A test of the fast-slow continuum model of life-history variation in the lizard Sceloporus grammicus

Background: The fast-slow continuum hypothesis (Promislow and Harvey, 1990) suggests that mortality rates are a strong selective factor that shapes inter-and intraspecific variation in life-history traits. At the fast end of the continuum, we expect higher mortality rates, faster growth rates, small size and/or early age at maturity, short lifespan, and large litters with smaller offspring. At the slow end, we expect the opposite. Question: Is intraspecific (spatial) variation in life-history traits associated with spatial differences in mortality rates, as the fast-slow continuum hypothesis suggests? Organism: The viviparous lizard Sceloporus grammicus (Sauria: Phrynosomatidae). Study system: Eight different populations of S. grammicus in central Mexico (latitudinal range: 19 degrees 11'N to 20 degrees 47'N). Methods: We used likelihood methods to calculate mortality rates from mark-recapture data. We also measured rates of body growth, size and age at maturity, litter size, offspring size, and offspring body condition from marked animals, preserved specimens, and females kept in captivity. We used a multi-model inference framework to examine inter-population variation in life-history traits and their potential association with mortality. Each model represented a different hypothesis about variation in life-history traits, and the relative fit of each was determined using AICc (adjusted Akaike Information Criterion). Conclusions: Life-history traits differed widely among populations, but litter size was the only trait that varied with site-specific mortality rates. Larger litters were associated with higher mortality, consistent with the fast-slow continuum hypothesis. Other life-history traits showed no clear evidence of influence of mortality rates.