• Primates are the most colorful group of mammals, according to our partner The Conversation.

  • A study shows that there is a relationship between the skin color of Japanese macaques and their social status.

  • The analysis of this phenomenon was carried out by Cécile Garcia, CNRS Research Officer at the National Museum of Natural History (MNHN) and Lucie Rigaill, Assistant Professor at Kyoto University (Japan).

The color red has always held a special place in human societies.

Associated with power and wealth during Antiquity, it then takes on multiple symbols depending on the period, evoking in turn the blood of Christ, love, beauty, but also violence or immorality ...

Nowadays, in the collective unconscious, the color red is often associated with sexuality and "sexitude".

Different studies have indeed shown that red increases sexual attractiveness.

It appears that men perceive women dressed or adorned in red as sexually receptive and attractive, with perceived sexual receptivity potentially responsible for the red-attraction link.

But what about other species, especially non-human primates?

Japanese macaques enjoying the hot springs of Jigokudani © Yosemite / Wikipedia CC BY-SA 3.0

Primates are the most colorful group of mammals, and males and females of many species develop a red coloration in different parts of the body such as the face, chest, anogenital area or hind legs.

Red as a sexual attractor

It seems that a red coloration of the skin also plays a role in sexual attractiveness in these species.

So far, most studies investigating the link between color and attractiveness have focused on the color of males.

These studies show that females are more attracted to the more colorful males, especially in the rhesus macaque.

Skin coloration in males has also been associated with other individual characteristics such as social status within the group, with the more dominant males being the most colorful. The development and maintenance of these colored ornaments can be explained by the theory of sexual selection developed by Charles Darwin as early as 1859 and which predicts that the presence of certain characters (sometimes extravagant or even disabling, with the classic example of the peacock's tail ) or their stronger expression in only one of the two sexes is explained by the advantage that such traits provide in access to sexual partners.

Thus, sexual selection could operate at two levels: between sexes, males and females choose the partner with the most attractive attributes (intersex selection);

and within the same sex, with competition between individuals of the same sex in order to access the partner (intrasexual selection).

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Grooming between Japanese macaques in Jigokudani monkey park © Frank Schulenburg / Wikimedia CC BY-SA 4.0

The coloration of male primates may have evolved through these two selection mechanisms, intra and intersex.

But what about female primates?

The expression of these ornaments could have been maintained during the evolution because they would confer a reproductive advantage to the "carrier" females who would be more attractive to the males.

Most studies of colored traits in female primates (e.g. mandrills, baboons) have focused on the color of the swelling of the genitals and sought to understand whether this readily visible signal could reliably indicate whether the females were fruitful.

An indicator of the time of ovulation

In our study in the Japanese macaque, we analyzed the role of another colored trait, the color of the face and hindquarters, in order to understand whether variations in the color of these ornaments could reliably indicate the time of ovulation and / or individual characteristics, such as social rank or parity (number of pregnancies).

In this species, which does not present sexual swelling in adults (a point in common with the human species - fortunately for us!), Both sexes display a red coloration of the face and hindquarters with a peak coloring during the breeding season.

A female Japanese macaque with a magnificent colorful face © Lucie Rigaill

By combining behavioral observations, photographic analyzes and hormonal data, our study in 12 female Japanese macaques shows that this coloration linked to changes in the concentration of sex hormones does not provide a reliable indication of the exact timing of ovulation: females are not redder or darker on the face and hindquarters when ovulating. Because of this, male Japanese macaques cannot know for sure when females are potentially fertile using this trait or other traits such as female behavior or vocalizations which are also unreliable indicators. of the time of ovulation, as we have shown in a previous study.

This is different from what happens in other primate species, notably the rhesus macaque, in which the color of the face (but not the hindquarters) seems to provide a reliable indication of the right time to copulate. Female Japanese macaques therefore seem to “hide” or not clearly display their ovulation, at least through the coloring of their face or their hindquarters. Thus, ovulating females are not monopolized by a male and can more freely choose their sexual partner, or even copulate with several different partners. If the skin color does not seem to be an indicator of when to ovulate in this species, what are the clues that males might use to know if they have a chance of reproducing and what is the function of this clue? visual?

An indication of social status

To answer the first question, although the color of the skin does not accurately indicate the time of ovulation, the hindquarters of females become lighter after ovulation than before ovulation. This visual trait therefore seems to be a crude indicator of the window in which it might be worth investing time and energy in copulating with a female who could potentially ovulate. These results are consistent with those obtained in a previous study showing that the face of females becomes darker during the first month of gestation vs the month before conception,suggesting that females could signal gestation via this color change that males could use so as not to waste energy copulating "unnecessarily" (with no chance of this resulting in conception) with pregnant females. This is what they seem to do, since the males of the wild population (Koshima) that we followed do not copulate with females already pregnant.

Japanese macaques taking a nap in the sun © Lucie Rigaill

Regarding the second question - what is the function of the red coloring of the face and hindquarters?

- our study shows that there is a relationship between skin color and social status: females of high social status have darker hindquarters which also tend to be redder.

This is the first time that this relationship has been established in female primates, because so far it had only been demonstrated in males. This color-social rank relationship could be particularly interesting for males who have just joined a new group or for males peripheral to the group (extra-group copulations are not uncommon in non-human primates, and in particular in Japanese macaques. ). Indeed, unlike resident males who benefit from knowledge concerning the social rank or reproductive history (the number of births per female) of the females in their group and therefore do not necessarily need an additional visual signal to confirm this knowledge,"naive" males might use the coloring of the female's skin to decide which females to mate with: the dominant females who are most likely to conceive.

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As a result of this research, some questions still remain unanswered: do males prefer redder or darker females in this species?

Rather, is it the combination of several clues (behaviors, smells, copulation calls, color of the face and hindquarters) that allows male Japanese macaques to decide when to copulate with a particular female?

The sexual communication of this species has not yet revealed all its secrets ...

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This analysis was written by Cécile Garcia, CNRS Research Officer at the National Museum of Natural History (MNHN) and Lucie Rigaill, Assistant Professor at Kyoto University (Japan).


The original article was published on The Conversation website.

Declaration of interests

Cécile Garcia has received funding from the CNRS.

Lucie Rigaill does not work, does not advise, does not own shares, does not receive funds from any organization that could benefit from this article, and has not declared any affiliation other than her research organization.

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