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Genetics

Bays and black horses photogrpahed on the central plateau, North Island. ~ during the early 1970s. Source: Morrow (1975) New Zealand Wild Horses. Millwood Press.

Bays and black horses photogrpahed on the central plateau, North Island. ~ during the early 1970s. Source: Morrow (1975) New Zealand Wild Horses. Millwood Press.

The coat colour variation described by Harvey Morrow [1] in feral horses of central North Island during and around the 1920s is not evident in today’s Kaimanawa horses. The colour illustrations of Kaimanawa horses in his early 1970s book also look to have todays more limited palate.

But Morrow’s photographs of the remnant but now extinct Roto Aira population around National Park just 35 kilometers from where the Kaimanawa population roams today shows a Tabiano pony behind two chestnuts and a bay (see photograph below). Have alleles for pattern and hue been lost or were they never present in the Kaimanawa herd?

Any of the few more colourful horses might have been taken from the population, being more attractive to the musterer’s eye. Harvey Morrow indicates as much – horses with unique coats were favoured for capture.

The steel-grey mare was still on her own when we made our usual survey from Wild Horse Lookout next morning. […] Her capture was only a matter of minutes. Chapter 7 – The Steel-grey Mare.

 Not long after the return of the grey mare to the run a new mare appeared. […] She was roan in colour and in due course produced a lovely roan colt. […] Her colt developed into a very attractive animal. … we did covet the colt. Chapter 8 – The Roan Colt.

The large musters from the Kaimanawa population during the 1950s and 60s for pet food too might have culled colour from the population. It is more likely, however, that alleles for hue and pattern were never bred into Kaimanawa horses.

Four feral horses around National Park ~early 1970s from Morrow (1975) New Zealand Wildlife Horses. Millwood Press.

Four feral horses around National Park ~early 1970s from Morrow (1975) New Zealand Wildlife Horses. Millwood Press.

The domestic origins of the Kaimanawa population are not as diverse as those that no longer exist in other parts of New Zealand. The Kaimanawa population are most recently descended from the release of cattle and sheep station (farm) and cavalry (military) horses that were less likely to be bred for brighter, patterned colours. Genetic studies confirm that Kaimanawa horses are most similar to thoroughbreds and thoroughbred crosses that made up most of the military and hill-country farm horses of the early 20th century [2].

Some surmise the genetic influence also of the Carlyon and Comet ponies in the Kaimanwa population today – ponies bred during the late-19th century for the demanding conditions of colonial New Zealand. And, the Carlyon and Comet were not colourful breeds either, being descended from imported Exmoor and Welsh stock renowned for their plain ‘browness’.

Such ‘brown’ horses are certainly not a genetic type but chestnuts or, less often, bays whose coat colour is much darker than is typical but it is not possible, without genetic testing, to know which. Indeed, indecipherable brown coat colours amongst the Kaimanawa horses, where they made up almost 10% of our studied population, might be attributed to this early English and Welsh genetic stock.

The Kaimanawa horse, is seems, is classically plain – its genome being largely dominated by black and chestnut alleles and bereft of alleles for hue and pattern.

Colours amongst the brown

And so it seems Harvey Morrow’s colourful wild horses and their descendants didn’t make it to the Kaimanawa Mountains or, if a few did, they didn’t elude capture or muster. Instead, the stoic, pragmatic choice for browness by first-colonists and then by cavalry and hard-country farmers, colours the Kaimanawa horse of today.

Nevertheless, over the four years that would follow seeing my first Kaimanawa horse, we would adapt descriptions of coat colour as part of our horse identikit and make the most of what variation there was. We developed detailed, sometimes unorthodox, descriptions of a horse’s pelage, especially amongst new foals to the study and unbranded horses. Sometimes descriptions of variations in the darkness of ‘brown’ across a horse’s body proved valuable. We recorded lighter, darker, chocolate and black browns. Some had lighter brown muzzles, breasts and barrels.

Looking closer now and for longer at my first band on that cool, clear August day in 1994, I began to differentiate amongst Kaimanawa colours. The mare was lighter than her foal – what I might describe now as a yellow chestnut with flaxen (bleached) mane and tail. Another mare was bay and the stallion was so dark as to probably be black. More brown than most they may be, but I would learn to see individuals and discover the colourful lives of Kaimanawa horses.

My initial impression, proved correct: the Kaimanwa feral horses are not nearly as colourful as their North American cousins. I did not see a blue or piebald coat amongst them, ever. Liver, roan and dun coats occurred but are not common. Most horses’ coats are best described as the classic bay, chestnut, or black. I once saw a couple of horses with grey pelage along the southern border of their range around Hautapu Stream, where long-haired fetlocks (feathered feet) were also seen – both evidence perhaps of more recent additions of domestic stock – but not in our study area centred in the Argo Basin or further north.

pelage for piechart [Total branded and not branded]In the present day population therefore, dominant white alleles are absent and grey alleles very rare. Alleles for spotting and tabiano patterns are probably absent, and roan alleles uncommon (see my post about coat colour genetics). Small white marks were present on just four adult horses, mostly across the loin or barrel. Perhaps this was evidence that alleles for patterning existed in the population. But clearly, if these alleles did occur, they were not common or influential enough to generate tabiano or sabino coats. However, the different shades of chestnut, bay and black, and especially the few liver and dun colours, I saw in our study population are evidence that subordinate alleles for hue are present to a small degree.

Instead, 90% of Kaimanawa horses could be described as bay (56%), chestnut-brown (26%) or black (7.7%) and so coat colours are dominated by just two alleles – red and black. What happened? Where are the grey, roan, blue, and piebald and skewbald from New Zealand’s wild horse past?

In my next post, I explore why coat colours were lost.

I wanted to know about coat colour genetics and inheritance in the horse because Kaimanawa Horses are so plainly brown much more so than historical accounts of wild horse colours that included grey, roan, blue, piebald and skewbald. As it turns out compared to other animals, horse coat colour is a fairly simple genetic problem.

Although selective breeding can produce incredibly diverse coat colours in horses, white, grey, black, red (chestnut) and bay horses are the basic colour types [1]. Which basic type a horse is-is governed by just four genes, each with their own site, called a locus, in the horse’s genome. There is one gene and locus each for white and grey horses, and two others determine the chestnut, bay and black hues.

Four genes can produce five different basic colours because each gene for coat colour has two different forms, called alleles. The allele at each loci can be either dominant or recessive. A recessive allele’s influence on coat colour is weakened or prevented entirely if there is a dominant allele at one of the other three loci. And so different combinations of dominant and recessive alleles at the four loci determine the coat colour of a horse.

coat colour graphic III

Crudely put, dominant alleles rule such that horses are black or chestnut if they are not white or grey. Dominant alleles at loci for white or grey make white and grey horses. A horse is black because it has a dominant allele at the loci for blackness but does not have a dominant red allele at its loci for redness. Chestnut horses do not have a dominant black allele, and so the red colour is uninhibited. Bay horses are an interesting combination of the two. They have a dominant allele at both the loci for redness and blackness, causing much of the black hue to be restricted to their extremities – mane, tail and feet.

After those four loci for the basic colour types, four other subordinate loci on the genome govern the different hues of black, bay and chestnut, such as palameno, buckskin, and dun. And another four loci govern coat colour patterns, like roan, leopard spotting, and tabiano (pinto, pie- or skewbald). But alleles at these eight loci for hue and pattern have no influence if the alleles for black and red are not also present. Different shades and patterns, therefore, are variations of fundamentally black, bay or chestnut horses.

Knowing how horse coat colour is derived and inherited, we can turn back to the question why are Kaimanawa Horses to plain brown? Perhaps they are not afterall? – my next post.

Bibliography

  1. Thiruvenkadan AK, Kandasamy N, Panneerselvam S. 2008. Coat colour inheritance in horses. Livestock Science 117: 109-129.
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