Based on the orientation of a fossil fetus in an adult specimen of Maiacetus, Gingerich and coworkers (2009) consider this scenario unlikely and think that Maiacetus came on land to give birth.
In 2000 and 2004, amongst a sea of dirt and currents of dust, a phenomenal window through time was discovered. Philip Gingerich and his associates had unearthed three fossilized remains of what are believed to be amphibious, primitive whales that roamed approximately 47.5 million years ago. Of the three specimens excavated, one fossil was enclosed within another, which Gingerich et al. (2009) suggested is a fetus. Importantly, the finds provide information on ancient whales regarding sexual dimorphism (before, it was difficult to ascertain the gender of fossil whales), location of birth (on land or in water), and precociality of neonates (how independent a newborn is).
The new species was named Maiacetus inuus, which means “mother whale,” and was placed in the family Protocetidae. Assignment to a new species was justified due to critical differences from other protocetid whales, such as solidly co-ossified left and right dentaries (lower jaws), variations in the ankle, and significant disparity in hind limb elements. The fossils show that this new species’ length is unimpressive relative to some extant (living) whales, but still, Maiacetus inuus measures a respectable 2.6 meters.
Of the fossil remains, one appears to be a female with child, as evidenced by a smaller fossil preserved within the larger specimen’s rib cage. The mother is characterized by relatively short hind limbs with long metapodials and digits that the authors interpret as being webbed in life. Gingerich et al. contend that this morphology correlates with foot-powered swimming, a characteristic trait of some semiaquatic mammals. In addition, Gingerich and his team argue that the vertebral/pelvic structure describes a whale that swam, lured by the ocean’s delicacies, but was still partial to dry breaks on land. The adult companion fossil also shares most of these traits, but the larger of the two adults is believed to be a male. This fossil is truly the Cadillac of fossil finds, as the bones are mostly intact and articulated - arranged touching one another in their natural state. From the nearly complete and articulated fossil, subtle differences, such as larger canine teeth and a V-shaped pubic notch in the pelvis can be appreciated.
These differences in physical traits may mark sexual dimorphism in Maiacetus inuus. Gingerich et al. found that the presumed male fossil is ~12% larger (linearly) than the female. This linear difference translates into a weight difference of ~39%. Following suit, the male canine tooth is ~20% larger than the suspected female. These variations in size and build may indicate the need for males to undergo courting competition to achieve sexual congress. For instance, in marine species, a threshold of ~16% in length difference between the sexes usually assures a territorial or harem mating system, wherein a group of females is aggressively guarded by a single male for copulation purposes. With only a 12% disparity in Maiacetus inuus, Gingerich et al. believe that only moderate male-male competition existed. One can infer then that food and shelter were dispersed in the Maiacetus inuus habitat. The dissimilarities in size and teeth in this species allowed the authors to hypothesize that sexual dimorphism existed early in cetacean evolution.
The fossil imprisoned in the rib cage was interpreted as a fetus, as there were no signs of damage to its skull; damage to the cranial aspect is highly likely for any ingested meal, as protocetids carry a battery of shearing molars that slice and crush their food. Most of the fetal skeleton - the skull, some cervical and thoracic vertebrae, ribs, and portions of the fore and hind limbs - were preserved. The skull, detailed by conspicuous teeth along the upper and lower jaws, and the rest of the skeleton are porous relative to the adult bones. The fetal orientation within the mother whale depicts a probable “head first” birth (as opposed to tail first births of many extant marine mammals today) that gives new insight into reproductive behaviors.
The fetal fossil was unearthed with the skull facing toward the tail end of the presumed mother and suggests a head first birth. Partially crowned molars imply that the fetus was close to term. A fetus close to term precludes the possibility of further fetal rearrangement within the womb prepartum (before birth), that is, a head to tail shift, where the head and tail ends of the fetus switch places in the womb. However, a spin along the longitudinal axis may occur. A cranial first entrance into the world resembles the near universal birthing posture in large-bodied land mammals. Thus, Gingerich et al. assert that although Maiacetus inuus appreciated the aqueous environment, it favored a terrestrial asylum for the birth of its young.
While the authors argue for a dry birth on land, they also infer a somewhat precocial newborn. The evidence is the upper molars that are partially crowned by enamel. According to Gingerich, the developed state of a baby’s teeth rates the newborn’s capacity to supplement its mother’s milk with other sources of nutrition. The calcification state of the fetal molars was compared to and was found similar to those of a fallow deer (a species with precocial neonates). Gingerich et al. argue that this is evidence enough to place Maiacetus inuus as a precocial mammal. This character is also consistent with a close evolutionary relationship between whales and Hippopotamidae (hippopotamuses), as baby hippos also lie near the precocial end of the birth spectrum.
It is presumed that a head first birth yields some advantages against a tail first birth for those born on land. Some of these include the elimination of choke danger through the umbilical cord and undesirable intake of mucus, blood, and amniotic material (Slijper, 1979). In addition, a head first strategy allows the nostrils to come in contact with air fairly early. In contrast, some believe that in aquatic mammals a tail first birth is advantageous as it prevents drowning and allows the newborn to swim with the mother and not away from her (Gingerich et al., 2009). If a fetus exhibits a headfirst orientation, Gingerich et al. suggest that birth was likely on land. However, this induction may lift a questioning or skeptic eyebrow or two. Despite the extraordinary preservation of this amazing fossil, it is difficult to determine the birthing strategy of an entire species with a sample size of one. For example, in extant whales, the newborn’s tail is often the first to cautiously emerge from its mother. However, there have been many instances where the head finds its way out before the tail (Slijper, 1979).
One must also be cautious when suggesting an association between two variables such as body orientation at birth and place of birth, as there may be other predictors that were not considered. For example, aside from headfirst birth being related to birth on land, fetal orientation also has been connected with litter size. Uniparous mammals (mammals that generally carry one fetus at a time, as in humans) commonly produce large offspring and present the cranial aspect during delivery. In contrast, multiparous mammals (those that usually give birth to multiple young at once, like a cat), whose offspring are smaller, exhibit young that are birthed 50% headfirst and the rest tailfirst (Slijper, 1979).
Accordingly, one may be bold enough to argue that a head first birth doesn’t necessarily indicate a land birth and that other factors are relevant. It appears that the final fetal position is also influenced by the shape and flexibility of the uterus along with the shape and size of the occupant. In many terrestrial, quadruped mammals (like a cow), the body of the uterus extends below the level of the cervix, which separates the uterus and vagina, towards the abdomen while occupied by a fetus. In addition, in many terrestrial mammals, the fetus exhibits a small head, a long neck, and a large body. Naturally, the heavier portion of the fetus (the tail end) will elect to perpetually stay near the abdominal side of the accommodations. Thus, the lighter and more flexible head region is forced to remain near the cervix (Slijper, 1979). Although the exact specifications of Maiacetus inuus’ uterine anatomy are unknown, the same principles would apply if the shapes and mechanism were similar. In extant whales, the uterine anatomy is comparable to ungulates, suggesting a head first birth. However, this assumption would be premature, as the shape of the fetus also dictates fetal position. In today’s whales, the center of mass lies close to the head and it is this region, including the thorax, that is the most bulky. Hence, it is the tail that habitually rests closer to the cervix (Slijper, 1979). In Maiacetus inuus, the parturition orientation, whether head or tail first, may have had little to do with being born on land or in water but instead may have been simply the result of fetal anatomy and uterine shape.
Gingerich, P. D., ul-Haq, M., von Koenigswald, W., Sanders, W. J., Smith, B. H., Zalmout, l. S. 2009. New protocetid whale from the middle Eocene of Pakistan: birth on land, precocial development, and sexual dimorphism. PLoS ONE. 4.
Slijper, E. J. 1979. Whales. Cornell University Press, New York.
By Joseph Chang (University of California Riverside undergraduate researcher in Gatesy lab)