Keywords

4.1 Introduction

Cetaceans (whales, dolphins, and porpoises) are derived from even toed ungulates (artiodactyls) and have a rich fossil history beginning about 50 million years ago. Early archaeocetes were quadrupedal, and within about 12 million years, the bodies of these whales became streamlined, pelves detached from the vertebral column, hindlimbs were reduced and encased within the body wall, and propulsion was provided by a novel appendage, the tail fluke (Thewissen and McLellan 2009). These changes in the post-cranial skeleton committed whales to life in the water as they were no longer able to bear their weight on land. This chapter reviews evolutionary changes in the pelvic girdle of archaeocetes, speculates on their potential consequences for muscle evolution and mating behaviors, and presents new ontogenetic data on dimensions of the pelves of modern bowhead whales. Taken together, these data create an evolutionary framework that allows readers to understand the morphological diversity and functional shifts that led to the mating systems of modern cetaceans.

4.2 Robust Pelves and Hindlimbs in Some Archaeocetes

4.2.1 Pakicetids, Ambulocetids, Remingtonocetids, and Protocetids

The earliest recovered fossil archaeocetes were collected in rocks of Pakistan and India that are about 48 million years old. These whales, called pakicetids and ambulocetids (Fig. 4.1b), were amphibious, and their robust hindlimb and tail may have aided in aquatic locomotion (Thewissen et al. 1994, 1996, 2001). The pelvis was fused to the sacrum and displayed a robust obturator foramen (Fig. 4.2) and a well-developed acetabulum for articulation with the femur. The femur and tibia were also thick and dense with mineral, a characteristic that likely allowed pakicetids and ambulocetids to achieve neutral buoyancy in water (Madar 1998; Thewissen et al. 2007). The feet of pakicetids and ambulocetids were broad and probably displayed soft tissue webbing between the digits (Madar 2007) that supported locomotion on soft substrates and increased the surface area of the foot during pelvic and hindlimb undulations during swimming. Mating could have occurred either on land or in water, and it is unclear what mating strategies could have been employed, other than an artiodactyl-like mating approach that included males mounting the females caudally.

Fig. 4.1
6 illustrations exhibit the evolution of skeletal morphology from land-to-sea transition in archaeocetes. The pelvic girdle and hindlimbs are reduced in modern cetaceans.

Skeletal evolution along the land-to-sea transition in archaeocetes (a–d) and morphology of the skeleton in the modern bottlenose dolphin (Tursiops) and bowhead whale (Balaena). Elements of the pelvic girdle, pelvis, and hindlimbs are shown in red. Other skeletal elements are shown in black. The fossil raoellid Indohyus (a, Thewissen et al. 2007) had a pelvic girdle typical of artiodactyls. This morphology was retained in archaeocetes, the earliest fossil whales, including Ambulocetus (b, Thewissen et al. 1996) and the protocetid Maiacetus (c, Gingerich et al. 2009). Basilosaurid archaeocetes showed a dramatic shift as the pelvis no longer had a bony connection with the vertebral column, and the hindlimbs were reduced to tiny vestiges, as seen in Basilosaurus (d, Gingerich et al. 1990). Modern cetaceans retain a reduced pelvis, and sometimes hindlimbs, immersed within the body cavity. The pelvis of the bottlenose dolphin (Tursiops truncatus, e) has a tiny pelvis that floats within the abdominal cavity and lacks hindlimbs (Cozzi et al. 2017). In contrast, some bowhead whales (Balaena mysticetus, f) retain a pelvis and reduced hindlimbs within the body cavity (Thewissen et al. 2009). All images are not to scale and are shown with equal body lengths to illustrate the relative size of the pelvis and hindlimb elements

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Fig. 4.2
5 drawings of the outlines of the pelvic evolution in the anterior view for Indohyus, Pakicetus, Ambulocetus, Basilosaurus, and bowhead whale. The labels in Ambulocetus are as follows. Ilium, acetabulum, obturator foramen, pubic symphysis, pubis, and ischium. The labels in Basilosaurus are as follows. Acetabulum, obturator foramen, and pubis.

Outlines of the pelves illustrating pelvic evolution in archaeocetes and the bowhead whale (Thewissen et al. 2009). Like what is found in terrestrial mammals, the earliest cetaceans (e.g., Pakicetus (composite of H-GSP 30395, 30213), Ambulocetus (H-GSP 18507)) and their relatives, including the raoellid Indohyus (Ranga Rao 256), had robust pelves that had a bony attachment to the vertebral column as seen in most terrestrial mammals. This attachment was lost at least in basilosaurids, as evidenced by the reduced pelvis of Basilosaurus (US National Museum 12,261). Basilosaurid archaeocetes were no longer able to bear their body weight on land and were obligatorily aquatic. In modern bowhead whales (pictured here is the pelvis of an adult male, Balaena mysticetus, NSB-98B5), the acetabulum and obturator foramen are lost, and the ilium is reduced. In modern cetaceans, and probably basilosaurid archaeocetes, the reduced pelvis is a site of muscular attachment for muscles associated with the genitals of both sexes

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Fossils of remingtonocetids were recovered from similar aged rocks from present-day India and Pakistan (Gingerich et al. 1997; Thewissen and Bajpai 2001). These archaeocetes were also amphibious but displayed a greater number of bones within their vertebral column, a more gracile skeleton, and a pelvis that was like that of pakicetids (Thewissen and Bajpai 2001). The limbs, although not as robust as those of pakicetids or ambulocetids, were sufficient to support terrestrial locomotion (Bajpai and Thewissen 2000). However, bones of the manus (hand) and pes (foot) are unknown. The pelvic and hindlimb morphologies suggest remingtonocetids probably mated like pakicetids and ambulocetids.

As in pakicetids, ambulocetids, and remingtonocetids, the pelves and hindlimb of protocetids, from 48–33 million years ago, retain a robust pelvic girdle that shared a bony attachment with the sacrum and functioned to support both terrestrial and aquatic locomotion (Fig. 4.1c). Protocetids differ from earlier families of archaeocetes in that they were cosmopolitan with skeletons documented throughout most of the globe. The protocetid whale, Maiacetus (Fig. 4.1c), is known from two skeletons that are about 12% different in size, leading to the conclusion of sexual dimorphism in total body size, with the males being larger than females (Gingerich et al. 2009). Peregocetus, a quadrupedal protocetid found in Peru from sediments dating to the middle Eocene, showed the sacrum was attached to the pelvis, the hindlimb was functional in both terrestrial and aquatic locomotion, and the digits were capped with small hooves (Lambert et al. 2019). Morphology of the vertebral column suggested aquatic locomotion was supported by a powerful tail that likely functioned in concert with the hindlimbs during aquatic locomotion, similar to modern otters (Geisler 2019).

In contrast, the pelvis of the protocetid Georgiacetus, recovered from late middle Eocene sediments dated to about 40 million years ago in North America, showed a pelvis that may have lacked a bony connection to the vertebral column and robust hindlimbs (Hulbert 1998). The concavity that supports pelvic articulation with the femur, the acetabulum, is well developed, suggesting that hindlimb locomotion was supported. Because Georgiacetus is unlike other protocetids in potentially lacking articulation between the pelvis and sacrum, it is reasonable to imagine that there were multiple lineages of protocetids swimming in the Eocene oceans and that some of these may have undergone an evolutionary shift toward a lack of function of the pelvis (Hulbert 1998).

Protocetids probably utilized caudal mounting as in other early archaeocetes. Curiously, the presence of a small skeleton partially within the body cavity of the adult protocetid Maiacetus has been interpreted as evidence of a head-first birth on land (Gingerich et al. 2009). This hypothesis has since come into question as the proposed fetus could also have been ingested by the whale during feeding or been a displaced fetus (Thewissen et al. 2009).

4.2.2 Basilosaurid Archaeocetes, Pelvic Detachment, and Hindlimb Reduction

Basilosaurids were the latest diverging lineage of archaeocetes from the late Eocene epoch (38–34 million years ago) and showed an altogether different trajectory in hindlimb and pelvic evolution. In these large-bodied whales (Fig. 4.1d), the pelvis (Fig. 4.2) was no longer in contact with the sacrum, and the hindlimbs were encased within the body wall (Uhen 1998). These were the first cetaceans to have lost their ability to walk and mate on land and were therefore obligatorily aquatic. Rather than using their limbs for propulsion, these archaeocetes used a tail fluke, and Basilosaurus may have employed whole body undulations while swimming (Gingerich 2003).

Basilosaurus isis was 16 meters long, but the pelvis (Figs. 4.1d and 4.2) was shorter than the lumbar vertebrae. The left and right sides of the pelvis articulated with another via a pubic symphysis, as in terrestrial mammals. The tiny hindlimbs, which contained representative elements of most of the limb, were thought to be encased within the soft tissues of the body wall; if they did protrude from the body wall, they might have been involved as a potential aid in positioning their elongated bodies (i.e., copulatory guides, Gingerich et al. 1990). The hindlimbs of all Basilosaurus and Dorudon (5 meters long) were too reduced to support body weight on land (Uhen 2004).

All basilosaurid archaeocetes lacked articulation between the pelvis and spine, resulting in necessary changes for copulation and birth. Copulation was no longer feasible on land, and these archaeocetes were uniquely released from the constraints of life on land. By losing bony connections between the pelvis and vertebral column and reducing the size of hindlimbs, the size of the fetus was no longer constrained by having to fit through the aperture of the bony pelvic girdle to be born big, thereby setting the stage for the novel evolution of extreme brain size (Smaers et al. 2021; Waugh and Thewissen 2021) and gigantism (Goldbogen and Madsen 2018) in later diverging cetaceans. Beyond setting the stage for larger fetuses and adults, the loss of the bony constraints on the size of the birth canal allowed for larger calves. In modern cetaceans, larger-bodied calves can survive colder water through heat conservation, thus enabling births in colder marine environments (Galatius 2005; Keener et al. 2018). Mating behaviors in these archaeocetes may have been like modern cetaceans in which mating pairs position themselves in the water with (a) touching ventral surfaces, (b) side by side in which the male arches his penis to fertilize the adjacent female (Slijper 1962), and/or (c) a male’s ventrum contacts the female’s flank, so the pair is positioned cross-wise during very rapid, energetic copulation (Keener et al. 2018; Webber et al. 2023, this book).

4.2.3 Even-Toed Ungulates: Artiodactyls

Cetaceans evolved from terrestrial even-toed ungulates (Mammalia: Artiodactyla) that were hoofed and quadrupedal. Their closest living relative is the large-bodied and amphibious Hippopotamus (Lihoreau et al. 2015), but their ancient artiodactyl relatives, including raoellids (Figs. 4.1a and 4.2) and dichobunids, had a more gracile body plan with fully functional pelves and hindlimbs. The sacrum in all artiodactyls remains fused to the vertebral column. Because evidence from the fossil record is limited to skeletal remains, assignment of sex of these fossils is difficult. However, fossil evidence shows that pelves of two lineages of Eocene artiodactyls with small body sizes compared to archaeocetes, raoellids (Figs. 4.1a and 4.2) (Cooper et al. 2011) and dichobunids (Thewissen and Hussain 1990), may have displayed two sizes, and the inference is that the female pelvis is smaller than that of males (Kaufmann et al. 2013) as in modern Hippopotamus (Shannon et al. 2021). Beyond differences in body size between the sexes, the large pelvis of males may offer a greater anchor for attachment of the muscles associated with penile erection (ischiocavernosus), and retention of the site of attachment may have driven retention of the bony pelvis of cetaceans even while undergoing hindlimb loss.

Unlike most other mammals, morphology of the vaginal wall of some female artiodactyls displays transverse folds that protrude into the lumen of the vagina, which are described as successive funnels (Slijper 1962; Nickel et al. 2004; Orbach et al. 2017b). These structures create undulating relief that may act as an impedance, impose selection on sperm, keep sea water out of the vagina, or enable females to control the depth of penile penetration and prospective paternity (Orbach et al. 2020). Vaginal folds have been found in Hippopotamus (Laws and Clough 1965), the closest modern relatives to cetaceans. Like cetaceans, Hippopotamus also mate in water (Dixson 2021). Other artiodactyls and cetaceans display these transverse folds (Slijper 1962; Kleinenberg et al. 1969; Orbach et al. 2017b; Tarpley et al. 2021), but they are not present in most non-cetacean marine mammals (e.g., seals, sea lions, manatees, and sea otters) (Orbach et al. 2021). Included in the epithelium of the folds are mucous cells that contribute to thick mucus lining the lumen of some of the folds, which lodged spermatozoa in the bowhead whale (Balaena mysticetus) (Tarpley et al. 2021). It could be that these folds originated in terrestrial artiodactyls and were exapted to also impede the passage of water into the vaginal canal (Orbach et al. 2020). Sea water is known to be fatal to the sperm of bottlenose dolphins (Tursiops truncatus) (Schroeder and Keller 1989). Among many other adaptations, it could be that the presence of these folds facilitated the ability of archaeocetes to successfully copulate in a saltwater habitat, thereby partially allowing for a completely aquatic lifestyle to have evolved in the Eocene epoch (Orbach et al. 2023, this book).

Hippopotamus are known to copulate in water, and intromission may last minutes, whereas intromission of the more terrestrial pygmy hippopotamus (Choeropsis liberiensis) lasts only seconds (Dixson 2021). The latter strategy includes a single copulatory thrust, and intromission that lasts for seconds is the more common copulatory strategy among some artiodactyls and modern cetaceans (e.g., boto (Inia), harbor porpoise (Phocoena), dusky dolphin (Lagenorhynchus), and killer whale (Orcinus orca)) (Brennan and Orbach 2020; Dixson 2021).

4.2.4 Modern Cetaceans: Pelves, Hindlimbs, and Genitals

Cetacea includes two suborders, baleen whales (mysticetes) and toothed whales (odontocetes). In all modern cetaceans, the pelvis is reduced and floating within soft tissues of the body cavity (Figs. 4.1e, f, 4.2, and 4.3). Pelves are ossified, except for Kogia, which may display cartilaginous pelves (Benham 1901), but this animal was young and with potentially incomplete ossification. This pelvis bone is typically dash, or comma-shaped, and has lost most morphological similarities with the pelves of the earliest archaeocetes (Figs. 4.1 and 4.2). Instead, this thin bone may only be 1–3% of the body length in embryos and adults (Hosokawa 1951). The pelvis of males is typically larger than that of females, and it retains an anterior surface that acts as an anchor for the muscle that supports penile erection (Dines et al. 2014). Among odontocetes, the pelvic bones can sometimes be palpated by a human researcher beside the genital slits in some beluga whales (Delphinapterus leucas). Male beluga whales display a pelvis that is greater in length compared to females. In females, pelves are connected to muscles of the vagina and may ensure tight closure of the vagina (Kleinenberg et al. 1969).

Fig. 4.3
Set of 13 schematics present Male bowhead whales' pelvic attachments near genital slits. Pelvis-femur and tibia connections differ with age and sexual maturity.

Pelves of male bowhead whales (Balaena mysticetus) and soft tissue attachments (modified from Thewissen et al. 2009, 2021). Pelves are found near the genital slits and are associated with a triangle-shaped femur and more rarely a tibia (b) and metatarsal (not shown). In males, the ischiocavernosus muscle and corpus cavernosum share an insertion on the anterior surface of the pelvis. (a) Schematic of a male bowhead in lateral view. (b) Results of a dissection of a juvenile male (NSB-06B4) showing the pelvis attaches to the femur with a synovial joint and the presence of a tibia. (c) The pelvis and femur of the juvenile pictured in b. (d) The pelvis and femur from NSB-98B5, an ~60-year-old male, for comparison. The pelvis (e) and femur (f) of a yearling female (NSB-1992B17). The pelvis (h) and femur (g) of an ~4-year-old female (NSB-1992B20). The pelvis (i), femur (j), and tibia (k) of a ~ 7-year-old male (NSB-1992B2). The pelvis (l) and femur (m) of an ~13-year-old male (NSB-2017B18). Only (d) is sexually mature

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Within baleen whales, some bowhead whales (Balaena mysticetus) have an exceptional lifespan over 200 years (George et al. 1999, 2021; Wetzel et al. 2017; Vazquez et al. 2022). This study reports an ontogenetic assessment of growth of the pelvic bones of both sexes in bowheads, based on the length of the pelvic bones recovered from deceased whales. In all measured pelves, length increased with age (Fig. 4.4, Table 4.1). As in other cetaceans (Struthers 1881), male bowheads display longer pelves compared to females perhaps as a structural anchor for the mechanical strains associated with contraction of the ischiocavernosus muscles. Results show pelves of some members of both sexes may display left-right asymmetry, but which side is larger varies (Fig. 4.4, Table 4.1). Unlike odontocetes, the hindlimbs of bowhead whales are ossified vestiges encased within the body wall and include a triangular-shaped femur with an occasional tibia (Fig. 4.3) that may or may not be ossified (Thewissen et al. 2021). At least the femur displays left-right asymmetry in bowheads, and this asymmetry does not always match that of the pelvis (Table 4.1). Rarely, a metatarsal is present. Occasionally, in at least juvenile bowheads, anomalous hindlimb buds may appear just caudal to the nipples, far lateral and caudal to the genital slit (Thewissen et al. 2021). Taken together, results show that left-right asymmetry in bowheads probably lacks a specific sidedness in which either the left or the right pelvis is typically larger than the other.

Fig. 4.4
A scatterplot for the pelvis length versus estimated age based on maximum baleen length plots legends for male left, right, female left and right, linear male left and right, linear female left and right. The regression line follows an increasing trend.

Modern bowhead whales (Balaena mysticetus) have pelves that increase in length with age. Pelves of males (diamonds) are generally larger than those of age-matched females (circles). As in other cetaceans, pelves show inconsistent asymmetry between left and right pelves within a single whale (ellipses). Y-axis is the pelvis length in centimeters. X-axis is the age of whales based on maximum baleen length and sex (Lubetkin et al. 2012). Data for males are shown in blue diamonds with the left pelvis shown in dark blue and the right pelvis shown in light blue. Data for females are shown in red circles with the left pelvis shown in dark red and the right shown in pink. Pelves were collected by Robert Suydam and J.G.M. Thewissen. Linear male left: y = 0.452x + 20.22, R2 = 0.6516. Linear male right: y = 0.4776x + 17.222, R2 = 0.785. Linear female left: y = 0.6475x + 12.082; R2 = 0.9423. Linear female right: y = 0.7384x + 9.9463, R2 = 0.8418

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Table 4.1 Length measurements of the pelvis and hindlimb elements (femur, tibia) of whales with ages estimated in years based on the longest baleen length (Lubetkin et al. 2008). Data are illustrated in Fig. 4.3. NSB = North Slope Borough, Division of Wildlife Management
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The penis can be meters in length in some species of whales (Slijper 1962). The penis of cetaceans consists of erectile tissue filled with collagen and elastic fibers, which differs from the spongy tissues in the penis of most mammals (Orbach et al. 2017a). The ischiocavernosus muscles in cetaceans are anchored by the pelvis (Fig. 4.2b) in males and attach near the distal end of the penis. This muscle aids in fluid retention within the paired corpus cavernosa during erection and may allow for the cetacean penis to move side to side as well as up and down (Dines et al. 2014). The pelvis is under selective pressure associated with larger penis size as males that practice polygynandry display greater-sized testes, ischiocavernosus muscles, greater-sized penises, and pelves (Dines et al. 2014). Potentially, because of the mechanical stresses associated with erection and directional movements of the penis, the pelves of males are generally larger than those of females. Males with greater-sized and more dexterous penises than other males are potentially able to overcome female resistance and deposit sperm deeper than others in the vaginal canal. Females of species that have larger male pelves also display larger pelves, potentially due to shared patterns of outgrowth and ossification of the pelves (Dines et al. 2014). Within females, the ischiocavernosus muscles attach to the clitoris and pelvis. In beluga whales, the ischiocavernosus muscles partially attach to the wall of the vagina and the pelvis (Kleinenberg et al. 1969).

Mating behaviors in modern cetaceans vary but are broadly associated with brief copulation (seconds to minutes) and with minimal or no pelvic thrusts (Slijper 1962; Orbach et al. 2014; Brennan and Orbach 2020; Dixson 2021). Mates may position themselves by (a) touching ventral surfaces in which the male can easily eject seminal fluid into the vagina; (b) aligning side by side in which males extend their long, curved penis and quickly eject seminal fluid into the vagina of an adjacent female (Slijper 1962); and (c) assuming a crisscrossed pattern in which the male’s ventrum comes in contact with the female’s flank and rapidly penetrates and ejaculates into her vagina (Keener et al. 2018; Webber et al. 2023, this book). At least some of these behaviors were probably used by basilosaurid archaeocetes, as they were the first cetaceans to copulate exclusively in water, and caudal mounting was impossible without hindlimbs. Short intromissions and fewer pelvic thrusts in basilosaurids may have prevented sea water from entering the vagina.

4.3 Embryonic Evidence of Pelvic Girdle Evolution

In vertebrates, limb buds protrude from the body wall, and as outgrowth proceeds, a greater number of skeletal elements are added until a full limb is formed. In embryonic dolphins, hindlimb buds form initially but are absorbed by the body before birth (Thewissen et al. 2006). Hindlimb buds of dolphins are present for a shorter amount of developmental time compared to those of bowhead whales (Gavazzi et al. 2023). As a result, the pelvic girdle of most dolphins includes just a pelvis, but in baleen whales such as bowheads, the pelvis can usually be associated with one to two additional elements near the middle of the pelvis, and these are presumed to be the femur and tibia (Fig. 4.5) (Eschricht and Reinhardt 1866; Hosokawa 1951; Thewissen and McLellan 2009). In dolphins, the truncated hindlimb buds stop synthesizing SHH, a protein that is essential for outgrowth and patterning of developing limbs, thereby shutting down limb outgrowth earlier compared to terrestrial mammals (Thewissen et al. 2006). In contrast, the hindlimb buds of embryonic bowhead whales probably undergo a greater duration of SHH signaling compared to dolphins and therefore develop an ossified femur and tibia. In all adult cetaceans, these hindlimbs are encased within the body wall and lack a role in locomotion. Functional hindlimbs were lost in archaeocetes about 40 million years ago in basilosaurids, and this could be due to truncated SHH expression (Thewissen et al. 2006).

Fig. 4.5
3 illustrations of modern cetaceans with labels as follows. A, left and right pelvis and hind limb. B, illustration of the pelvis and hind limb is labeled with pubis, ilium, femur, and tibia. C exhibits a cartilaginous pelvis.

Modern cetaceans retain a small pelvis and sometimes parts of a reduced hindlimb immersed within their bodies. Within the bowhead whale (Balaena mysticetus, NSB-2000B3F), fetal specimens (a) show a cartilaginous pelvis that connects to some bones of the hindlimb (b, femur, tibia) but lacks a bony connection with the vertebral column. Within pantropical spotted dolphins (Stenella attenuata, LACM 94285), fetal specimens (c) show a cartilaginous pelvis that lacks an associated hindlimb and lacks articulation with the vertebral column. Images show fetal specimens with most soft tissues removed and connective tissues are stained such that bone is red and cartilage is blue. Scale bars are 1 cm in length

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Left-right asymmetry in the pelvis of some cetaceans is a characteristic of stickleback fish and manatees (Nganvongpanit et al. 2020) with modified PITX-1 expression (Shapiro et al. 2006; Chan et al. 2010). It could be that PITX-1, or a similar gene(s), could be associated with the impressive left-right asymmetry found in the pelves of many cetaceans. Female cetaceans also display left-right asymmetry, suggesting that this is perhaps a consequence of asymmetrical growth, from an outgrowth pattern that is no longer under selection, and the functional consequence of this is unknown. It also could be that left-right asymmetry of the pelvic bone of male cetaceans may be associated with curvature of the penis (Orbach et al. 2020), although this hypothesis has yet to be tested with quantitative evidence linking curvature of the penis of adults with sidedness of the pelvic bones.

4.4 Conclusion

During the first 12 million years of cetacean evolution, archaeocetes underwent an exceptional land-to-sea transition, and the pelvic girdle radically transformed from an organ of locomotion and reproduction to an organ solely supporting muscles associated with genitalia. Small pelves and associated hindlimbs, if any, were relocated within the body wall, and as a consequence, basilosaurid archaeocetes were no longer able to mate on land. Mating via caudal mounting, like in terrestrial artiodactyls, was no longer possible. Based on evidence taken from the modern relatives of archaeocetes, including terrestrial artiodactyls and cetaceans, this study speculates on the copulation behaviors of ancient whales. Males may have had dexterous penises capable of depositing sperm in females that were oriented ventrally or along their flanks. Duration of intromission and pelvic thrusts were probably minimized, partially to protect the vaginal canal and sperm from intrusion of sea water. Transverse folds within the vaginal canal of these archaeocetes may have been exapted to also offer protection from the intrusion of sea water into the vaginal canal.

Consequences of the size reduction and relocation of the pelvic girdle probably released constraints on the fetus. By losing bony connections between the pelvis and vertebral column and reducing the size of hindlimbs, head or body size of the fetus of basilosaurid archaeocetes was no longer constrained by having to fit through the pelvic girdle (aperture). A larger body size could have provided a thermodynamic advantage as larger calves of modern cetaceans are known to fare better in colder water. Moreover, this expansion of the birth canal may have laid the stage for the eventual expansion in brain size in modern odontocetes (e.g., dolphins, beluga whales) and gigantic body sizes, including blue whales, the largest mammals ever.