Age at first reproduction in wolves: different patterns of density dependence for females and males

Age at first reproduction constitutes a key life history trait in animals
and is evolutionary shaped by fitness benefits and costs of delayed versus
early reproduction. The understanding of how intrinsic and extrinsic
changes affects age at first reproduction is crucial for conservation and
management of threatened species because of its demographic effects on
population growth and generation time. For a period of 40 years in the
Scandinavian wolf (Canis lupus) population, including the recolonization
phase, we estimated age at first successful reproduction (pup survival to
at least three weeks of age) and examined how the variation among
individuals was explained by sex, population size (from 1 to 74 packs),
primiparous or multiparous origin, reproductive experience of the partner,
and inbreeding. Median age at first reproduction was 3 years for females
(n = 60) and 2 years for males (n = 74), and ranged between 1 and 8-10
years of age (n = 297). Female age at first reproduction decreased with
increasing population size, and increased with higher levels of
inbreeding. The probability for males to reproduce later first decreased,
reaching its minimum when the number of territories approached 40 to 60,
and then increased with increasing population size. Inbreeding for males
and reproductive experience of parents and partners for both sexes had
weak or no effect on age at first reproduction. These results allow for
more accurate parameter estimates when modelling population dynamics for
management and conservation of small and vulnerable wolf populations, and
show how humans through legal harvest and illegal hunting influence an
important life history trait like age at first reproduction.