Some Labradors and flat-coated retrievers can't help being fat, scientists say, due to a genetic mutation that makes them constantly hungry while burning fewer calories.
The mutation was found in one-in-four Labradors and two-thirds of flat-coated retrievers.
Speaking to the BBC, Dr Eleanor Raffan from the University of Cambridge explained that dogs, like humans, have genes that influence hunger and metabolic rate.
"If we get dealt a genetic hand of cards that makes us feel hungry or always want to eat, it takes greater effort to stay slim," she said.
"Dogs with this genetic mutation face a double whammy: they not only want to eat more, but also need fewer calories because they're not burning them off as fast."
More than 80 pet Labradors took part in the study, given various tests including the "sausage in a box" test, where they were allowed to see and smell a treat before it was hidden. Those with the mutation tried much harder to get at the sausage than the dogs without it, indicating they were hungrier.
Sleeping in a special chamber that measured the gases they breathed out, the dogs with the mutation were shown to burn 25% fewer calories than those without it.
Labradors have the highest rates of obesity and have been shown to be more obsessed with food than other breeds.
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