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A model was constructed to predict monthly birth probabilities using mammalian fertility data. We used a sample of 147 female capybaras (Hydrochoerus hydrochaeris) hunted on a farm on Marajó Island, Brazil. In the model each month was treated as a multinomial with six cells representing the six possible reproductive states (five months gestation). A hypothesis test was carried out to see whether a cosine curve would fit the birth probabilities. The results offer no support for a seasonal component (F2,9= 1.84, P = 0.21), whereas results from a direct census do (F3,23= 87.29, P < 0.01). Some hunting techniques were biased towards killing pregnant females (Χ21= 7.2, P< 0.01), thereby spreading reproduction throughout the year (F2,9= 1.84, P = 0.21). The model remained a powerful predictive tool to be used with mammalian fertility data as long as the data are not biased towards pregnant females.

Original publication




Journal article


Brazilian Journal of Biology

Publication Date





1127 - 1131