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University of Helsinki, Department of Ecological and Environmental Sciences, Lahti, Finland
1 Correspondence: University of Helsinki, Department of Ecological and Environmental Sciences, Niemenkatu 73, FIN-15140 Lahti, Finland. E-mail: aki.sinkkonen{at}helsinki.fi
ABSTRACT
Typically, mammalian umbilical cord forms a tiny, stable, and asymmetrical scar. In contrast, humans have a clearly visible umbilicus that changes with age and nutrients gathered. Based on this, I propose that umbilicus, together with the surrounding skin area, is an honest signal of individual vigour. More precisely, I suggest that the symmetry, shape, and position of umbilicus can be used to estimate the reproductive potential of fertile females, including risks of certain genetically and maternally inherited fetal anomalies. The idea is supported by a comparative study where symmetrical t-shaped and oval-shaped umbilici of fertile females were considered the most attractive. Further support comes from observations that abnormal velocity of umbilical cord has been associated with fetal brain development, diabetes, and other fitness-related properties with a strong genetically or maternally inherited component. In addition, umbilicus and the umbilical skin area may reveal nutrimental competitive ability, and need for social care in small children and pregnant females. The novel hypothesis explains why umbilicus has aesthetic value, and why umbilicus has had a distinctive role in different cultures. If further research confirms the signalling hypothesis, female umbilici may be routinely measured to detect risk pregnancies of several fetal abnormalities.
Key Words: fetal development • honest signal • nutrition • pregnancy • symmetry
THE FETAL END of the umbilical cord of most mammals forms a tiny asymmetrical scar that remains unchanged throughout life. In those species, umbilicus has no visual role in sexual selection or in other social contacts between individuals. Our ancestors became bipedal millions of years ago (1)
. Thereafter, the previously hidden umbilicus has been visible to conspecifics. Nowadays, the shape and form of umbilicus and umbilical skin area are dependent on several factors (2
, 3)
. Umbilical skin area changes as a child grows, as a woman is pregnant, and as the amount of nutrient reserves changes (4)
. Despite its great variability and exceptional visibility, umbilicus is commonly considered as a textbook example of a remnant organ. The reason is that the shape and form of umbilicus has been argued to be a consequence of the cutting of umbilical cord, which obviously cannot be genetically or maternally inherited.
In this context, it is surprising how the remnant scar tissue has inspired tremendous attention throughout human history. Prehistoric female figurines found in Austria (Venus of Willendorf, circa 30,000–25,000 BCE) and Russia (Kostenky settlement, 23,000–21,000 BCE) have a distinctive umbilicus. Greeks and Romans drew special attention to the shape and position of umbilicus in art, and renaissance and baroque painters mimicked natural variation of female umbilici astonishingly (Fig. 1
). Even virginal Adam and Eve are usually drawn with perfect umbilici (5)
. As in Europe, ancient sculptors gave special attention to symmetric natural or symbolic umbilici in other parts of the world (Fig. 1)
. In modern art, the absence of umbilicus is used consciously to draw attention to artistic anomalies (6)
and tiny paintbrush movements can give an impression of a navel that reveals nutrient reserves and individual’s vigor (7
, 8)
. The importance of umbilicus in social context is supported by the facts that several medicinal companies perform umbilical surgeries to increase the aesthetic value of their customers’ abdomens (9)
, and that umbilicus has been characterized as a sexual symbol in psychology (10)
. For these reasons, I suppose that a symmetric umbilicus signals high fitness.
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In this essay, I present a novel hypothesis that umbilicus is either an honest signal or a cue that reveals individual’s vigor (11)
. More specifically, I propose that the shape and form of umbilicus and the umbilical skin area relate to the reproductive value of humans of fertile age. I reconsider the results of a study where mature humans preferred certain types of female umbilicus over others. I refer to medicinal literature that has linked the fetal end of umbilical cord to physiological properties that are genetically or maternally inherited and linked to fitness estimates. In addition, I speculate whether umbilici of pregnant women and small children are similar because they both have needed social care by conspecifics. Finally, I propose ways to utilize the novel hypothesis in gynecology and pediatrics.
UMBILICUS MAY SIGNAL FEMALE FITNESS POTENTIAL
I hypothesize that umbilicus—together with the surrounding skin area—is an honest signal of individual vigor. Adults consider symmetric female umbilici more attractive than asymmetric umbilici. In a recent study (4)
, the aesthetic value of vertically oriented or T-shaped umbilici with superior hooding were scored the highest, whereas the presence of any degree of distorted shape or protrusion and a horizontal orientation lowered the score. Further, those with a large umbilicus were scored below those with a smaller configuration (4)
. Such clear tendencies in aesthetic value are usually associated with evolutionarily important traits, not with remnant and meaningless tissues, and they cannot be explained only by avoidance of fluctuating asymmetry. In other words, males do not prefer certain visual traits in females over alternative ones unless the phenotypes differ in reproductive potential (11)
. Therefore, I propose that vertically oriented or T-shaped umbilici can be associated with an above-average reproductive potential.
For most of the history of our species, umbilical cord has been left to erase naturally, and delayed cord clamping is considered beneficial (12
, 13)
. Even if umbilical cord is detached immediately after birth or after a short time period, the size of the upcoming scar obviously depends on the diameter and other properties of the umbilical cord. These, in turn, have been observed to be abnormal in certain diseases that have a strong genetically inherited component (such as diabetes) (14
15
16
17)
. Further, recent medicinal literature links umbilical cord and especially umbilical artery to blood venous hemodynamics that are crucial in fetal brain development, fetal anemia, and restricted intrauterine growth (18
19
20
21)
. Intriguingly, umbilical vein constriction has been associated with reduced umbilical cord cross-sectional area and Wharton’s jelly in female fetuses, while those parameters were not affected in male fetuses (22)
. This indicates that there may be gender-specific differences in the visual appearance of the upcoming scar, i.e., umbilicus. Intriguingly, maternal effects are crucial fecundity determinants in human females (23)
. It is thus possible that female umbilicus honestly signals not only genetic but also maternal effects, and that females and males therefore evaluate umbilicus differently in sexual selection. Links between fetal brain development and the velocity of umbilical cord may have been especially crucial in the evolution of human umbilicus because most visual cues in our body are correlated with physical properties, not with intelligence.
UMBILICUS MAY SIGNAL NEED FOR SOCIAL CARE
At all ages, the form of umbilicus depends on nourishment (4)
. Vast nutrient reserves are seen by a wide umbilicus, while malnourished child’s umbilicus has an outward position on top of a round stomach. It is thus possible that humans utilize umbilicus as a cue of nourishment, which often is related to reproductive potential. It is noteworthy that the shape of the umbilical area of pregnant women is reminiscent to small children’s umbilical area. During our evolutionary history, malnourished children and pregnant women have usually needed continuous care. I hypothesize that the reason for the similarity described above is that umbilicus—together with umbilical skin area—is an honest within-species signal that aids conspecifics to estimate individuals’ vigor and need for social care.
QUESTIONS THAT SHOULD BE ADDRESSED
Based on the studies reviewed above, there are several reasons why humans may unconsciously utilize umbilicus and umbilical skin area while estimating the vigor of conspecifics. Although a number of studies have already dealt with fetal hemodynamics, there are no surveys on the relationship between fetal hemodynamics and the size, form, and symmetry of the upcoming umbilical scar, i.e., umbilicus. Similarly, it is not known whether maternal umbilical characteristics, such as fluctuating asymmetry, are connected to risks of certain syndromes during fetal development or early childhood. If any relationships, causalities, or even a slight degree of heritability is found, it may become possible to identify high-risk pregnancies and possibly to increase the likelihood of favorable outcomes of such pregnancies.
CONCLUSIONS
Although the idea of umbilicus as a fitness cue or signal is novel in gynecology, pediatrics, and evolutionary ecology, the same idea has been intuitively commercialized for decades in plastic surgeries. Since humans naturally estimate the fitness and social status of conspecifics, I surmise that umbilicus and the surrounding skin area have been used as a fitness cue soon after the onset of bipedality. This may have given a boost to the evolution of umbilicus from a fading scar to a clearly visible symmetric organ. In this essay, I have gathered evidence in support of the umbilical-signaling hypothesis. My main arguments are that certain types of female umbilici are considered as the most attractive, that the velocity and other properties of umbilical cord affect both fetal development and the upcoming umbilicus, and that no physiological or evolutionary facts lend support to the traditional view that umbilicus is an evolutionary meaningless organ. My main conclusion is that investigations testing the umbilical signaling hypothesis should include the search for causalities and relationships between this easily measurable organ and severe physiological syndromes.
FOOTNOTES
The opinions expressed in editorials, essays, letters to the editor, and other articles comprising the Up Front section are those of the authors and do not necessarily reflect the opinions of FASEB or its constituent societies. The FASEB Journal welcomes all points of view and many voices. We look forward to hearing these in the form of op-ed pieces and/or letters from its readers addressed to journals{at}faseb.org.
REFERENCES
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