRs obtained from literature were calculated using metabolic rate estimates predicted by Klieber (1975) for other marine mammals. (Noren, 2010)

FMR During Growth

FMR during growth is a measurement of the energy required for all physiological processes while the animal is still growing and developing.

FMR of a growing adolescent male killer whale (3702 kg) was measured to be around 182,721 kcal/day or 49.4 kcal/kg/day. (Noren, 2010)

FMR of a growing adolescent female killer whale (3017 kg) was measured at around 156,727 kcal/day or 51.9 kcal/kg/day. (Noren, 2010)

FMRs for growth were measured using similar metabolic rate estimates as adults, based on previous findings that FMR in marine mammals is not significantly affected by growth. However, seeing as these are closer to the upper limit of adult FMRs is expected due to the increased energy demand of growth (Noren, 2010)

FMR During Lactation

FMR during lactation is exclusive to female mammals and is a measure of the energy required for all physiological processes, including producing milk to feed their young.

The FMR of an adult female lactating killer whale (7.7 m body length) was calculated to be around 297,310 kcal/day. In an average adult female killer whale (4700 kg), this works out to around 63.3 kcal/kg/day. (Williams et al., 2011)

These values were calculated using an equation provided in the literature, relating the body length of an individual to the daily energy requirement. The calculated FMR is a little higher than the predicted cost (lactation = 5.5 x BMR), which may be a reflection of the high nutritional content in their milk.

Equation used to calculate FMR: ln(E) = ln(a) + b*ln(L) where E is daily energy requirement, ln(a) = -4.92 for lactating females, L is body length and b is a constant with a value of 2.84. (Williams et al., 2011)

Energy of Prey

Chinook Salmon
Oncorhynchus tshawytscha

Energy Analysis

Proximate Analysis of 100g portion: Obtained from: (USDA. 2019)

Water = 71.6%

Protein = 19.9%

Lipid = 10.4%

Metabolizable energy (ME) was calculated using atwater coefficients according to the equation: ME = [(4*%protein) + (4*%carbohydrate) + (9*%lipid)]*10 (Asaro et al., 2017)

Plugging in our values: ME = [(4*19.9) + (4*0) + (9*10.4)]*10

Therefore, the calculated ME of chinook salmon is 2207 kcal/kg

Prey requirements for BMR of adult male killer whale (6000 kg)

Looking back on the previous section, BMR for adult male killer whales is 8.8 kcal/kg/day, thus, a 6000 kg orca has a BMR of around 52,800 kcal/day.

The average chinook salmon is around 8.5 kg, and based on our ME calculation, contains around 18,760 kcal ME.

Given this, the number of salmon required to satisfy BMR can be calculated as:

# of salmon = 52,800/18760 = 2.8

Therefore, at least 2.8 chinook salmon must be consumed each day to satisfy the BMR of an adult killer whale weighing 6000 kg.

Prey requirements for FMR of adult male killer whale (6000 kg)

The average FMR of adult male killer whales is 47 kcal/kg/day, thus, a 6000 kg orca has an FMR of around 282,000 kcal/day.

The average chinook salmon has around 18,760 kcal ME. (as above)

Given this, the number of salmon required to satisfy FMR can be calculated as:

# of salmon = 282,000/18760 = 15.0

Therefore, at least 15.0 chinook salmon must be consumed each day to satisfy the FMR of an adult male killer whale weighing 6000 kg.

Ash = 1.3%

Carbohydrates = 0%

Fiber = 0%

Steller Sea Lions
Eumetopias jubatus

Energy Analysis

Proximate Analysis of 100g portion: Obtained from: (USDA. 2019)

Water = 56.8%

Protein = 22.1%

Lipid = 14.6%

Metabolizable energy (ME) was calculated using atwater coefficients according to the equation: ME = [(4*%protein) + (4*%carbohydrate) + (9*%lipid)]*10 (Asaro et al., 2017)

Plugging in our values: ME = [(4*22.1) + (4*5.6) + (9*14.6)]*10

Therefore, the calculated ME of steller sea lions is 2420 kcal/kg

Prey requirements for BMR of adult male killer whale (6000 kg)

As calculated under chinook salmon, a 6000 kg adult male orca has a BMR of around 52,800 kcal/day.

The average steller sea lion is around 40 kg, and based on our ME calculation, contains around 96,800 kcal ME.

Given this, the number of sea lions needed to satisfy BMR can be calculated as:

# of sea lions = 52,800/96,800 = 0.54

Therefore, at least 0.54 steller sea lions must be consumed each day to satisfy the BMR of an adult killer whale weighing 6000 kg.

Prey requirements for FMR of adult male killer whale (6000 kg)

As previously calculated, a 6000 kg adult male orca has an FMR of around 282,000 kcal/day.

The average steller sea lion has around 96,800 kcal ME. (as above)

Given this, the number of sea lions needed to satisfy FMR can be calculated as:

# of sea lions = 282,000/96,800 = 2.9

Therefore, at least 2.9 steller sea lions must be consumed each day to satisfy the FMR of an adult male killer whale weighing 6000 kg.

Ash = 0.9%

Carbohydrates = 5.6%