Detecting killer whale pregnancies from the air

To continue over a decade of collaborative killer whale research alongside colleagues at NOAA, SR3, and Ocean Wise, Raincoast’s cetacean scientists spent two months in the field last year collecting aerial images for photogrammetry.

Photogrammetry, a versatile technique used across various disciplines, is useful in biology to enable precise measurements of animals from photographs. At Raincoast, we utilize this method to capture detailed images of threatened Northern Resident killer whales using a drone launched from our research vessel, Achiever. When we return from the field each year, we take meticulous measurements from these photos to determine each whale’s body condition. This evaluation of body condition in killer whales not only helps us as conservation scientists in understanding a population’s vitality, but it also  provides insights into the whales’ reproductive health by detecting pregnancies. 

How we can determine body condition and pregnancies from drone photos

It is remarkable that we now have the ability to assess the body condition of killer whales simply by analyzing their ‘eye patch ratios’. This method entails measuring the distance in pixels between the anterior tips of the white eye patches and the distance between the eye patches at 75% of their length. If we divide the lower measurement by the upper measurement, the resulting ratio indicates how far the eyepatches angle outwards. A higher eye patch ratio indicates a fatter whale, and a lower eye patch ratio a skinnier whale.

The aerial images are also a useful tool for detecting pregnancies in killer whales. As they spend most of their lives below the surface, pregnancy can often go unnoticed until a female gives birth to a calf. Photogrammetric measurements of the width of the female’s abdomen can provide a relatively simple and non-invasive method for determining when a whale is pregnant long before this point, which would be valuable for assessing rates of miscarriage. 

Mapping eye patch ratio over time

To identify individual killer whales, a member of the field team takes lateral identification photos of the whales with a DSLR camera during drone flights.. We can then match unique markings on the dorsal fin and saddle patch in the aerial and lateral images.

Because we can identify individual whales in this way, we are able to track how each whale’s condition changes over the years. We can then test which factors are associated with an increase or decrease in body condition. While much of our work focuses on how environmental variables, such as prey abundance, affect whale body condition, other factors can also influence condition. For adult females, a major driver of condition is their reproductive cycle.

For example, we measured A86, an adult female, every year between 2016 and 2020. You can see how her eye patch ratio changed over that period in the graph below.

After becoming pregnant with her first calf in 2017, A86 gained fat throughout her pregnancy before giving birth in early 2019. Following the birth of her calf A120, A86 quickly lost fat, with her eye patch ratio declining to below her pre-pregnancy condition. This likely reflects the high energetic costs of producing milk to feed her calf. Female killer whales are known to nurse their calves for up to two years, and studies have shown that a lactating female consumes 42% more prey than a non-lactating female. In 2020, more than a year after A120 was born, A86 had not yet recovered to pre-pregnancy fat levels, a pattern typical for breeding females. 

Some killer whales have higher reproductive success than others. While the Northern Resident killer whales show higher birth rates on average than the endangered Southern Residents, individual females vary widely in the number of calves they produce. A86 appears to be highly successful, as right after she weaned A120, she got pregnant again! This summer in 2023, the photogrammetry team was able to gather aerial photos of A86 with her second calf, born in winter 2023. In the years before she was seen with her second calf, her eye patch ratio followed the same pattern, increasing to its highest level before she gave birth. 

The important role of killer whale mothers

Killer whales are very social animals that live in highly structured female-led societies. In resident killer whales, both male and female offspring will typically remain with their mothers for their entire lives. These extended families are known as matrilines, and will travel together and share food. Female provisioning of offspring is particularly important, especially the older female matriarchs who play an important role in leadership and foraging.

While it’s not surprising that pregnancy and motherhood have a profound effect on the body condition of female killer whales, we are also interested in how this might affect the other whales in their matrilines. For example, do pregnant and nursing females reduce the amount of food they share with their relatives – causing them to experience a decline in body condition – in order to feed themselves and their growing calves? Raincoast researchers are currently investigating the roles killer whale moms may play in their tight-knit families. By using body condition as a proxy for nutritional status, we can start to understand some of the mechanisms driving what makes a healthy killer whale family.

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