When Labrador retrievers were first recognized by the national Kennel Clubs in England in 1903 and the USA in 1917, black was the predominant coat colour. The first appearance of chocolate Labradors can be traced back to a litter born in 1892 and sired by Buccleuch Avon: the first yellow Labrador on record is Ben of Hyde, born in 1899 [1, 2]. While there has been a rapid rise in the popularity of yellow Labradors, fans of Labrador retrievers may have noticed that chocolate Labradors are less common than black or yellow Labradors. Anecdotally, chocolate-coloured Labradors have a reputation for being less trainable and more hyperactive and aggressive than their black or yellow peers. To date there is little scientific evidence to support this belief.
It is well known that people can have preconceived ideas about the personalities of dogs based on their appearance. For example, earlier work has shown that dogs perceived as being cute are more likely to be perceived as amicable . Based on appearance alone, dogs with yellow coat colour are assessed as being more agreeable, conscientious and emotionally stable than dogs that are otherwise identical other than having a black coat . Similarly, dogs with floppy ears are considered to be more agreeable and emotionally stable than dogs with pointy ears .
Coat colour is determined by melanocytes producing either phaeomelanin resulting in a yellow or red coat; or eumelanin resulting in a brown or black coat. The three recognized coat colours in Labrador retrievers are black, chocolate and yellow. Observable within-breed variation among coat colours is determined by two genes: MC1R (melanocortin 1 receptor) and TYRP1 (tyrosinase related protein 1). Black is the dominant colour at the TYRP1 locus while brown coat colour is recessive. Yellow Labradors are homozygous for a recessive MC1R mutation (R306ter), a nucleotide alteration that causes a premature stop codon at amino acid 306 of MC1R, resulting in blocking of eumelanin production allowing only the phaeomelanin reds and yellows to show [5,6,7]. This is also known as the E locus. Labradors with the recessive bb genotype at the B locus of TYRP1 can be chocolate or yellow, while those with genotypes BB or Bb can be black or yellow. Several mutations at the brown locus may cause the brown phenotype .
Yellow has recently overtaken black as the most popular colour in registered Labradors in the UK (Kennel Club registration data). The popularity of chocolate Labradors has consistently been far lower than black or yellow, but does vary: 7% in 1988, 22% in 2008 and 9% in 2018 (Kennel Club registration data). Unfortunately similar data for Australian or American Labradors was not available, preventing regional comparisons. A UK study found that chocolate Labradors (21%) weighed, on average, 1.4 kg more than black (49%) and yellow Labradors (27%) . Recent work suggested that, of 2074 Labradors with health records, chocolate Labradors were more likely to have otitis externa and pyo-traumatic dermatitis than either black or yellow Labradors. Data on longevity for 173 Labradors revealed that the median lifespan for chocolate Labradors was significantly less than for non-chocolate Labradors (10.7 years compared to 12.1 years) . Whether the increased propensity for skin or ear infections is related to longevity is yet to be determined. It is also currently unclear whether the same differences are characteristic of Labradors in other countries.
The mechanism by which coat colour may affect behaviour is yet to be definitively determined, but various hypotheses abound. Melanocortins such as adrenococorticotropic hormone (ACTH) and melanocyte stimulating hormone (MSH) bind to the melanocortin 1 receptors in the skin responsible for coat colour, but also bind to other melanocortin receptors . MC2R mediates the effect of ACTH on steroid secretion in the adrenal gland. MC3R and MC4R are expressed in the brain, especially the hypothalamus. MC5R has been linked to aggressive behaviour in mice . Consequently, melanocortins may be involved in many behavioural and physiological functions. Lines of mice carrying TYRP1 mutations have also exhibited behavioural abnormalities. For instance, Tyrp1b-1FGHLc/Tyrp1b-1FCHLc strains of mice exhibit brown pigmented eumelanin, decreased eye pigment, decreased body size, and are described as nervous. (http://www.informatics.jax.org/allele/genoview/MGI:3719250?counter=1).
The goal of this study was to test claims regarding chocolate Labrador retrievers and temperament differences relative to other coat colours. To do this, we used dogs that have results available for both array-based genotyping data and behaviour characteristics assessed by their owners using a standardized questionnaire. By using the allelic haplotypes at the two relevant genes rather than simply the observed phenotypic coat colour, we were able to observe the association of allele dosage at each locus, and the relative impacts of the different genetic backgrounds on behaviour.