Living slow and dying young? Life-history strategy and age-specific survival rates in a precocial small mammal

1. Mammalian life histories can be ordered along a slow-fast continuum from slow-developing, long-lived species with low reproductive rates at one end to rapidly developing, short-lived species with high reproductive rates at the other. Body size and mortality rate are strong correlates of the slow-fast axis, whereby juvenile and adult mortality rates correlate positively but are linked to certain life-history traits with differing strengths. The strong covariation of life-history traits renders it difficult to evaluate the adaptive value of single life-history elements. Species that deviate from the common pattern might help to identify specific selected strategies. 2. Caviomorph rodents show an unusual combination of life-history traits. Like slow-living mammals, they produce few precocial young after long gestation periods, but they have the early maturation potential of their fast-living counterparts. This combination of life-history traits suggests low levels of juvenile mortality but high adult mortality rates, and thus an unusually low ratio of juvenile to adult mortality compared to other small mammals. To test these predictions, we modelled age-specific survival probabilities using capture-recapture data from a wild population of Cavia magna. 3. Patterns of age-specific survival rates were in accordance with the predictions, but atypical for small mammals. Levels of adult survival were low, and comparable to those of much smaller rodent species. Juvenile survival probabilities were high relative to those reported for other small mammals and reached adult levels after only 1 month. The ratio of adult to early juvenile survival depended strongly on the date of birth. Whereas survival of young from the first birth cohort in spring did not differ from that of adults, early survivorship of later cohorts was lower. 4. Our results fit expectations from life-history theory and suggest that high levels of adult mortality selected for the early onset of reproduction in wild cavies. The comparatively low juvenile mortalities are probably a consequence of the precocial state of the cavy offspring. We suggest that the reproductive strategy of cavies represents a different solution to the trade-off between fecundity and juvenile survival compared to altricial small mammals.