A pseudoseasonal reproductive strategy in a tropical rodent, Peromyscus nudipes

A population of cloud forest mice (Peromyscus nudipes) at latitude 10 degrees N near Monteverde, Costa Rica, was sampled four times by live-trapping twice during the 7-8 month wet season and twice during the 4-5 month dry season in 1989 and 1990. Body weights were lower during the early part of the dry season in males and throughout the dry season in females than at other times. Testes and seminal vesicles were somewhat lighter early in the dry season, but epididymal spermatozoa were abundant in most males throughout the year. Adult females ovulated, mated and became pregnant in the wet and dry seasons, but young were produced only during the wet season. Most embryos failed to implant during the dry season, and the few that did complete implantation were reabsorbed before midpregnancy. Apparently, every year, the females in this population spend several months actively engaged in a behavioural and metabolically costly process that is doomed to be unsuccessful. This reproductive strategy is termed pseudoseasonal, because reproductive success is highly seasonal, but attempts to reproduce are nonseasonal. Implantation failures similar to those seen in the wild were induced in the laboratory using mild restriction of food or water. Field evidence points to food restriction as the more important cause of pregnancy losses in the wild. Exposure to the gradually changing daylengths typical of Costa Rica had no effect on the production of young by adults, and maintenance on light cycles of 8 h light: 16 h dark, 11 h light: 13 h dark, 13 h light: 11 h dark and 16 h light: 8 h dark had no effect on the reproductive development of young animals of either sex.