The sheep, another domesticated Ruminant, has a life even shorter. According to Grindon, sheep do not live longer than 12 years as a rule, but may reach 14 years, which in their case would be extreme age, as they generally lose their teeth at from 8 to 10 years.
Some Ruminants, such as camels and deer, apparently live longer than sheep or cattle, but I do not know exact facts about them.
The short life of domesticated carnivorous animals is well known. Dogs seldom live more than 16 or 18 years, and even before that, at an age of from 10 to 12, they usually show plain signs of senility. Jonatt has mentioned as an extreme rarity a dog of 22 years of age, and Sir E. Ray Lankester (Comparative Longevity, p. 60) cites another instance, in this case the age being 34 years. The oldest dog that I have been able to procure died at the age of 22.
It is generally believed that cats do not live so long as dogs. The average age which they may attain is usually thought to be 10 or 12 years, but certainly a cat of that age has not the decrepid appearance of an old dog. Thanks to the kindness of M. Barrier, the Director of the Ecole d’Alfort, I have had in my possession a cat 23 years old. It appeared to be quite vigorous, and died from cancer in the liver.
Most rodents, particularly the domesticated kinds, are extremely prolific and very short lived. It is extremely rare for a rabbit to reach the age of 10 years, whilst 7 years is the utmost limit for a guinea-pig. Mice, so far as I can ascertain, do not live more than 5 or 6 years.
It is plain from the facts that I have brought together, that mammals, whether they are large or small, as a rule, have shorter lives than birds. It is probable, therefore, that there is something in the structure of mammals which has brought about a shortening in the duration of their lives.
Whilst most of the lower vertebrates, and all birds, reproduce by laying eggs, the vast majority of mammals are viviparous. As the tax on the parent organism is greater when the young are produced alive than when eggs are laid, it might be thought that in this difference lay the cause of the shorter life of mammals. It is well known that an animal may be made feeble by too great fecundity, and it is conceivable that the kind of parasitic life of the embryos within the body of the mother may weaken her system.
There are many facts, however, which make it impossible to accept such a view. The longevity of mammals is nearly equal in the two sexes, although the tax on the organism caused by reproduction is much greater in the case of females than in males. Longevity, however, cannot be regarded as a character stable in each species and necessarily identical in the two sexes. The animal kingdom presents many cases of disparity in this respect, the difference in longevity in the two sexes being specially striking in species of insects. Generally, the females live longer than the males, as, for instance, amongst the Strepsiptera, where the females have 64 times the duration of life of the males. On the other hand, amongst butterflies, there are cases (e.g., Aglia tau) where the males live longer than the females. In the human race, there is a difference in the longevity of the sexes, the females having the advantage.
As in most cases of disparity in the duration of life the female lives longer than the male, it is plain that the difference cannot be assigned to the drain on the organism caused by reproduction, which, of course, is much greater in females.
Moreover, a closer scrutiny of the facts shows that although mammals do not live so long as birds, the reproductive drain is greater in the case of birds.