Pregnancy Pigmentation as a Candidate Human Reproductive-Status Cue

Abstract

The linea nigra is usually described as a benign physiological consequence of pregnancy: a hormonally induced darkening of the abdominal midline. This explanation is almost certainly correct at the proximate level, but it may be incomplete. Hormonal mediation does not exclude adaptive function; in many vertebrates, hormone-dependent color changes serve as reliable cues or signals of reproductive state, dominance, aggression, fertility, pregnancy, or social status. I propose that the human linea nigra deserves consideration as a candidate reproductive-status cue, and possibly as an evolved or exapted signal. The trait is common during pregnancy, temporally linked to gestation and the postpartum period, located on the body region most directly altered by pregnancy, often continuous with the pubic region, and associated with other pregnancy-related pigmentary changes. Comparative primate evidence shows that pregnancy coloration can affect male attention, appeasement behavior, mating effort, and female aggression in macaques and baboons. The human linea nigra may have carried a close-range message of current or recent pregnancy, perhaps reducing courtship or sexual harassment, reducing female-female aggression, recruiting protection or support, or advertising reproductive history. This hypothesis does not require that the trait originated as a signal. It may have begun as a hormonally induced pigmentation cue along a pre-existing anatomical midline and later become socially meaningful after human body-hair reduction made abdominal skin more visible. I outline behavioral, comparative, cellular, genetic, and developmental tests that could distinguish a neutral byproduct account from cue, exaptation, and evolved-signal accounts.

Keywords: linea nigra, pregnancy, pigmentation, melanin, reproductive signaling, human evolution, pregnancy coloration, social cue, primates, melanogenesis

Introduction

The linea nigra is one of the most familiar visible changes of pregnancy. A dark vertical line appears along the abdominal midline, usually running from the pubic region toward the umbilicus, and sometimes continuing upward toward the sternum. Clinically, it is treated as harmless physiological hyperpigmentation, a normal product of pregnancy hormones acting on melanocytes. That description is valuable, but it answers only one kind of question. It tells us how the trait appears. It does not tell us whether the trait ever carried information that mattered to other humans.

In evolutionary biology, many traits that appear to be ordinary physiological changes acquire social meaning because they are reliable indices of internal state. Hormonal coloration in animals is often interesting for precisely this reason. A hormone-regulated color patch may reveal reproductive status, rank, aggression, health, sexual receptivity, or pregnancy. The fact that a trait is endocrine-dependent does not make it nonadaptive. It may make it more honest.

The question considered here is therefore simple but neglected: if the linea nigra is not merely a byproduct, what message might it carry?

I propose that the linea nigra may be a hormonally honest, melanin-based cue of current or recent pregnancy. Its possible function was probably not to attract suitors in any simple sense, although it could advertise proven fecundity. More plausibly, it may have helped regulate the behavior of others toward a pregnant or recently postpartum woman. The relevant receivers could have included potential sexual pursuers, current mates, female competitors, kin, alloparents, and possibly infants. The message may have been something like: pregnancy has occurred; sexual, aggressive, protective, or supportive behavior should be recalibrated.

This should be framed as a hypothesis, not as an established conclusion. The hypothesis is attractive because the trait has several features that make it evolutionarily suspicious: it is common, visible, temporally linked to pregnancy, located on the reproductive body, continuous with the pubic region, and part of a broader pregnancy pigmentation complex. It is also attractive because comparative primate work has already shown that pregnancy-related color changes can alter social behavior. The hypothesis is limited because the linea nigra is often hidden by clothing, is not perfectly specific to pregnancy, and may be fully explainable as generalized pregnancy hyperpigmentation acting along the linea alba. These limitations make the idea testable.

The Biological Phenomenon

The linea nigra is a physiological form of hyperpigmentation commonly associated with pregnancy. DermNet describes it as a dark vertical line running down the middle of the abdomen and notes that it can be among the earlier visible indicators of pregnancy. It is associated with increased pigmentation in other body regions during pregnancy, including the nipples, areolae, and genitals. DermNet further reports that pregnancy-related pigmentation affects about 90% of pregnant women and is particularly common or prominent in people with skin of color, especially Fitzpatrick skin types IV through VI. (DermNet®)

The line is usually described as a darkening of the linea alba, the pre-existing midline connective tissue seam of the abdominal wall. Cleveland Clinic describes the linea nigra as beginning near the pubic bone and extending toward or through the umbilicus, sometimes continuing toward the chest. It usually becomes visible around 20 weeks of pregnancy, may darken as pregnancy progresses, and generally fades gradually after birth over weeks or months, although it may not disappear completely in every case. (Cleveland Clinic)

The proximate mechanism is generally attributed to pregnancy-related changes in melanogenesis. DermNet attributes linea nigra and related pregnancy hyperpigmentation to placental hormones, metabolic factors, and immunological factors, and it specifically notes roles for increased melanin production, estrogen, progesterone, and placental bioactive sphingolipids that may upregulate tyrosinase and related melanogenic proteins. (DermNet®) Experimental work on human melanocytes has shown that sex steroids can regulate melanin synthesis through nonclassical membrane-bound receptors. Natale and colleagues reported that physiological estradiol increased melanin production in primary human melanocytes, and that sex steroids can access the classical pigmentation pathway through receptors such as GPER and PAQR7. (eLife)

Recent genetic work strengthens the case for treating the linea nigra as a biological phenotype rather than a random dermatological curiosity. A 2025 study, “An Exploration of the Genetic Architecture of Pregnancy-Related Linea Nigra and Its Relationship With Pigmentation Phenotypes,” investigated heritability and genetic relationships between pregnancy-related linea nigra and other pigmentation traits. The authors reported that common genetic variation explained a substantial portion of variance in the tendency to develop pregnancy-related linea nigra, and that polygenic scores for pigmentation traits contributed to the phenotype. (PubMed)

None of this proves adaptive function. However, it establishes that the linea nigra is common, hormonally organized, genetically modulated, temporally linked to pregnancy, and visually patterned. Those are exactly the sorts of properties that make a trait worth considering from the standpoint of animal communication.

Cue, Signal, and Exaptation

The central theoretical distinction is between a cue and a signal. A cue is a feature that contains information, whether or not it evolved for communication. A signal is a trait shaped by selection because it changes the behavior of a receiver in a way that affects the fitness of the signaler. In animal communication theory, signals often evolve from pre-existing cues through a process in which receivers first exploit reliable information and selection subsequently modifies the trait, the receiver response, or both. (ScienceDirect)

The linea nigra could fit several models. Under a pure byproduct model, pregnancy hormones increase melanogenesis, the abdominal midline darkens incidentally, and no evolved receiver psychology responds to it. Under an informational cue model, the line began as a byproduct but reliably indicated pregnancy or recent pregnancy, allowing others to use it as probabilistic information. Under an exaptation model, the line originated as a physiological byproduct and later became socially meaningful after changes in human ecology, body visibility, or social structure. Under an adapted signal model, the trait itself was shaped or maintained because its visibility altered the behavior of males, females, kin, or helpers in ways that benefited pregnant or postpartum women and their offspring.

The exaptation model may be especially plausible. The linea nigra need not have originated as a communicative display. It could have begun as a consequence of pregnancy hormones acting on a developmental midline. Once humans became relatively hairless, visually attentive, socially interdependent primates, a previously incidental mark could have become informative. Selection could then act on receiver psychology alone, or it could act on both receivers and the pigmentation response.

Human body-hair reduction is relevant because a pigment mark under dense fur is unlikely to function as a visual signal. Jablonski’s work on human skin pigmentation emphasizes that the loss of most body hair in early Homo exposed skin to ultraviolet radiation and was associated with the evolution of heavily melanized skin. (PMC) If a pregnancy-related midline pigmentation response existed before substantial body-hair loss, it may have been mostly hidden. If it arose or became elaborated after hair reduction, it would have been available as a visible cue on the anterior body.

Why the Location Is Interesting

The linea nigra is not a random patch of hyperpigmentation. It appears along the abdominal midline, the region most visibly transformed by pregnancy. It often runs from the pubic area toward the umbilicus, which places it in a zone that could be scrutinized during sexual, intimate, grooming, medical, or caregiving contexts. It is anatomically tied to the linea alba, but its visual effect is of a dark reproductive midline connecting the pubic region and the pregnant abdomen.

This location suggests several possible receiver contexts. A potential sexual partner might see the line during close-range sexual inspection. A current mate might see it during intimacy and pregnancy care. Female relatives or group members might see it during bathing, grooming, birth preparation, infant care, or communal living. An infant might eventually be exposed to the line while lying on the maternal abdomen, although the more obvious infant-relevant pigmentation system is the areola-nipple complex.

The association with areolar, nipple, and genital darkening is also important. Pregnancy pigmentation does not affect the abdominal midline alone. It appears as part of a larger reproductive pigmentation complex. Areolar changes have often been interpreted as relevant to breastfeeding, and there is strong evidence that Montgomery gland secretions elicit newborn responses. Doucet and colleagues found that native areolar gland secretions intensified newborn inspiratory activity and appetitive oral responses more than comparison stimuli, suggesting that areolar glands help orient newborn behavior through olfactory cues. (PMC) The visual-target hypothesis for darker areolae is plausible, although the strongest experimental evidence appears to be chemosensory rather than visual.

The linea nigra may be the adult-social counterpart of this broader pregnancy pigmentation system. The areola may help coordinate mother-infant interaction. Genital and areolar darkening may reflect reproductive tissue sensitivity to pregnancy hormones. The abdominal line may provide a close-range visual index of gestational or recent postpartum state to adult observers.

Comparative Background: Pregnancy Coloration in Primates

The strongest reason to take the hypothesis seriously is that pregnancy coloration has already been studied as a communicative phenomenon in non-human primates. The human linea nigra is not identical to macaque facial coloration, baboon pregnancy signs, or marmoset genital color changes. However, these cases establish the broader principle that visible pregnancy-related color changes can alter the behavior of conspecifics.

In Japanese macaques, Rigaill and colleagues studied multimodal advertisement of pregnancy in free-ranging females. They reported pregnancy-related changes in sexual behavior, facial color and luminance, and possibly vocal cues. The authors suggested that males may use these cues to reduce wasted mating effort directed toward females who are already pregnant. (PMC) This is directly relevant to the hypothesis that pregnancy cues can reduce unnecessary or costly male pursuit.

In rhesus macaques, Gerald and colleagues tested whether pregnancy coloration affects attention and behavior. They presented adult macaques with color-manipulated images of female faces in which pregnancy coloration was present or absent. Males attended more to pregnancy-colored female faces, and both sexes showed more appeasement behavior toward stimuli with pregnancy coloration. The authors suggested that pregnancy coloration may act as an attention-grabbing stimulus for males and a warning stimulus for both sexes. (ScienceDirect)

In olive baboons, Bailey and colleagues tested whether pregnancy coloration reduces female conspecific aggression. They found that free-ranging pregnant females experienced significantly less aggression from non-kin females after the onset of the pregnancy sign compared with the period before the sign. (PMC) This finding is especially relevant because it connects pregnancy coloration not merely with male mating behavior but with female-female social dynamics.

In marmosets, Moreira and colleagues reported chromatic and achromatic changes in genital and thigh skin around parturition and suggested that visual cues may indicate the timing of birth to male and female conspecifics. (PMC) This example is particularly interesting because marmosets are cooperative breeders. A visual cue that helps group members anticipate birth would have obvious potential relevance to paternal and alloparental behavior.

Together, these studies provide a comparative scaffold for the human hypothesis. Visible pregnancy coloration can be informative. Receivers can attend to it. It can affect mating effort, aggression, appeasement, and preparation for offspring. The human linea nigra should be evaluated in this broader biological context rather than treated only as a dermatological oddity.

What Message Might the Linea Nigra Carry?

If the linea nigra carried a message, it was probably not a single message to a single receiver. A visible pregnancy cue could have different meanings depending on who observed it.

To potential sexual pursuers, the message may have been that immediate mating effort was less likely to produce conception. Pregnancy and early lactation reduce immediate fertility, and a visible cue of current or recent pregnancy could reduce wasted courtship, coercive pursuit, or harassment. This is one of the more plausible adaptive functions because it has direct analogues in the Japanese macaque literature. If males can detect pregnancy cues, they may redirect mating effort toward females with higher immediate conception probability. (PMC)

To a current mate or likely father, the message may have been that investment was now valuable. Human pregnancy and infancy are energetically costly, and human offspring are unusually dependent. A close-range cue of pregnancy or recent birth could increase protection, tolerance, provisioning, and sexual restraint. It could also help maintain attention to the woman’s reproductive state during the long months when pregnancy is developing and in the postpartum period when the mother-infant dyad remains vulnerable.

To kin and alloparents, the message may have been that support would increase offspring survival. Humans are cooperative breeders compared with most mammals, and maternal success depends heavily on social support. A visible cue of pregnancy or recent birth could elicit food sharing, workload reduction, protection, childcare assistance, or reduced social demands. If the line persists after birth, it could function as a probabilistic marker of recent reproductive burden, not simply current pregnancy.

To female competitors, the message may have been to reduce aggression or alter competitive behavior. Female-female aggression and reproductive competition are often underappreciated in human evolutionary speculation, but the baboon evidence makes this possibility especially important. If pregnancy coloration can reduce non-kin female aggression in baboons, then a human pregnancy cue could plausibly have affected female social treatment, even if subtly. (PMC)

To long-term mates or potential future mates, the message could also have included proven fecundity. Recent pregnancy indicates that conception has occurred. In some contexts, this might increase perceived reproductive value or mate value. However, this is an ambiguous message because recent pregnancy also implies current investment obligations, possible existing pair bonds, lactational infertility, and dependent offspring. For that reason, the strongest version of the hypothesis is not that the linea nigra evolved primarily as an attractiveness display. It is more likely a reproductive-status cue that regulates behavior.

The Meaning of Visibility Across Skin Tones

The linea nigra is often more prominent in darker complexions, which is significant. The hypothesis does not depend on a pale-skin background. The trait is a contrast phenomenon: the line is darker than surrounding abdominal skin. DermNet reports that pregnancy pigmentation is particularly common or prominent in darker skin types, and Cleveland Clinic similarly states that the linea nigra is usually more pronounced in people with darker complexions. (DermNet®)

This is important for ancestral plausibility. The trait would not have required light skin to be visible. If early Homo had darkly pigmented skin after body-hair reduction, a localized increase in melanin could still have been detectable as contrast along the abdomen. A human pregnancy signal need not be optimized for modern lighting, clothing, or racial categories. It would only need to alter probabilities under ancestral viewing conditions, especially at close range.

Modern human racial categories are recent and socially constructed relative to deeper human evolution. The more biologically meaningful point is that pregnancy-related midline hyperpigmentation appears across human populations and is tied to shared endocrine and pigmentation biology. That broad distribution is consistent with an ancestral mechanism, although it does not by itself establish adaptation.

Cellular and Genetic Predictions

The strongest future test of this hypothesis may not begin with observer psychology. It may begin with tissue.

The central cellular question is whether the abdominal midline is biologically special during pregnancy. Does skin over the linea alba have a distinctive pregnancy-sensitive melanogenic program, or does the line appear simply because generalized pregnancy hyperpigmentation happens to be more visible along an existing anatomical seam?

A serious research program would compare midline linea skin with adjacent lateral abdominal skin in the same individuals. Samples would ideally include pregnant, postpartum, nulliparous, and non-pregnant parous women, with careful attention to gestational stage, lactation, parity, ancestry, skin tone, sun exposure, age, and endocrine profile. Ethical constraints would require minimally invasive sampling, opportunistic sampling during clinically indicated procedures, and noninvasive imaging wherever possible.

The relevant assays would include histology, melanin chemistry, melanocyte density, melanosome transfer, single-cell RNA sequencing, spatial transcriptomics, ATAC-seq, DNA methylation profiling, proteomics, and hormone-receptor mapping. The research would ask whether melanocytes, keratinocytes, fibroblasts, endothelial cells, immune cells, or extracellular matrix structures differ along the midline. It would also ask whether pregnancy changes these differences.

Candidate pathways include melanogenic regulators such as MITF, TYR, TYRP1, DCT, MC1R, KITLG/KIT, EDN1/EDNRB, ASIP, OCA2, SLC24A5, SLC45A2, and HERC2/OCA2, along with estrogen and progesterone-sensitive pathways such as GPER and PAQR7. These candidates are not proposed as a fixed mechanism, but as obvious starting points for testing how pregnancy hormones might interact with local skin biology. The eLife work showing sex-steroid regulation of melanocyte pigmentation through nonclassical receptors provides one mechanistic foundation for this approach. (eLife)

Skin organoids and skin-on-chip systems would be especially valuable. Researchers could derive melanocytes, keratinocytes, and fibroblasts from midline and lateral abdominal skin and expose them to pregnancy-level estradiol, progesterone, placental lipids, melanocyte-stimulating signals, UV exposure, inflammatory mediators, and mechanical stretch. If midline-derived skin darkens more strongly than lateral skin under pregnancy-like conditions, that would support a local specialization model. If no midline-specific response appears, a neutral byproduct account becomes more plausible.

Mechanical stretch should also be tested. Pregnancy stretches the abdominal wall, and the skin over the linea alba experiences changing tension as the uterus expands. A combined mechanism is possible: pregnancy hormones increase melanogenic potential throughout the body, while mechanical stress, connective tissue remodeling, or local fibroblast-keratinocyte signaling makes the midline especially responsive. Such a mechanism would explain why a pigmentation cue appears exactly where pregnancy is physically transforming the body.

Genetic studies should move beyond presence or absence and quantify the phenotype continuously. Variables should include line contrast, width, length, onset week, peak intensity, postpartum persistence, recurrence across pregnancies, relation to areolar and genital pigmentation, and relation to melasma. The 2025 genetic study provides an initial indication that pregnancy-related linea nigra has a complex genetic architecture, but much larger and more diverse samples will be needed. (PubMed)

If the trait has design-like features, one might expect consistent developmental timing, localized pregnancy sensitivity, heritable variation in intensity or persistence, and perhaps genetic overlap with reproductive pigmentation traits. If it is only a byproduct, the genetic architecture may reduce to general pigmentation propensity without any special midline biology.

Comparative Tests in Other Apes

It remains unclear whether other great apes have a hidden analogue of the human linea nigra. I am not aware of a well-established literature documenting a human-like visible pregnancy line in chimpanzees, bonobos, gorillas, or orangutans. The absence of reports is not decisive. A midline pigmentation response under fur could easily be missed, especially if researchers are not looking for it.

The comparative test is straightforward in principle. During routine veterinary care, researchers could use noninvasive imaging to examine the abdominal midline of pregnant and non-pregnant great apes. Fur could be parted, and in cases where shaving is already medically indicated, hyperspectral imaging could be performed. The same individuals could be followed longitudinally across reproductive states. If tissue is ever available from medical procedures or necropsy, midline and lateral abdominal skin could be compared cellularly.

Several outcomes would be informative. If other apes have a hidden pregnancy-related midline pigmentation response, then the human linea nigra may represent an ancient ape trait that became visible after body-hair reduction. If other apes lack such a response, the human trait may have been elaborated after the human lineage became more hairless and visually social. If other primates show pregnancy coloration mostly in sexual skin, while humans show conspicuous pigmentation in the areolae, genitals, and abdominal midline, then humans may have shifted some reproductive-status information to anterior body surfaces.

This question connects to broader comparative reproductive biology. Bonobo pregnancy endocrinology, for example, shares several features with other apes and humans, including chorionic gonadotropin during pregnancy, but detailed visible pigmentation studies are not the focus of most ape pregnancy research. (perinataldiscovery.ucsd.edu) A systematic comparative atlas of primate pregnancy pigmentation would be valuable in its own right.

Behavioral Tests

The behavioral question is whether humans actually respond to the linea nigra. The simplest experiment would use standardized images of female abdomens in which the only manipulated feature is line presence, darkness, width, or length. Belly size, age, body composition, lighting, skin tone, pubic-hair visibility, clothing boundary, and posture would need to be tightly controlled. The images should include ambiguous cases, because a cue may be most useful when pregnancy is not otherwise obvious.

Observers could be asked to judge pregnancy, recent pregnancy, lactation likelihood, sexual availability, attractiveness in short-term and long-term contexts, vulnerability, need for help, social protectedness, and likely existing pair bond. More importantly, experiments should measure behavior rather than only explicit ratings. Participants could allocate money, food, time, help, protection, punishment, or courtship messages to targets with or without the line. Virtual reality could simulate social proximity while manipulating the mark.

Eye tracking would be valuable. The hypothesis predicts that observers may attend to the lower abdominal midline even without consciously identifying the trait. Speeded categorization tasks could test whether the line affects rapid inference. Masked or brief presentations could test whether the mark shifts judgments before explicit verbal reasoning. Memory tests could ask whether observers better remember targets with linea nigra under pregnancy-relevant contexts.

Different receiver classes should be tested separately. Men may show reduced short-term sexual pursuit toward targets with the line, especially when pregnancy is ambiguous. Women may show altered aggression, competition, or support responses, particularly in social contexts involving reproductive competition. Parents and individuals with pregnancy experience may show stronger explicit recognition. Individuals without explicit knowledge of linea nigra may still show implicit shifts if the cue taps a deeper receiver bias.

The strongest behavioral finding would not be that people can identify the line when asked. The stronger finding would be that the line changes social behavior even when people do not know why.

Cross-Cultural and Developmental Tests

A hypothesis about evolved receiver psychology must be tested across populations. Modern clothing, media exposure, medical knowledge, and cultural norms could strongly affect responses to pregnancy pigmentation. A cross-cultural design should include societies with different degrees of abdominal visibility, different postpartum practices, different breastfeeding norms, different alloparental structures, and different levels of explicit knowledge about linea nigra.

If the response is entirely learned, effects should depend heavily on prior exposure and explicit cultural knowledge. If the response reflects a more general receiver bias, people may infer pregnancy or alter behavior even without knowing the term or having been taught the association. The likely answer may be mixed: a biologically prepared sensitivity could be sharpened by cultural learning.

Developmental studies could ask when children or adolescents begin to associate abdominal pigmentation with pregnancy. If the cue is learned only through observation, sensitivity should increase with exposure to pregnant bodies and reproductive knowledge. If there is an evolved perceptual bias, some sensitivity may appear earlier or more uniformly, although ethical constraints would require careful design.

Cross-cultural work should also attend to skin tone and contrast. A line that is visually obvious in one pigmentation context may be subtle in another. Hyperspectral and psychophysical methods could estimate how detectable the line would be under natural light, firelight, shade, sweat, ochre, body hair, and different viewing distances. The ancestral signal value of a trait depends on actual perceptual ecology.

Alternative Explanations and Falsification

The pure byproduct account remains viable. Pregnancy hormones produce widespread pigmentation changes, and the linea nigra may simply be a visible consequence of melanogenesis along the linea alba. It may have no adaptive function and no special receiver response. High prevalence alone does not prove adaptation. Many common traits are side effects of developmental or endocrine systems.

A protective pigmentation account is also possible, although less obvious. Melanin protects against ultraviolet radiation, and pregnancy changes may alter skin vulnerability. However, the thin vertical geometry and midline location of the linea nigra seem poorly suited to broad UV protection. The trait’s signal-like plausibility comes more from timing, location, and contrast than from a protective surface function.

An infant-directed account is possible but secondary. Darkened areolae and Montgomery gland secretions have a clearer link to newborn orientation and breastfeeding. The abdominal line could conceivably help orient the infant on the maternal torso, but there is no strong evidence for this, and the spatial relationship between the line and nipple is indirect. If the linea nigra has a social function, adult receivers are probably the primary candidates.

The social-signal hypothesis would be weakened if the line fails to predict pregnancy or recent postpartum status after controlling for pigmentation background and other cues. It would be weakened if observers do not notice or respond to it in controlled experiments. It would be weakened if midline skin shows no special pregnancy sensitivity compared with adjacent skin. It would be weakened if comparative primate data show no homologous or analogous pregnancy pigmentation pattern. It would be weakened if all observed responses are explained by explicit cultural knowledge.

The hypothesis would be strengthened if the line is a reliable probabilistic index of pregnancy or recent birth, if observers respond to it implicitly, if it shifts courtship, aggression, helping, or protection, if the response occurs across cultures, if midline skin has specialized pregnancy-sensitive melanogenic biology, and if comparative work reveals similar pregnancy pigmentation functions in other primates.

A Proposed Model

The model can be stated simply. Pregnancy produces endocrine, placental, immune, metabolic, and mechanical changes. These changes increase melanogenesis in specific tissues, including the abdominal midline, areolae, nipples, and genitals. In the case of the linea nigra, pigmentation appears along a pre-existing anatomical seam running through the reproductive abdomen. The line becomes a visible cue of pregnancy or recent pregnancy. Receivers may use that cue to infer reproductive status. Those inferences may alter sexual pursuit, aggression, appeasement, paternal investment, kin support, or alloparental care. If those altered behaviors affected maternal or offspring fitness, selection could favor receiver sensitivity, trait elaboration, or both.

This model permits several levels of interpretation. At the lowest level, the linea nigra is a hormonal byproduct. At the next level, it is an honest cue. At a stronger level, it is an exapted signal. At the strongest level, it is an evolved signal shaped by selection for communication. The value of the hypothesis is that these levels can be separated empirically.

Conclusion

The linea nigra is usually treated as a harmless dermatological consequence of pregnancy hormones. That account is probably correct, but it may not be complete. Hormonal causation and adaptive function are not mutually exclusive. In many animals, hormone-dependent coloration is informative because it is tied to internal physiological state. Pregnancy coloration in macaques, baboons, and marmosets provides a comparative foundation for asking whether human pregnancy pigmentation might also have social meaning.

The most cautious hypothesis is that the linea nigra is a hormonally honest, close-range cue of current or recent pregnancy. It may have originated as a byproduct of pregnancy endocrinology acting on a pre-existing abdominal midline. After human body-hair reduction, bipedality, and increased visual exposure of the anterior body, this cue may have become available to other humans. Its message may have been less about ornamentation than about reproductive status: pregnancy has occurred, and behavior should be adjusted.

The possible adjustments are biologically plausible. Potential sexual pursuers might reduce courtship or harassment. Current mates might increase protection and investment. Kin and alloparents might increase support. Female competitors might reduce aggression. Future mates might infer proven fecundity. These possible functions are not mutually exclusive. A single cue can carry different meanings for different receivers.

The strongest version of the hypothesis is not that the linea nigra was designed from the start as a deliberate advertisement. It is that a common pregnancy pigmentation pattern may have become socially meaningful in a visually oriented, cooperative, sexually complex primate. The task now is to test whether the line is merely visible, or whether it is seen.

If the linea nigra is an evolved signal, its message may be: I am pregnant, or I have been pregnant recently. Treat me accordingly.