The big bad wolf seems to permeate fairy tales, haunting children in their sleep. Ever wondered what contributed to the fitness of the villainous wolf? In truth, like many other mammal species, the gray wolf’s morphological, behavioral, and life-history traits are of adaptive value. A 14-year study was performed on Canis lupus in Yellowstone National Park to understand the influence of these traits on reproductive success. UCLA biology professors Daniel Stahler and Robert Wayne, Utah State wildlife ecology assistant professor Daniel MacNulty, UCLA graduate student Bridgett vonHoldt, and Yellowstone gray wolf restoration project leader Douglas Smith together monitored over 1000 wolf individuals in about 38 packs. Age, body mass, and maternity were determined, and individual, group, and pack level factors were accounted for in the data analysis.
Both adaptive traits and environmental conditions impact reproductive success, but it is unknown as to which traits are favored by natural selection. The study digs into this relationship, as not previously done before. Traits include body size, genetic heterozygosity, cooperative breeding, and age-specific performance. Environmental conditions include disease prevalence (top-down), resource availability (bottom-up), and population density (bottom-up). A multi-level analysis was used to (statistically) control these multiple factors’ effects, and a sensitivity analysis measured the strength of the factors across biological levels.
Wolves are known to be large, social, territorial carnivores with short generation times (four to five years), high fecundity (five to six pups), early first reproduction (two years old), fast development, and short life spans (five to six years). The evolution of sociality is influenced by factors like territorial and food defense, group hunting, kin selection, and cooperative breeding. With this background information, five hypotheses were tested: 1) age-specific reproductive profile shows concave-down pattern and a peak around age five; 2) there is a positive correlation between female body mass and reproductive success; 3) there is a positive correlation between multi-locus heterozygosity estimates and reproductive success; 4) there is a differential reproductive performance between gray and black females; and 5) there is a positive correlation between pack size and reproductive success. Also, group size was predicted to be the most influential.
Best-fit GLMM models (female age, female body mass, adult pack size, population size, and disease) were done on litter size and litter survival. Larger females in turn produced larger litters; there was a threshold at which pack size affected litter size; litter size was constrained by population size and disease. Larger females’ pups had larger chances of survival; pack size was a positive influence on litter survival; once again, population size and disease were important constraints.
The comprehensive analysis of wolf reproduction showed significant effects of factors at all levels (individual, group, and population). Contrary to the hypothesis, individual-level traits were most influential. Estimates of heterozygosity (internal relatedness) did not significantly affect reproduction, most likely due to high genetic variation and little inbreeding of YNP wolves. Individually, increased female body mass and gray coat color (versus black) attributed to improved reproductive performance. As for groups, the effects of pack size on reproduction were contrasting but not linear (conditional). At the population level, CDV was less influential than individual and group factors but still significantly impacted female reproduction (strong selective force).
In conclusion, this study’s finding on trait and environmental-specific patterns is a step to improving models on age, social, density, and disease structures/patterns. Wolf reproduction turned out to be similar to other mammalian reproductions. In following studies, it would be useful to test if phenotypes are also more influential (in comparison to environmental conditions) in other taxa. Knowledge of fitness-promoting traits is vital to understanding wild populations’ responses to eminent events like global climate change, disease outbreaks, habitat change, and human exploitation.
Stahler, D. R., MacNulty , D. R., Wayne, R. K., vonHoldt, B., Smith, D. W. (2012), The adaptive value of morphological, behavioural and life-history traits in reproductive female wolves. Journal of Animal Ecology, DOI: 10.1111/j.1365-2656.2012.02039.x