In science, every answer prompts new questions. Having found the genes associated with smallness, scientists were eager to find other traits that might be related to size. For example, little dogs live longer than big dogs. Or so it seems. Is there a genuine connection between size and longevity and, if so, where is the link in the DNA? Longevity has been documented only since the mid-1980s. However, because life span was never selected for by breeders, any link in the DNA between size and life span would be a result of random chance rather than artificial selection.
Researchers found no evidence indicating that the largest individuals within a breed die earlier than smaller ones (Jones et al. 2008; to date, studies have been done on only a few breeds. Anecdotal evidence indicates this may not be the case in other breeds). But when they mapped traits across certain breeds of extreme size difference, they found that the genetic subset that influences size also plays a role in longevity. As the authors wrote, “This peculiar inverse correlation between longevity and size … is strictly a between-breed phenomenon and provides an excellent example of a trait that can be approached with across-breed mapping … [A] subset of loci, which control body size, also contribute to longevity, with some playing a greater role in the aging process than others.”
Is temperament linked to longevity? A 1997 analysis of rates and causes of death in more than 222,000 insured Swedish dogs provided baseline mortality data on 250 breeds from birth through 10 years of age (Bonnett et al. 1997). Using the population-based study and then attributing temperament traits to breeds, researchers found evidence suggesting that a breed’s typical boldness is also related to longevity (Careau et al. 2010).
In 2003, researchers found evidence indicating that dogs engineered for the chase, with long thin legs for running, have narrow heads and long muzzles because the width of the leg bone and the length of the skull are controlled by the same group of genes (Chase et al. 2003). Recently, scientists in the same labs published additional findings (Jones et al. 2008): two aspects of size and shape of the muzzle are linked; tail length, ear erectness and size appear to be related; and ear and tail shapes are linked, as are head and neck size. In addition, they identified a candidate gene associated with the degree of tail curvature and short coat.
Geneticists suspect that many multiple- trait relationships were developed under extreme selection and are linked to a few single genes that regulate systems controlling most types of growth, some of which have been favored by breeders, and others — like longevity — the result of unintended consequences. Breed development is rapid because, as noted earlier, most selected traits are tied to a few big bundles of prepackaged traits; very few characteristics are offered a la carte. Unless breeds have truly unique features (like the Lundehund’s triple-jointed polydactyl toes), most breeds could be recreated by bringing together the right combination of genes from related breeds.
The Brain Game
Characterizing behavioral traits is an inexact science at best. Ask 10 experts to define fear-aggression and you’ll likely get 10 different descriptions. However, a few behaviors are magnified to the point of being easily characterized: the intense gaze and stalking movement certain herding breeds use to move livestock; the stylized position pointing dogs take to indicate the presence of game; the retrievers’ act of fetching game and returning it to the hunter. Strung together in sequence, these behaviors make up the bulk of the predatory response wolves employ to hunt and survive. But, in some amazing way, artificial selection extracts and segregates fragments of the sequence.