Abstract

SOBRAL, Gisela  and  OLIVEIRA, João Alves de. Annual age structure and reproduction in the Caatinga red-nosed mouse, Wiedomys pyrrhorhinos (Rodentia, Sigmodontinae). Therya [online]. 2014, vol.5, n.2, pp.509-534. ISSN 2007-3364.  https://doi.org/10.12933/therya-14-194.

Introduction: We analyzed the variation in the age structure, sex ratio, proportion of reproductive individuals, age and size at sexual maturity, and litter size in a population of the sigmodont rodent Wiedomys pyrrhorhinos. We compared the observed distributions with the monthly rainfall for the same period, to identify the reproductive period and to assess the effects of the rains on the age structure of the population during the ensuing months. We further compared our results with age structure patterns recovered from other documented rodent outbreaks to identify possible causes of a presumptive population outbreak represented by this series. Methodology: Museum samples had been obtained between July 1953 and February 1955 from 40 sites in Caruaru, Pernambuco, Brazil (Fig. 1, Appendix). Records from a total of 2,280 individuals, 1,834 of which represented by their skulls, were analyzed. Skulls were sorted into seven age classes based on molar eruption and wear (Fig. 3). Monthly rainfall during the entire collecting period (Fig. 2) was analyzed for putative correlations with monthly frequencies of specimens (Figs. 4 and 5) and of indicators of reproductive condition, tabulated for each sex and age class. Results: Monthly frequency distributions of age classes presented separated and consecutive modal peaks, conforming to an age structured population (Fig. 6). Monthly proportions of pregnant females were highly correlated with the amount of rainfall of the same and of the previous month (Fig. 7), although records of pregnant females were obtained every month between July 1953 and December 1954. Pregnant females were found from age class 2 on. From January to April, 1954, reproductive females belonged to age classes 5 to 7. From May to September a wide range of age classes was documented among pregnant females, and from October to December only those of age classes 2 to 4 were recorded (Fig. 8). Litter size varied throughout the year, with larger averages and ranges at mid-wet season (June-August) (Fig. 10) and was highly correlated with female weight, female length and with the amount of rainfall of the previous 30 days. Higher means in litter size were also in phase with higher proportions of pregnant females and with the age class 1 peak. Discussion and Conclusion: The population peak occurred four to six months after the rainfall peak (September - November) and was primarily comprised of young adults. After this period, the pregnancy rates were very low, ceasing completely by January, 1955, when most of the individuals belong to older age classes. This shift in age structure from younger to older individuals during population decline indicated that the samples documented an outbreak episode in 1954. This episode was possibly related to the unusual rainfall in November 1953, apparently sufficient to support two closely-spaced breeding seasons, with the second amplifying the effect of the first.

Keywords : bionomy; Brazil; breeding season; life history; litter size; outbreak; Pernambuco; population dynamics; pregnancy rate; ratada.

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