Maternal and pup contributions to different patterns of pup growth in Phodopus species.

A cross-fostering approach was used to compare and quantify patterns of pup growth in Phodopus sungorus and P. campbelli. Standard litters of five pups were reared by a foster mother of the same or the opposite species. Individual pups were weighed daily, and maternal and pup body condition were quantified. Studies were conducted at an ambient temperature of 23 degrees C, which constrains maternal contact with pups in P. campbelli but not P. sungorus, to maximize the difference in pup growth curves between the species. Results confirmed that the similar Day 18 weights in the two species of Phodopus were a complex result of laboratory conditions that masked different patterns of pup growth. The primary determinants of that pup growth were 1) constraints on maternal investment by P. campbelli mothers, which arose from the thermoregulatory stresses of a 23 degrees C ambient temperature and may have involved different milk energetic value or water content; 2) an intrinsic pup difference in the threshold weight for independent thermoregulation, which increased energetic expenditures for P. sungorus pups several days earlier than for P. campbelli pups; and 3) a longer lactation in P. campbelli, which improved access to dry food. Results also confirmed that within-litter variation in pup weights was considerably larger in P. campbelli, but forced rejection of the hypothesis that the difference arose from different maternal investment strategies during lactation. Species-typical patterns of variation were already established on Day 1 after birth.