Seasonal variation of family group structure of plateau pikas (Ochotona curzoniae)

From May to August of 2005 and 2006, the age structure and sex ratio within family groups of plateau pikas (Ochotona curzoniae) were studied in Maqin County, Qinghai Province, by mark-recapture method. The result indicated that in May the family groups were composed of adults and first-litters (male: Adult, 47%; Juvenile, 53%; female: Adult, 40%; Juvenile, 60%), and in August were mainly composed of first-litters (male, 58%; female, 51%). Composition of plateau pika families averaged 5.1 adults (2.4 males and 2.7 females) and 13.0 juveniles (5.2 males and 7.8 females) that survived long enough to be caught. There were significant differences in age structure within family groups among different months. Individuals of adults and female first-litters in May were significantly larger than those in other months. Individuals of male first-litters in May and June were significantly larger than those in July and August. Individuals of second-litters in June were significantly larger than those in July and August; individuals of male second-litters in July were significantly larger than those in August. Individuals of first-litters in August were significantly larger than those of second-litters. The result also showed that these was no significant difference from 1: 1 in sex ratio during whole breeding season, whereas sex ratio of adult in May was significantly higher than that in July and August. It is suggested that plateau pikas living in environmental conditions where it have a long winter time, short breeding seasons, rigorous climate and limited food supply adjust their family group structure to improve reproductive success.