Visitors to St Kilda will immediately notice three striking features of the sheep.
First, Soay sheep are tiny. In August, mature females average around 24kg in weight while mature males are around 38kg, making them about one third the size of most modern domestic sheep.
Second, they are highly variable in appearance. While many Soays have the ‘classic’ Soay coat colour, which we call ‘dark wild’ (with ‘wild’ being short for ‘wildtype’ the coat pattern that features a light belly and rump patch), we recognise three other varieties (‘dark self’, ‘light wild(type)’ and ‘light self’ (where ‘self’ refers to ‘self-coloured’ – a coat pattern with the same colour all over including belly and rump patch). Our studies have shown that the dark/light colouration is due to a gene called tyrosinase-related protein 1, or TYRP1 which is on sheep chromosome 2 and genetically dark is dominant to light, while the wild/self colouration is due to the locus Agouti on sheep chromosome 13 and wild is dominant to self.
In addition, a small proportion of Soays have variable amounts of white colouration.
We also define three different horn types: normal horns (carried by 85% of males and 35% of females), scurred horns, which are small and mis-shapen (15% of males; 37% of females), and polled – having no horns at all (28% of females). The horn variation is mostly controlled by a so far unidentified gene on sheep chromosome 10 and the different frequencies of the types in the two sexes arises as a result of the genetic factor for normal horns being more dominant in males than females.
All this variation is also present with the source population on the island of Soay, St Kilda.
The third feature visitors will notice is that virtually all the sheep in Village Bay (but not the rest of the island) are tagged. Identification of individual sheep is key to the research we conduct on St Kilda. About one day after birth, we catch and tag most lambs and reunite them with their mothers; any we miss are caught in later expeditions. This individual identification allows us to follow each animal throughout its life. It is the variation in individual life histories, and the causes of this variation, that generate the population dynamics of the population.
What’s in a tag?
Each Soay is tagged with a single colour and number showing in both ears, back and front. All the lambs born in the same year have the same colour – for example the 2011 lambs have green tags. We use six main tag colours, and so we also prefix the tag colour to indicate each set of six years. The 2011 lambs are the so-called beta greens or BGs. This distinguishes them from the 2005 cohort which are the alpha greens or AGs. Occasionally, we catch a sheep as an adult and cannot estimate its age; in these cases it gets pink tags and is known as an Old Pink or OP.
Some famous sheep
Old Green 23 – early champion
OG023 was born in 1984, survived the population crashes of 1986 and 1989 and lived until 1992. For much of his life he was the largest ram living in Village Bay and consequently enjoyed high mating success, with well over 40 offspring to his name. In consequence, many of the sheep we study today are his descendents. Locally, we called him Clark, after Clark Kent, a.k.a. Superman!
New Green 007 – a ram with no horns
Just occasionally, rams are born with such extremely scurred horns that they have no observable horns at all. NG007, also known as Bond, was such an animal. He lived from 1992 to 1997 and despite his handicap, managed to father at least five lambs.
Old Green 042 – superewe
OG042, here pictured with her twins, YY115 and YY116, was born in 1984 and died in 1995. She had her first lamb, a singleton, in 1985, and then a lamb in every year, up to 1994, including twins in '87,'88,'90,'91 and '93 yielding a total of 15 lambs in all.
Photo Credits: Arpat Ozgul