Breeding lay young-of-the-year female deer mice: Why weight?

Female deer mice (Peromyscus maniculatus) in the Kananaskis Valley, Alberta seldom breed during the summer of their birth despite having a breeding se ason which appears to be long enough for them to sexually mature. Thirteen years of live-trapping data indicate that only 3.6% of all females recruite d into the trappable population bred as young-of-the-year (YY). Exclusion o f those YY females who were born too late in the season to have bred and th ose who may have bred outside the trappable population indicated that only 14% of YY females with the opportunity to breed did so. Most females who br ed as YY conceived within days of emerging from their natal nest. Breeding by YY females was associated with longer breeding seasons, which were lengt hened through later cessation rather than earlier initiation of breeding, i ncreased nestling growth rates and female-biased litters. The results of th is study support the conclusion that nestling growth rates are important fo r YY breeding in these populations. Further, it appears that nestling growt h varies annually, as well as among litters within breeding years, but not among individuals within breeding litters.