Washington: Sheep are more clever than you thought -- they are able to memorise faces of up to 50 fellow sheep and can recognise them two years later from their smell, a new study has found.
An international team, led by the University of Western Australia, says despite having a cultural reputation for being unintelligent, sheep have excellent memories.
According to researchers, sheep use this capacity for long-term face recognition to distinguish between flocks and learn which sheep in their own flock are friendly and which are aggressive -- this helps sheep position themselves within their flock's dominance hierarchy.
Team leader Professor Graeme Martin says more so than humans the olfactory memory of sheep -- particularly in ewes -- is extremely developed and is the primary mechanism used by ewes to recognise their young.
Ewes identify their young through an  odour which is produced within the placenta and contained in the amniotic fluid in which the newborn is drenched.
The odour of the amniotic fluid is highly attractive to ewes only for the first two hours after birth and it is within this timeframe that ewes memorise their offspring’s scent and are thus able to identify which lambs they allow to drink milk from their udders, say the researchers.

The lambs themselves remember their mothers because the colostrum they drink causes their stomach to fill with milk and stretch -- this bodily sensation intensified by the warmth of the udder is stored in the lamb's memory.
Colostrum is yellow and thicker than normal ewe milk is produced 48 hours immediately after the birth of a lamb. A new-born lamb must receive colostrum within 18 hours of birth, otherwise it has only a 50:50 chance of survival.
The colostrum along with the chemicals released from the ewe's teat become associated with the smell of their mother and the lamb remembers that particular ewe as the one from which food is provided, according to the study.
Professor Martin believes that farmers will benefit from understanding and applying this newly acquired knowledge because it can be used to improve farming practices inorder to increase on-farm lamb survival rates.