Failure to estimate reliable guanaco sex-ratios from road-survey data.

PEDRANA, J.; RODRÍGUEZ A.; BUSTAMANTE, J. ; TRAVAINI, A.; ZANÓN MARTÍNEZ, J.I.

Canadian Journal of Zoology

NRC Research Press

Año: 2009 vol. 87 p. 886 - 894

1480-3283

The guanaco (Lama guanicoe) is a monomorphic polygynous mammal whose adult sex-ratio is expected to be balanced or biased towards females. Remarkably male-biased sex-ratios of adult guanacos are often estimated from road surveys. We analyzed the distribution of guanaco social groups recorded during road surveys in Patagonia, Argentina, to test the hypotheses that group assignation based upon behavioral traits is not unequivocal and can be biased by survey factors. Guanacos are organized into three social units: family groups, male groups, and solitary males that are identified by their grouping behaviour. We recorded 992 guanaco groups, and estimated an adult sex-ratio of 3.2 males per female. We used Generalized Additive Models (GAMs) to test the null hypothesis that the probability of recording a group as a family group was constant. Alternatively, this probability could decrease when juvenile abundance and/or detectability was low. The most parsimonious model showed that the probability of classifying a “family” group increased with date, and decreased with group size, distance to the observer, and time of day. Our results do not support the null hypothesis and suggest that road surveys are unsuitable to estimate reliably the social structure or sex-ratio of guanaco populations.