Earlier thermoregulation and consequences for pup growth in the Siberian versus Djungarian dwarf hamster (Phodopus)

In spite of the shared reproductive timetable and phenotypic similarity of the two Phodopus species, previous studies had suggested that P. sungorus pups might thermoregulate at an earlier age and might experience decreases in growth rate as a consequence of that energetic demand. The current study investigated thermogenesis and thermoregulation in single pups and huddles separated from adults. Thermoregulation was earlier (Day 9 vs Day 12 for the earliest individuals) in P. sungorus than in P. campbelli and occurred at a lower body weight (6 vs. 8 g). Earlier development of insulating pelage in P. sungorus was not responsible. Both guard and underhair components of the pelt were completed earlier in P. campbelli development. Pup weights from five independent studies of each species were used to compare patterns of growth. Although birthweights were similar, P. sungorus pups grew faster during early lactation. Nest temperatures were probably involved in that faster growth. Maternal hyperthermia raised nest temperatures > or =4 degrees C above nonbreeding levels and left P. sungorus huddles 1.0 degrees C warmer than P. campbelli huddles throughout lactation. Comparisons between studies also suggested that laboratory conditions adversely affected pup growth in each species. In P. campbelli, cool ambient temperatures and biparental care improved early growth. In P. sungorus, providing fresh apple as a water source prevented a major decline in pup growth rate during mid to late lactation. Thus, as predicted, P. sungorus pups grow faster, thermoregulate earlier, and weigh more at Day 18 (when water is available to the pups).