Are Hair Colour and Behaviour Linked?

When Francis Galton in 1883 wrote of the "prevalence of dark hair colour among men of atrabilious and sour temperament," referring to Cromwell’s Ironsides, his readers at the time may well have thought it a rather obvious statement (1). Today it would probably be regarded as an unfair slur on dark-haired people and, given the wide availability of hair dyes and tints, a somewhat impractical way of discovering a person’s character. However, in some animals it seems there may be a definite association between hair colour and behaviour.

Some fifty years ago Clyde Edgar Keeler and Helen Dean King found that black Norway rats, obtained by selective breeding from wild ancestors were remarkably tame compared with those with wild-type hair colour (2). Wild rats have hairs which are mainly black but have a narrow band of yellow near the tip. This type of hair, known as agouti, is also found in several other wild animals including mice, rabbits, guinea pigs, and grey squirrels. For them it presumably provides effective camouflage, the result of many generations of natural selection.

The gene for agouti hair is dominant and in animals which lack it the yellow band of phaeomelanin in the hair is replaced by eumelanin making their coats a uniform black. Such rats only rarely occur in nature and are timid, cringing creatures compared with agouti rats. Docile behaviour has also been found in rabbits and foxes with non-agouti coats. In contrast a mutation in the Deer-mouse known as orange-tan in which the amount of phaeomelanin is increased appears to make them more aggressive. It seems likely that there is an association between an animal’s behaviour and the amount of phaeomelanin in its fur.

There is also some evidence of an association between eumelanin and reproductive behaviour. Non-agouti Icelandic sheep produce more lambs out of season than agouti sheep. A mutation which increases the amount of eumelanin in the feathers of pigeons is associated with a longer breeding season. Pale Arctic skuas are more aggressive than the dark form but the dark males complete their courtship earlier which may be an advantage in some of their habitats.

Genetically there are two possible explanations for the apparent association between hair colour and behaviour. They could be controlled by distinct genes which are very close together on the chromosome but however close they are they should occasionally separate giving rise to individuals which are agouti and timid. This does not appear to happen.

The alternative is the phenomenon known as pleiotropism where a gene has more than one, apparently independent effect on the organism. If this is what is happening in the association of hair colour and behaviour it is possible that the primary effect of the gene is on the animals’ hormones which in some, at present unknown, way affect both their melanin synthesis and their reactions to external stimuli.

Perhaps fortunately, none of this appears to apply to humans possibly because although there can be slight gradations of colour along our hairs we do not have definite pigment banding like agouti hair. Which could explain why although Cromwell’s Ironsides may have had darker hair it does not seem to have made them any less aggressive than their enemies.

John Timson

References

1.Galton, Francis, Inquiries into Human Faculty, 1883.

2.Keeler, C.E & King, HD Journ. Comp. Psychol. vol 34, p. 241, 1942.