Maternal expenditure in the polygynous and monomorphic guanaco: suckling behavior, reproductive effort, yearly variation, and influence on juvenile survival

We investigated patterns of maternal expenditure and its influence on juven ile survival in the polygnous monomorphic guanaco (Lama guanicoe) in southe rn Chile from 1990 to 1994. Birth weight and growth rate (until age 1) of m ales and females were similar Suckling rates of males and females were not significantly different, although mothers of males rejected suckling attemp ts more often than mothers of females during fall and winter. Mothers with sons terminated suckling bouts in equal proportion as did mothers with daug hters. Our estimated level of reproductive effort for guanacos falls within the range of species exhibiting no sex-biased maternal expenditure on offs pring. Mean yearly birth weight was negatively correlated with population d ensity;. Mean suckling time throughout the year differed among cohorts, as did the mean number of suckling attempts and rejected suckling attempts per hour throughout the year. Juvenile survival was estimated until age 1. Of the model with five covariates including juvenile sex, birth weight, adult female aggression toward taggers, mean suckling time, and population densit y, only mean suckling time and population density were significantly relate d to survival. The risk ratio for mean suckling time indicates that the ris k of mortality increases as suckling time increases, whereas the risk ratio for population density indicates that the risk of mortality decreases as p opulation density increases. Under some conditions increasing population de nsity may be correlated with lower offspring birth weight, yet enhanced juv enile survival. This effect on survival was possibly associated with the nu mber of predators on the studs area from year to year.