Mice were fed diets of skeletal muscle meat from young and old farmed red deer (3 years old versus 25). The diet consisting of meat from old deer shortened the lifespan of female mice by 10%. The lifespan of male mice was shortened but was not statistically significant. The researcher thinks this is because he did not have a large enough sample size of male mice. Also, the mice were fed a normal diet until 1 year of age (basically middle age for mice) then were shifted to the meat from old vs. young deer. There may be a stronger effect if they were fed the meat from birth. The same thing was found in flies and worms: protein from old animals shortened the lifespan of male and females by 10%. It is thought that damage accumulation in the meat from older animals is introduced to the organism through the diet and this may contribute to aging.