Keywords

22.1 Introduction

Living entities have evolved to maximize their long-term survival including selecting and specializing in niches where they can effectively compete with other species in their habitats. Bowhead whales (Balaena mysticetus) have taken this specialization to an extreme level by living year-round in a challenging geographic region where no other closely related species reside. To ensure survival of individuals in this isolated and unpredictable environment, bowhead whales have adopted conservative reproductive and growth strategies. They can live to be 200+ years old, and sexual and physical maturity is delayed compared to other species. They delay raising a calf for decades, producing young only when their energy reserves are sufficient to maximize survival of a calf and its mother (George et al. 2021b). Growth in young whales emphasizes the head and feeding apparatus at the expense of postcranial skeletal structures. The size of the head relative to the body and the length of baleen plates is greater than most other mysticetes so that they can efficiently feed on small low-density invertebrates. This chapter describes what we currently know about the above strategies with respect to reproduction in this slow-growing and long-lived mammal.

22.2 Age and Length at Sexual Maturity

Bowhead whale growth is slow compared to other cetaceans (Koski et al. 1993; George et al. 2011). They appear to forgo growth in favor of storing energy reserves that allow them to survive long periods with a negative energy balance (i.e., during periods when they use more energy for normal activities than they obtain from feeding) (George et al. 2021a, b). Male bowhead whales mature at a length of 12–13 m when they are about 25 years old (O’Hara et al. 2002; Tarpley et al. 2016). Female bowhead whales become sexually mature at 14.2 m when they are estimated to be about 25 years old (Koski et al. 1992) based on samples from harvested whales collected up to 1992 (Tarpley et al. 2016). More recent harvest data, however, have identified sexually mature females 12.6–13.9 m, suggesting that there may be a trend toward sexual maturity at a younger age in recent years (Tarpley et al. 2021). The appearance of smaller sexually mature females in the harvest at Utqiaġvik (formerly Barrow), Alaska, was thought to be due to more favorable feeding conditions that have resulted from increases in productivity with longer and more extensive areas of open water each year. However, young female bowhead whales migrate past Utqiaġvik later than older females (Koski et al. 1993). Thus, the appearance of small sexually mature female bowhead whales in the harvest in recent years may be due to the migration being earlier. Koski et al. (1993) identified bowhead whales as small as 12.3 m with calves in the Eastern Beaufort Sea during summer in the early to mid-1980s when the smallest pregnant females harvested at Utqiaġvik were 14.2 m (Tarpley et al. 2016). For comparison with bowhead whales, North Atlantic right whales (Eubalaena glacialis) become sexually mature much earlier at about 10 years (Knowlton et al. 1994; Rolland et al. 2005; NMFS 2022), blue whales (Balaenoptera musculus) at 10 years for females and 12 years for males (Sears 2002; NAMMCO 2020a), fin whales (Balaenoptera physalis) at 7–12 years for females and 6–10 for males (Aguilar 2002; NAMMCO 2022a), humpback whales (Megaptera novaeangliae) at 5–15 years (Clapham 2002, 2018; Gabriele et al. 2007; Best 2011), sei whales (Balaenoptera borealis) at 8–10 years (Horwood 2002; NAMMCO 2020b), and gray whales (Eschrichtius robustus) at 6–12 years (Jones and Swartz 2002; Swartz et al. 2023, this book).

George et al. (2011) examined the question of whether corpora accumulation is, on average, the same for left and right ovaries. They found no significant difference in the number of corpora in each ovary; however, they did find large variation where the number of corpora could vary by up to 12 scars. Ohsumi (1964) examined corpora-accumulation curves and noted baleen whales were “Type 1” ovulators where right and left ovaries attained maturity at the same time and overall accumulation rates were similar. Bowheads appear to be consistent with Ohsumi’s characterization of baleen whales. George et al. (2021b) examined sex differences in the maximum body lengths of bowhead whales. They noted that among several hundred whales harvested since 1972, the 10 longest were females. The maximum reliable lengths for females were about 19 m while the maximum lengths for males were 16–17 m. However, maximum lengths of whales measured photogrammetrically are about 8.2% shorter than those of hunter-harvested whales, as there appears to be some “stretching” when whales are hauled onto shore (George et al. 2004). The longest whale measured in photographs was 17.57 m (Koski et al. 2006).

Reproductive senescence is poorly understood for bowhead whales, mainly because relatively few whales over 100 years have been aged. The current method of aging old bowhead whales relies on aspartic acid racemization (AAR; Wetzel et al. 2017), with exact age estimates insecure. There is no evidence for reproductive senescence in males. A large male bowhead estimated to be 159 years old had seminal fluid extruding from its penis (George et al. 1999). There is, however, information on possible senescence in female bowhead whales based on examination of reproductive tracts and age estimates using AAR (Wetzel et al. 2017). The oldest female that has been harvested with a fetus was estimated to be 121 years old. The three oldest female bowhead whales that were harvested had AAR age estimates of 133, 139, and 149. They all had small, regressed corpora albicantia and no corpora lutea, suggesting that they were reproductively quiescent or senescent (George et al. 2021b). Although these age estimates are not precise, they suggest that at least some female bowhead whales might be able to produce calves over a period of about 100 years.

22.2.1 Mating Period and Location

Bowhead whale behaviors associated with mating have been seen throughout the year, but a recent study (Fortune et al. 2017) found that some of this behavior is associated with stimulation of the epidermis during the molt, and therefore is not a reproductive activity (Würsig and Koski 2021). In particular, close physical presence and rubbing with pectoral appendages seem to assist with removal of dead skin and stimulation of new skin. Most calves are born over a very short period from late April to late May (Koski et al. 1993, 2008), suggesting that most successful mating occurs over a short time as well. On April 13 and 14, 2005, hundreds to perhaps one thousand or more bowhead whales were seen and photographed mating in the Bering Sea north of St. Lawrence Island (Figs. 22.1 and 22.2; Koski et al. 2005). Activities associated with mating have also been reported in autumn (Koski et al. 1993; Würsig and Clark 1993), but it is unknown how much of this behavior might be non-procreative. This behavior may have been rubbing relative to sloughing skin stimulation or it may have been non-reproductive mating attempts (Würsig and Koski 2021; Ham et al. 2023, this book; Würsig et al. 2023, this book). Several observations of presumed sexual activity in autumn were clearly sexual in nature, with extruded penises visible (Würsig et al. 1993). However, integration of variable lines of observational information and information below on gestation suggests that the main mating period is in late March to mid-April when most Bering, Chukchi, and Beaufort seas (BCB) bowhead whales are in the Bering Sea (Reese et al. 2001; Koski et al. 2005). Data from examination of ovaries suggest that female bowhead whales may ovulate up to three times during a reproductive cycle if the first and second ovulations do not result in a pregnancy (Tarpley et al. 2016). This likely explains observations of smaller numbers of whales engaged in mating activities during mid-April to late May than in early-to-mid April.

Fig. 22.1
A top-view photograph of a group of bowhead whales swimming in the water amidst the ice floes.

Mating behavior in a group of bowhead whales photographed on April 13 and 14, 2005, ~50 km north of St Lawrence Island, Bering Sea. The female (upside down at the top of image) is being aggressively pursued by several males. The resting whales on the lower right are probably males “catching their breath.” From Koski et al. (2005)

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Fig. 22.2
A photograph of a female bowhead whale surrounded by 5 male bowhead whales. The penis of one of the males is indicated by a circle.

Five male bowhead whales chasing a female (diving). with the penis of one of the males clearly visible

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In addition to an overall decline in the number of whales seen engaged in mating activities later in the spring, the group sizes of whales engaged in mating behaviors have declined from early April to late May. Carroll and Smithhistler (1980) described mating behavior during May near Utqiaġvik and typical group sizes have been 4–6 animals. Similarly, during the many years of ice-based whale census observations west of Utqiaġvik, mating behavior was described by observers on several occasions and group size has been 4–6 animals as seen from the ice edge. The smaller group sizes seen during May (4–6 animals) than during April (up to 24+; Koski et al. 2005) could be due to waning sexual behavior because most ovulating sexually mature female bowhead whales have become pregnant during their first ovulation, which appears to be in early-to-mid April.

22.2.2 Mate Selection

A hypothesis advanced by Kenagy and Trombulak (1986) and summarized for mammals in general by Clutton-Brock (2016) relates large testis size (generally by weight) to body size/weight in polygynous/polygynandrous (multi-mate) mating systems. This hypothesis is likely operating in bowhead whales. Males need a large volume of sperm to mate with as many females as possible during a proscribed mating season. In multi-mate systems, where the females are inseminated by more than one sexual partner, a higher volume of sperm also enhances the likelihood of paternity (Lüpold 2013). This relationship of sperm competition by volume was further advanced for gray whales, right whales, and bowhead whales by Brownell and Ralls (1986; see Gerber and Krützen 2023; Würsig et al. 2023, this book, for a summary for cetaceans); all of these species have large testes relative to their body size. Tarpley et al. (2021) mention that the largest single testis measured so far for a bowhead whale weighed 211 kg and was 1.5 m long. The estimated body mass of that whale was about 54,000 kg, making the testes/body weight 0.0078 (combined testis weight 422/per body weight 54,000). A relatively small number of harvested male bowhead whales had a combined testicular weight over 150 kg (n = 9) and only 5 had combined testes weights over 200 kg. The largest combined weight was 422 kg in a 14.6 m whale. A sexually mature male bowhead whale from the Eastern Canada-West Greenland (EC-WG) stock that was 14.1 m long had a single testis size of 42 kg (Heide-Jørgensen et al. 2012). Spermatozoa volume is massive due to the large size of the epididymis that can rival the testis (Haldiman and Tarpley 1993). Sperm competition can also be at sperm viability level; that is, some sperm will survive longer in the uterus making them more likely to impregnate their mate (Lüpold 2013); this has not been explored in bowhead whales, to our knowledge. In the closely related North Pacific right whale (Eubalaena japonica) the largest combined testes weight of 972 kg was in a 16.3 m whale and the next largest combined weight was 955 kg in a 17.0 m whale. Both whales were from the western North Pacific (Omura et al. 1969).

Very little has been published of the penis of the bowhead, but it is fibroelastic type like that found in most artiodactyls. From our field observations of a few large males, the penis is long (to 3.0 m) and slender (see also Fig. 22.2). As in right whales, penis length in bowheads, as a percentage of total body length, appears to be greater than that of balaenopterids. Little information is available on the morphology of spermatozoa in baleen whales. Tarpley et al. (2021) provide a photograph of spermatozoa from a 16.6 m adult male bowhead whale, but the morphology and size are not provided. The apparently strong physical sperm competition by volume may mean that male bowhead whales are rather indiscriminate in their choice of mates, but aspects of mate selection for both sexes are unknown. We explore mate selection further in the following section on mating behavior.

22.2.3 Mating Behavior

Bowhead whale sexual behavior entails much scramble competition, with males maneuvering to gain access to females, possibly almost always near the surface (Würsig and Clark 1993). In the Bering Sea and near Utqiaġvik, where the water is clear, we saw chasing well below the surface (perhaps to 20–30 m), but we do not know how much farther below the surface it may occur. Similar to observations of right, gray, and humpback whale sexual interactions, there can be many whales involved (mostly males; Figs. 22.1 and 22.2) with at least one female, and much white water being created, making observations by humans as to specifics particularly difficult (Würsig et al. 1993). These interactions are generally termed surface-active groups (SAGs), following a definition for humpback whales (Clapham 1996). However, in humpback whales, testes to body size ratios are much smaller than in the apparent sperm competing bowhead whales, and in humpbacks there are very aggressive male–male interactions, termed contest competition Clapham 1996, 2018). In the apparent sperm competitors, while there is swift action, males appear to attempt to outmaneuver each other without much overt aggression (Dines et al. 2015). There is even the (unproven) provocative suggestion that males at times help each other to gain access to females, by one male pushing down on a ventrum (belly)-up female and another attempting to insert his penis (as shown for southern right whales, Eubalaena australis, Payne 1995). If true, this also makes sense for apparent strong sperm competitors; if only by helping each other, each male receives a better chance at inseminating the female.

While males are jostling for position, females tend to hold their breath while ventrum-up, making it difficult for males to mate with them (Würsig et al. 1993). That female inverted behavior is likely to represent a form of female choice, where perhaps the most adroit, experienced, largest, longer-lasting male has a better chance to inseminate the female. When the female needs to breathe, she rolls dorsum-up while rapidly surging ahead and diving, again appearing to evade the males. Würsig et al. (1993) provide several sequential line drawings of SAG activities; also see Figs. 22.1 and 22.2). It is not clear how often female bowhead whales mate with different males, but in southern right whales, the same female has been seen multiple times in different SAGs (Payne 1995; Brown and Sironi 2023, this book); it is unknown whether she is physiologically reproductively active throughout this time. At any rate, sperm competition does not negate the probability of polygynandry in these animals with super-large testes and can presently be hypothesized for bowhead whales. SAGs may also consist at times of bowhead whale male homosexual activity (Finley 1990), as in other whales (Darling 1977 for gray whales; Kraus and Hatch 2001 for northern right whales (Eubalaena glacialis); and Sironi 2004 for southern right whales; see also Brown and Sironi 2023, this book).

Surface-active groups (SAGs) of many males and one to several females (Würsig and Clark 1993; Koski et al. 2005) are “commonly” seen in spring in the Beaufort Sea, but also occur at other times of the year (e.g., Everitt and Krogman 1979; Würsig et al. 1993). It is not known for what reason extra-seasonal apparent matings or mating attempts occur, but it has been hypothesized that while some of it is homosexual, it may also represent especially young animals learning mating patterns and physical maneuvering needs (Würsig et al. 2023, this book; Fig. 22.3). Not all apparent mating attempts occur in SAGs, and at times (and reported infrequently, probably as it is difficult for humans to witness), apparent mating has been reported for only two or three animals in much more leisurely rolling and touching activities than in SAGs (Würsig et al. 1985). While it is very likely that female bowhead whales mate with multiple males during estrus, this has been confirmed for right whales (Brown and Sironi 2023, this book). In right whales, two males with penises inside one female at the same time have even been reported (Mate et al. 2005 for northern right whales; Würsig 2000 for southern right whales), with strong evidence for competition being at sperm volume level, and perhaps with other physiological advantages. In Würsig (2000), the larger male seemed to be preferred by the female, as she tilted toward the larger male and thereby pulled the smaller male’s penis out of her; the larger male went to apparent orgasm, with clear muscular contractions from base toward tip of penis. The gentle-seeming interactions, including caressing by flippers after the mating event, indicate that at least at times female choice may not be simply among several males in large aggregations, but may be more of a one-on-one interaction of choice. While the above vignettes are of right whales, the similar large testes and apparent polygynandrous system of bowhead whales allow us to predict—with caution—that multiple mating strategies may also be involved in bowhead whales.

Fig. 22.3
A stacked bar graph illustrates the number of whales versus length category for B O S 18 and 19. The length categories between 13.0 to 13.5 and 14.0 to 14.5 meters have the highest value, with four whales in each category.

Sizes of bowhead whales observed engaged in presumed sexual activity during behavior studies conducted in the Central Beaufort Sea on September 20, 1998. Lengths were determined by aerial photogrammetry conducted using the methods of Koski et al. (1993). Figure prepared by William R. Koski, and used here with permission

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While detailed analyses of bowhead whale sounds and song have not been unequivocally linked with sexual behavior, it is likely that this link exists (Würsig and Clark 1993). Stafford et al. (2012) analyzed bowhead whale song recorded in Fram Strait between Greenland and Svalbard. Song-type calls were initiated in late October, with near-constant singing from late November until early March. Singing occurred somewhat intermittently through March-early April and ended abruptly in late April. While the number of new songs had decreased by March, the highest diversity of different song types occurred in March. It has been speculated that bowhead whale song is a male reproductive display similar to that of humpback whales (Stafford 2022; Eichenberger et al. 2023, this book). Peak singing coincides with the peak breeding/conception period in late March and April (see above) for BCB bowhead whales.

22.2.4 Gestation

The bowhead whale gestation period appears to be slightly more than 1 year based on the main mating period in late March to mid-April and the peak of calving in late April to late May (Koski et al. 1993). This assertion is supported by small embryos (3 cm) and near-term fetuses (366–455 cm) having been found in harvested whales during the spring hunt at Utqiaġvik, Alaska (Tarpley et al. 2016, 2021). Reese et al. (2001) proposed a ~14-month gestation period like the 13.5-month gestation in gray whales (Dale Rice). However, bowhead and gray whale gestation is slightly longer than of blue (10–12 months; Sears 2002; NAMMCO 2020a), fin (11 months; Aguilar 2002; NAMMCO 2022a), and humpback whales (11–12 months; Clapham 2002; NAMMCO 2022b), and considerably longer than sei whales (7 months; Horwood 2002, 2018; NAMMCO 2020b).

22.2.5 Birthing Period and Location

Bowhead whale calves are born in spring during the northward migration from the southwestern Bering Sea to the Eastern Beaufort Sea and mother and calf pass northern Alaska during the latter part of the migration in water with extensive ice cover that is <0 °C (Angliss et al. 1995; Koski et al. 2004, 2008). Although it seems like a stressful time to calve, bowhead whales avoid encounters with their main natural predator, the killer whale (Orcinus orca), and calving occurs a short time before the mothers arrive at prime feeding areas in the Eastern Beaufort Sea (Citta et al. 2015).

The question of how neonates withstand parturition into <0 °C sea water has not been investigated but does not appear to result in mortality of the calf. Based on between-day sightings of recently born calves in mother–calf pairs near Utqiaġvik, it appears that mothers interrupt their migration for a few days after they calve to recover from the stress of calving and to permit calves to become strong enough to migrate (Koski et al. 2004; Arnold Brower, Sr, pers comm to JCG). When mothers and newborn calves begin their journey toward summer feeding areas, the calf has at times been seen to ride on the back of the mother (Würsig et al. 1999). The calf sits on the lower back of the mother and appears to be able to maintain this position passively due at least in part to the Bernoulli effect. The riding behavior by very young calves is probably a method to reduce the energetic requirements of the calf during migration and hence to reduce the stress of migration on the young calf.

Most calves appear to be born in the Chukchi and Western Beaufort seas based on few sightings of calves in the Bering Sea, the relatively high occurrence of mature females 11/64 (17%) harvested at Utqiaġvik in spring carrying a full-term fetus (NSB unpublished data), and only one sighting of a young calf in the eastern Beaufort Sea during summer (Koski et al. 1988). However, some calves are born in the Bering Sea. Noongwook et al. (2007) reported that Saint Lawrence Island whale hunters have reported seeing migrating mother/calf pairs in April and May, but the frequency of such sightings was not quantified: “Mothers with calves travel as part of the large-whale group and are seen as early as April, but with greatest frequency in mid-May. Calves may be born as late as June. Hunters say they are harvesting pregnant females more frequently now than years ago, which is consistent with observations of increasing numbers of whales and particularly small whales” (Noongwook et al. 2007). In addition, Koski et al. (1993) estimated that 14% of calves were born before 1 April, when most whales are still in the Bering Sea, and 11% were born after 1 June, when most whales have passed Utqiaġvik. Some sexually mature females harvested at Utqiaġvik in May had near-term fetuses and these ranged in total length from 366 to 455 cm (Tarpley et al. 2016, 2021). It appears that first-time mothers calve late in the spring after they pass Utqiaġvik, based on a comparison of the sizes of mothers seen during the summer with those passing Utqiaġvik in late April to early June during the 1980s to early 1990s (Koski et al. 1993).

22.2.6 Calf Size at Birth

There are limited data on the lengths of newborn calves, but the estimated near-term fetal lengths ranged from 400 to 450 cm in Nerini et al. (1984). Koski et al. (1993) suggested a mean size at birth of 430 cm based on the estimate of Nerini et al. (1984) and photogrammetric measurements of young calves photographed near Utqiaġvik. More recent data from near-term fetuses harvested from 29 April to 15 June had a mean size of 413 cm (George, field observations). Twelve of the 16 near-term fetuses were >400 cm (Tarpley et al. 2021) and the largest fetus was 455 cm. Based on photographic data from late spring and early summer, calves with their mothers had total lengths of around 4.4 m; however, most of these calves were more than a few days old so this overestimates the size of a newborn calf. The mean 4.4 m calf size is about 30% of the 14.7 m mean length of all photogrammetrically measured mothers (Fig. 22.4a; W Koski, unpubl data). Agbayani et al. (2020) reported that average size at birth of gray whales was 4.6 m, or 39% of the mean size of mature females (11.7 m; Rice and Wolman 1971).

Fig. 22.4
Two top-view photographs of a bowhead whale mother with a calf in the water.

(a) A bowhead whale mother and newborn calf photographed near Utqiaġvik AK in spring 2004. Photo taken by W. Koski; (b) A bowhead whale mother with a 4–5-month-old calf photographed in Cumberland Sound during August 2019. Photo taken by Ricky Kilabuk

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22.2.7 Calf Dependency

Bowhead whale calves remain with their mothers for about 10–11 months (Koski et al. 1993, 2004, 2008). Calves are primarily born during April to early June, and only a few yearlings have been seen with their mothers near Utqiaġvik in spring (Koski et al. 2012). The vast majority of yearlings are solitary when they pass Utqiaġvik toward the end of the spring migration, well after other immature whales (Koski et al. 2012). While few calves born the previous year are still with their mothers in spring, milk in the stomach of one yearling harvested in spring (it was probably born late in the previous year) indicates that it was still nursing (George and Suydam 2014; Tarpley et al. 2021). Stomach analyses of 12 calves of the year that were harvested in autumn found that nine had only milk in the stomach, two of them also had invertebrates, and one was empty. This indicates that calves begin to feed on invertebrates when 5–7 months old, but their main source of nourishment is milk (George and Suydam 2014; Sheffield and George 2021). Calf dependency is quite similar in related southern right whales, where females and their 1-year-old young return to the general winter-time mating area just before females and young separate (Taber and Thomas 1982). Other species of baleen whales are weaned at an earlier age than bowhead whales and after they arrive on their feeding grounds with their mothers (6–8 months for blue, fin, sei, gray, and humpback whales; Aguilar 2002; Clapham 2002; Horwood 2002, 2018; Jones and Swartz 2002; Sears 2002; NAMMCO 2020a, b, 2022a, b). Despite a longer period of dependency on their mothers, bowhead whale calves are generally less than half the length of their mother when weaned (Koski et al. 1993), while the other species of mysticetes are generally about two-third to three-quarter the length of their mothers at weaning (NAMMCO 2020a, b, 2022a, b). Gray whales, for instance, are weaned at about 8.7 m (Jones and Swartz 2002), which is about 74% of the average length of a mature female gray whale (Rice and Wolman 1971).

22.2.8 Calving Intervals

Early photographic studies suggested that bowhead whale calving intervals were mostly 3 and 4 years (Miller et al. 1992; Rugh et al. 1992; Koski et al. 1993). Consistent with the calving intervals during those years were reproductive cycles of 2 years with very low calf production followed by 1 year with moderate calf production and 1 year with high calf production (Koski et al. 1993). Tarpley et al. (2021) summarized the considerable literature on bowhead whale calving intervals and calving rates for bowhead whales of the Bering–Chukchi–Beaufort (BCB) stock that was collected during post-mortem examination of harvested whales, aerial surveys, and progesterone spikes in baleen of mature females (Rolland et al. 2018). Tarpley et al. (2021) included data from more recent years (1993–2013) than Tarpley et al. (2016) and concluded that the average inter-birth interval is just over 3 years with an average pregnancy rate of about 0.32 calves/year. George et al. (2018) reported a positive and statistically significant trend in pregnancy rate during their study period; however, these findings are being reanalyzed. Right whale calving intervals are similar to bowhead whales (Davidson et al. 2018; Watson et al. 2021), but calving intervals for North Atlantic right whales have increased since the 1980s to about 5 years (Kraus et al. 2001, 2007). In comparison to bowhead and right whales, calving intervals of other baleen whales are generally 2 years (gray whales—Jones 1990; Jones and Swartz 2002; humpback whales—Clapham 2002; Baker et al. 1987; fin whales—Aguilar 2002) and are 2–3 years for blue whales (Sears 2002).

It is worthwhile to compare lactating bowhead whales with other migratory baleen species. Lactating gray whales in Mexican overwintering lagoons were in better body condition than other gray whales (Christiansen et al. 2021); however, it is not known whether the better body condition was maintained to weaning of the calf, which is several months later and after they leave the wintering lagoons. It is also not known whether females that had calves the previous year were more likely to die than other sexually mature gray whales during mass mortality events that periodically have occurred recently (Perryman et al. 2002, 2021). Survival of sexually mature bowhead whales is extremely high (98.4–99.6%, Zeh et al. 2002; Givens et al. 2017), so few bowhead whale mothers are likely to have died as a result of giving birth to a calf and raising it to weaning.

22.2.9 Growth Rates

During the nursing period, bowhead whale calves grow rapidly (Fig. 22.4a, b; Koski et al. 1993), but between weaning and approximately year 5, bowhead whales display sustained baleen and head growth and an almost unprecedented pattern of limited growth in the rest of their bodies (i.e., the postcranial portions). Large autumn calves and small yearlings or even 2+-year-old bowhead whales can overlap in body length (Koski et al. 2012; George and Suydam 2014). During this period, they withdraw resources from the skeleton, in particular the ribs, which may lose 40% of bone mass (George et al. 2016). The body lengths of whales 2–5+ years old overlap, with little annual increase in body length (Lubetkin et al. 2008; Koski et al. 2012). This emphasis on baleen and head growth is unique among baleen whales and prepares the young whales for efficient feeding on tiny invertebrates in an environment with low densities and unpredictable presence of their primary prey. Following this hiatus in growth, bowhead whale growth becomes more rapid (Koski et al. 1992; Lubetkin et al. 2012) but is still slow compared to cetaceans inhabiting warmer waters (George et al. 2021b). Once growth resumes in year 6, it takes 13–20 years to grow to the age of sexual maturity (i.e., at about 25 years old) and another 30 years or more to attain physical maturity at an age of 50–60 (Koski et al. 1992; Lubetkin et al. 2012).

In other baleen whales, growth is rapid until they are weaned and then slows down or stops after they depart their feeding grounds. The second growth phase starts when they return to their feeding ground at about 1.5–2.5 years old, whereas in bowheads, the slow growth extends until they are 5+ years old. Sexual maturity in right whales is around 9 years (Hamilton et al. 1998), blue whales 8–10 years (Sears 2002), fin whales 6–8 years (Aguilar 2002), sei whales about 10 years (Horwood 2002), humpback whales about 5 years (Clapham 2002), and gray whales 6–12 years (Jones and Swartz 2002).

Bowhead whales store energy reserves in their blubber to ensure survival during periods of low food abundance rather than applying these reserves more immediately to growth as do other cetaceans (Burns 1993; George 2009). George et al. (2021a, b) speculated that these apparently conservative growth and reproductive strategies appear to be possible only for geographically isolated species such as the bowhead whale.

22.3 Conclusions and Future Directions

Most of the information summarized above has come from harvest and photographic data collected from the BCB stock of bowhead whale. These data have been used for management of the EC-WG stock and should be validated by studies conducted on that latter stock. Estimates of growth rates and reproductive parameters rely heavily on accurate estimates of age. Calves and yearlings can be accurately identified in photographs based on their morphology (Koski et al. 1993, 2012), and whales up to about age 10 can be reliably aged based on the length of their baleen (Lubetkin et al. 2008). The latter information can only be obtained from harvested whales, which results in much smaller sample sizes for analyses and makes analyses impossible for stocks other than the BCB stock. Approximate age estimates for whales older than about 10 years come from AAR analyses (Wetzel et al. 2017). DNA methylation has been used to estimate the ages of a few EC-WG bowhead whales (Li et al. 2021; Parsons et al. 2023), and this method appears to provide more precise age estimates than AAR based on data from known age animals of other species in captivity. We recommend that tissues already aged using AAR and tissues from recently harvested whales be aged using DNA methylation. This will permit us to obtain better estimates of growth and reproduction parameters.

Biopsy samples from bowhead whales can be used to determine the age of sampled whales using DNA methylation (Parsons et al. 2023), the sex of sampled whales (Linsky et al. 2022), pregnancy rates for sexually mature whales (Pallin et al. 2018), and their diet (Marcoux et al. 2012; Pomerleau et al. 2018). From combinations of these data and with samples from a large number of whales, the ages at sexual maturity and senescence of female bowhead whales can more accurately be determined, and the population structure can be determined as well. Bowhead whales segregate by size class in the summer feeding areas (Koski et al. 1988) and so the diets of different size classes are expected to be different. Analysis of stable isotopes from biopsy samples will provide information on the differences in diet and energetics of different size classes of bowhead whales.

Genetic analyses of biopsy samples to determine the parents of bowhead whales will provide information on reproduction history of the parents. For example, if the age of a young animal is obtained and there is information for its mother about the years when she had calves, the information for the young animal could contribute to the sequence of years when the mother had or did not have calves. If a male bowhead is found to be the father of many animals, it would provide information on the existence of “super males” or males that father a disproportionately high proportion of the recruits to the population (Gerber and Krützen 2023, this book).

Drones have become a valuable tool for observing the behavior of animals while minimizing disturbance (Fortune et al. 2017; Ramos et al. 2023, this book). Recent studies of bowhead whales in eastern Canada have provided more detailed information on behavior than earlier studies from fixed wing aircraft because camera resolution has improved, and whales do not appear to be disturbed by battery-powered drones flying at low altitudes above them (W Koski, pers obs). During the same studies, accurate information on whale sizes and body condition can be obtained from still photographs.

Also, as noted by Brownell and Ralls (1986) there is still a need to collect additional anatomical data on harvested bowhead whales. Special attention should be given to the following: (1) size of testes and weights, (2) epididymides, (3) sperm morphology, (4) total length and girth of the penis, and (5) the size of the vagina and the number of vaginal folds (see also Orbach et al. 2023, this book). In addition, details are lacking on the morphology and viability of the sperm.

Bowhead whales have adopted many growth and reproductive strategies that have enabled them to survive in a challenging environment where no other mysticetes co-occur throughout their principal range. It appears that these strategies permit bowhead whales to survive long periods with negative energy balances by storing energy reserves in a thick layer of blubber. To obtain this energy in an environment with overall low annual productivity, they have developed an efficient feeding apparatus that can filter tiny organisms from the water column, as right whales of the related genus Eubalaena do. To ensure that they have adequate energy reserves to survive periods with low productivity, they delay reproduction until energy reserves are sufficient to successfully wean their young and at the same time enhance the survival of the mother. As a direct result of this cautious approach to using food resources for growth and raising a calf, females do not become sexually mature until they are in their mid-twenties, in comparison to other baleen whales which become sexually mature at 5–12 years (Hamilton et al. 1998; Aguilar 2002; Clapham 2002, 2018; Horwood 2002; Jones and Swartz 2002; Sears 2002). They counterbalance late sexual maturity and long periods between giving birth to calves with their extraordinary longevity. Bowhead whales are the longest living mammal discovered to date, apparently living to ages of 200 years or more and female bowheads may be capable of producing calves for 100 years, which is longer than other mysticetes are thought to live. Blue and fin whales, for instance, are believed to live to 80–90 (Aguilar 2002; Sears 2002; NAMMCO 2020a, 2022a), humpback and right whales to 70 (Hamilton et al. 1998; Clapham 2002, 2018), and sei whales to 50–70 years (Horwood 2002; NAMMCO 2020b).