Whale migration has long been described as an annual, round‐trip movement between summer foraging grounds in high latitudes and winter calving grounds in low latitudes (see Introduction). According to Matthews (1978), “We now know that the migrations are closely correlated with the two fundamental necessities of cetaceans, as indeed of all animals, those of feeding and breeding.” However, why whales travel to the tropics to breed has remained a mystery. Below, we summarize our findings on the movements and residency patterns of antarctic killer whales and offer some new insights into the nature of, and reason for, killer whale migrations. We provide additional support for the argument that antarctic killer whales migrate to lower latitudes to promote skin molt and discuss how this might apply to the migrations of other whale species as well.

4.1 Antarctic killer whale migration

Our updated satellite‐tracking results indicate that, among the four killer whale ecotypes that are known to regularly occur in antarctic waters (i.e., types A, B1, B2, and C), at least three (B1, B2, and C) undertake long‐distance, high‐speed, nonstop, round‐trip migrations to lower latitudes. These three ecotypes comprise a mammal‐eating prey specialist (B1), a fish‐eater (C), and a penguin predator that is suspected to feed mainly on fish (B2; Durban et al., 2016; Pitman & Durban 2010, 2012; Pitman & Ensor, 2003; Pitman et al., 2018). Furthermore, this migratory behavior is geographically widespread in Antarctica having now been documented off the WAP (types B1 and B2), and in western Ross Sea (type C; Figure 1). It now seems likely that all antarctic killer whales are highly migratory, although the migratory patterns of type A killer whales (mammal‐eaters that frequent open waters; Fearnbach et al., 2019; Pitman & Ensor, 2003), while broadly similar to the other ecotypes, differed in some key details and will be discussed separately below.

Antarctic killer whales performed some remarkable feats of migration. A type C killer whale from the western Ross Sea traveled 48° of latitude (from 78°S to 30°S), a round trip of over 11,000 km and the longest killer whale migration documented to date (Figure 5c). A type B2 killer whale tagged off the WAP migrated to the waters off southern Brazil and back; it then spent just 71 days in antarctic waters before repeating the same trip (Figure 4d). This is to our knowledge the only record of any animal species completing the same, long‐distance, round‐trip migration twice in the same year. Furthermore, these two migrations were completed within a 5.5‐month period, suggesting that antarctic killer whales could, potentially, complete this migration 3–4 times per year.

These findings support the contention of Durban and Pitman (2012) that, although killer whales commonly occur in antarctic waters during summer when prey is abundant and accessible, their subsequent migrations to lower latitudes are not for purposes of feeding or breeding (except possibly type A; see below). The observed pattern of directional movement, elevated speed, decreased dive depths, and limited time spent at the northern terminus of migration argues against a hypothesis that killer whales migrate to lower latitudes for feeding. They are not spending enough time along their migratory route to obtain enough energy to offset the cost of migration. Additionally, except for type A (see below), tagged migrants generally did not approach islands, continental shelf areas, seamounts, or other obvious topographic or oceanographic features that might have afforded enhanced feeding opportunities (Figures 3-5; cf. Reisinger et al., 2015). Instead, they traveled rapidly north and then turned abruptly around and headed south toward Antarctica (sometimes on the same track they took northward or, in most cases, just east of it; Figure 4 and Figure 5). Furthermore, all four individuals that were tracked through complete, round‐trip migrations (one type A, Figure 2a; three B2s, Figure 4a,c,d) returned to the same general areas where they started their northward migrations, after being gone only about 6 weeks—a further indication that they probably did not leave antarctic waters due to a lack of prey.

Dive‐depth profiles provided additional evidence that antarctic killer whales forego normal foraging behavior during migration. Mammal‐eating killer whales (types A and B1) feed on air‐breathing prey, and, accordingly, most of their dives were less than 50 m, both on the feeding grounds (Figure 7a,b) and during migration (Figure 7a; no dive data for type B1 during migration). In contrast, type C killer whales (fish‐eaters) foraging in McMurdo Sound during the summer had an average mean dive depth of 110.0 m and regularly dove 300–500 m with a maximum of 696 m (Figure 7d). However, after they left the Sound on their northbound migration, their average mean dive depth immediately decreased to 38.0 m, with a maximum dive of 292 m (Figure 7d), as whales simultaneously increased their speed and directional coherence (Figure 6d). Although type B2 killer whales have been observed feeding only on penguins (Pitman & Durban, 2010), their dive depth profiles on the feeding ground are much more similar to the fish‐eating type C killer whales than the two mammal‐eaters (types A and B1; Figure 7a,b), and it appears that B2 whales are also primarily fish‐eaters. Although large baleen whales are normally assumed to fast during migration to lower latitudes (Lockyer & Brown, 1981), they will feed if the opportunity arises (e.g., Andrews‐Goff et al., 2018; Findlay et al., 2017; Geijer et al., 2016; Owen et al., 2017). It seems likely that killer whales, being much smaller and traveling continuously at relatively high speeds, for 6–8 weeks, would probably also take advantage of any feeding opportunities in route, but their directed tracks suggest that they normally do little or no feeding during migration.

It also seems unlikely that antarctic killer whales migrate to low latitudes for calving. On average, migrating killer whales, except type A, traveled 2–3 times faster than they did when they were on the feeding grounds (Figure 6). Their high‐speed, almost nonstop, travel would make it difficult for a neonate calf to keep up. In addition, we also now know that killer whales are capable of successfully giving birth in antarctic waters. Gill and Thiele (1997) photographed killer whales (type C, based on their images) well within the pack ice during late winter (10 August 1995), and they cited the presence of a small calf as the first evidence of a cetacean being born in antarctic waters. Figure 9 shows two different type B2 killer whale mothers and their newborn calves photographed near the WAP in March 2016 and January 2018, respectively. Both mothers had a thick film of diatoms, but their calves were clean—clear evidence that the calves were recently born in antarctic waters, with insufficient time to accumulate diatoms (see below). Figure 9b shows a very small calf, less than a third of the length of its mother; it had a floppy dorsal fin and fetal folds and was probably no more than a few days old (JWD and HF, unpublished data). Additionally, a type C female observed at TNB on 15 January 2015 was traveling with a neonatal calf that had conspicuous fetal folds. The mother and calf appeared to be with a group that subsequently migrated northward. Birthing in antarctic waters would allow newborn calves to forgo traveling thousands of kilometers, at high speed, immediately postpartum, while living in an area where the mother has access to abundant prey resources. More observations will be needed to determine when and where most calves are born, but the evidence at hand suggests that birthing in antarctic waters is a viable, and probably desirable, option for antarctic killer whales.

Figure 9 Open in figure viewer PowerPoint Two different female ecotype B2 killer whales with a heavy yellowish coating of diatoms all over their bodies; their newborn calves have no diatoms and we infer that they were recently born in antarctic waters (see text); (a) Wilhelmina Bay, Western Antarctic Peninsula, 14 March 2016, photo by A. Schulman‐Janiger; (b) a calf with fetal folds still visible; Gerlache Strait, Western Antarctic Peninsula, 3 February 2018; photo by J. Durban and H. Fearnbach, collected from >30 m above the whales using a unmanned hexacopter, under NMFS Permit No. 19091 and Antarctic Conservation Act Permit ACA 2017–029.

There are several sets of observations to suggest that killer whale occurrences in Antarctica are aseasonal and that migration could be facultative. The first is that at least some killer whales can be found in antarctic waters throughout the year. A caveat here is that our field research had a strong seasonal bias: we tagged animals only during the austral summer months (i.e., December–February; Table 1), and mean tag life was only 56.3 days. Nevertheless, based on a combination of tagging and photo‐identification data, we recorded killer whales around the WAP from at least October through June (Table 1; Fearnbach et al., 2019). Further evidence of killer whale occurrences in the WAP during winter (i.e., June–August) came from a tagged B2 whale that completed a round‐trip migration and returned to the WAP at the onset of winter on 1 June (2010; Figure 4a) and another B2 that nearly completed a southbound trip that would have returned it to the WAP on or about 7 August (2016; Figure 4d). Because many (and perhaps all) tagged killer whales spent at least 2 months on the foraging grounds (Table 1), these last two individuals likely would have remained in antarctic waters until at least July and September, respectively. Other evidence for winter occurrences comes from observations recorded during the relatively few winter surveys conducted in Antarctica. These include 4 July (1987), type unknown, Anvers Island, WAP (RLP personal observation); 10 August (1995), type C, fast‐ice edge south of Tasmania (Gill & Thiele, 1997); and 13–16 August (1955), type B1 or B2, Carlson Island, Weddell Sea (Taylor, 1957). Plötz, Weidel, and Bersch (1991) reported three sightings of killer whales (type unknown) “roving in large open leads” in the northeastern Weddell Sea also in July and/or August (1986; exact dates unspecified). Santora (2014) reported sightings of three groups of type B (i.e., B1 or B2; mean group size: 6.0), “hanging around the Bransfield and near Elephant Island” in August 2012.3 We conclude that killer whales can be found in antarctic waters year‐round, although a more complete parsing of relative abundance, by month and by ecotype, awaits further investigation.

Perhaps related to the year‐round occurrence of killer whales in antarctic waters, the migration departure dates of tagged whales were protracted. This was evident both in the variable amounts of time that individual whales spent on the antarctic foraging grounds and in the wide range of their departure dates. Normally, we were unable to determine how long individual killer whales remained on their antarctic foraging grounds because, as mentioned previously, all 62 of the whales we tagged had been present for an unknown period of time prior to tagging, and among those tagged only about half (48%) initiated migration before their tags stopped transmitting. However, as mentioned previously, the one B2 individual that was tracked through two, nearly complete migrations (Figure 4d) spent 71 days in antarctic waters between migrations, while another B2 individual spent a minimum of 146 days in antarctic waters after being tagged and had not initiated migration when its tag stopped transmitting (Table 1).

The variable amount of time that killer whales spent on the antarctic foraging grounds was also reflected in the wide range of migration departure dates for all tagged ecotypes combined, which spanned a minimum of almost 6 months—from 31 December (type A, Figure 2a, 2013) to 25 June (type B2, Figure 4d, 2016; Table 1, Figure 10). Again, these departure dates represent a minimum range because of the seasonal (summer) bias in our tagging efforts and a mean tag life of only 56.3 days—only one tag transmitted more than 150 days, and it recorded the latest departure date.

Figure 10 Open in figure viewer PowerPoint Tagging dates and migration departure dates for 22 killer whales of four different ecotypes, satellite‐tagged in antarctic waters during 2009–2016. Tagging occurred from 4 December (2014) to 2 March (2016); departures were recorded from 31 December (2013) to 25 June (2016).

A wide range of migration departure dates was also recorded within individual killer whale ecotypes. Departure dates for tagged type A killer whales ranged from at least 31 December (2013) to 23 March (2014), although another whale had not departed by 2 April (2010) when its tag stopped transmitting (Table 1). A type B1 killer whale departed on 28 January (2009), and another had not departed by 4 March (2015). Departure dates for type B2 ranged from 11 March (2016) to 27 June (2016; two consecutive migrations by the same whale). Departure dates of type C killer whales from the western Ross Sea ranged from at least 25 January (2015, Terra Nova Bay) to 18 March (2015, McMurdo Sound; Table 1). As an illustration of variability in migration schedules within an ecotype, the two type Cs cited above departed 51 days apart in the same year, from locations in the Ross Sea only approximately 100 km apart. Furthermore, when the first of these two whales stopped transmitting on 24 February, it was on track to be back in Terra Nova Bay by 8 March, 10 days before the second whale departed McMurdo Sound (Table 1).

Asynchrony in migration times was also evident in the varying degrees of diatom infestation evident among different ecotypes and among different groups of the same ecotype, often on the same day, and sometimes even intermingled within the same sighting (RLP, JWD, and HF, unpublished data). All these observations taken together suggest that the migration schedules of types B1, B2, and C killer whales were flexible (perhaps facultative) and protracted—extending well beyond the austral summer (i.e., December–February). More tagging (or, possibly, intensive photo‐identification studies) will be required to determine range and variability in residency times and departure dates of killer whales on antarctic feeding grounds, both within and among ecotypes, and for individual groups.

Although type A killer whales also made long‐distance, round‐trip migrations to lower latitudes, their tracks were notably different from the other killer whale types. Type As traveled northward on both sides of South America (Figure 2), while types B1 and B2 killer whales tagged near the WAP traveled northward only on the eastern side of the continent (Figure 3 and Figure 4), in the direction of the nearest warm water (Figure 8b,c). Compared to the other ecotypes, type As migrated at slower speeds (mean 5.4 vs. 8–9 km/hr; Table 1, Figure 6) and meandered more readily (i.e., greater difference between MTD and MGD; Figure 2), suggesting that they may have been foraging (or possibly calving) during migration. By contrast, the tracks of types B1, B2, and C killer whales were rapid and directional, with no indication of feeding. Migration by type A whales also took more time than those of types B and C: the only type A tracked through an entire round trip took 59.2 days (Table 1; Figure 2a), almost 10 days longer than any of the other killer whale types tagged off the WAP. Another type A (Figure 2b) reached its turning point after 49.6 days; at that rate, it would have taken an estimated 99.3 days to complete its migration.

In addition to having different migratory patterns, type A killer whales differed from the other antarctic ecotypes in other ways. In general, they appeared to be less derived, behaviorally and morphologically, than the other killer whale ecotypes in Antarctica. Type A has a typical black and white killer whale color pattern, and in Antarctica, they forage only in open water, avoiding sea ice altogether (Fearnbach et al., 2019; Pitman & Ensor, 2003). In contrast, types B1, B2, and C killer whales have a distinctive, two‐toned gray and white color pattern, a conspicuous dorsal cape, markedly different eye patches, and they forage extensively in and around sea ice (Durban et al., 2016; Pitman & Durban, 2012; Pitman & Ensor, 2003; Pitman et al., 2018). These “caped forms” appear to be endemic to Antarctica, and additional studies, including histology and genetic analyses (e.g., Foote et al., 2011), will be necessary to determine if they are better adapted than type A to cope with the extreme antarctic environment, and how such adaptation might contribute to observed differences in migratory patterns.

In summary, the migration schedules of antarctic killer whale ecotypes B1, B2, and C were protracted, perhaps year‐round, and possibly facultative, with groups capable of making multiple, 6–8 week, round‐trip migrations per year. Departure dates were highly variable among and within ecotypes, and the amount of time spent on antarctic foraging grounds for individual whales ranged from 71 days to at least 146 days, at least for type B2 killer whales. We infer that at least some of the caped forms (i.e., types B1, B2, and C) can be found year‐round in Antarctica and that they are capable of calving there (at least type B2 and probably C), making it unlikely that killer whales migrate to warmer waters for breeding purposes. It is currently unknown if type A killer whale(s) give(s) birth in antarctic waters, or if they occur there during winter. Migration by type A whales appears to be longer, slower, and more meandering than the other types, perhaps due to feeding or calving. Type A killer whale(s) could be a more recent arrival to antarctic waters and therefore may be less well adapted physiologically (Foote et al., 2011). These observations provide further support for the Durban and Pitman (2012) hypothesis that most Antarctic killer whales migrate to warm waters for reasons other than feeding or breeding, which we further discuss below.