By Alex Kirby
BBC News Online environment correspondent
Some animals can feel and think in ways not too dissimilar from us, welfare campaigners say.
They say there is evidence of altruism, with some animals acting disinterestedly for the good of others.
Animals which live in communities, they say, often exhibit signs of morality which resembles human behaviour.
They say there is scientific backing for their claims, with huge implications for human use of animals.
The campaigners are from Compassion in World Farming (CIWF), a UK group which accepts that farm animals will be killed for their meat but argues they should be treated humanely.
CIWF is holding a conference in London on 10 May entitled Understanding Animals. Its theme is animal awareness, emotions and intentions.
The concept that animals are sentient - possessing a level of conscious awareness, and able to have feelings - was recognised by the European Union in 1997.
In a briefing paper, CIWF says: "There is evidence that some animals do have some level of morality and some concern over other animals.
Pigs are very sensitive to fear
"Living within a group requires a moral code of behaviour... Most animals that live in communities exhibit similar moral codes to humans.
"Zoologists who have spent their professional lives studying animal behaviour, either by observation or by experiments to test their mental capacities, believe that many animals feel and think."
Joyce D'Silva, chief executive of CIWF, told BBC News Online: "The whole climate over whether to accept sentience has changed hugely in the last 15 years.
"It has huge implications for all the ways we use animals. It implies all farm animals are entitled to humane lives and deaths - and millions are denied them."
Dr Jackie Turner, research director of the CIWF Trust, told BBC News Online: "There's far more rationality and mental complexity in farm animals than we acknowledge.
"But our attitudes to them are tremendously culturally determined - look at the different ways we feel about dogs and pigs."
Sheep can remember 50 other sheep faces for several years
The claim of scientific backing for the concept of animal sentience has its critics, who say it is simple anthropomorphism, the projection of human traits onto animals.
A spokesman for the Countryside Alliance told BBC News Online: "There seems to be a trend towards anthropomorphism throughout society.
"It's leading people to suggest animals can feel sensation and emotion in the same way as humans, and this is obviously nonsense."
But Dr James Kirkwood, chief executive and scientific director of the Universities' Federation for Animal Welfare (Ufaw), gives qualified approval to CIWF's approach.
He told BBC News Online: "Animal sentience has been a matter of debate down the centuries.
"We can't prove absolutely even that another human being is sentient, though it would obviously be unreasonable to assume they are not.
"But the weight of scientific opinion is that it's certainly right to give the benefit of the doubt to all vertebrates."
Pig and sheep images courtesy of CIWF