Endangered Species Recovery Program

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Reproduction and growth of a confined population of endangered riparian brush rabbits

Presented to:

2nd World Lagomorph Conference, Vairão, Portugal, July, 2004.

Daniel F. Williams, Elizabeth A. Williams, Laurissa P. Hamilton, and Patrick A. Kelly

Endangered Species Recovery Program, Department of Biological Sciences, California State University, Stanislaus, Turlock CA 95382, USA

Abstract

We propagated endangered riparian brush rabbits (Sylvilagus bachmani riparius) in 3 outdoor pens, each of approximately 0.50-0.57 ha. Pens contained natural vegetation dominated by Himalayan blackberry bushes (Rubus discolor). Rabbits were free-ranging within pens and, because of the high density of blackberry thickets, could be studied only by radio-telemetry and infrequent trapping. In November 2001, six wild-caught rabbits (3f, 3m) were released into one pen. These adults were removed in June-July 2002 and the pen was emptied of progeny starting in July and ending in October. In December 2002, 18 wild-caught rabbits were moved into the breeding pens—3 of each sex in each pen. The first 2 offspring were captured on 22-23 February 2002 and were estimated to be 18 and 23 days old. Offspring about 90 days old or greater exhibited evidence of reproduction, including 12 of the young females in 2002. In 2002, we estimated conservatively that there were 22 pregnancies by 15 females producing 65 young that lived long enough to be trapped and marked, probably 14 days or more from birth. Mean number of young per pregnancy surviving to this milestone was 2.9. Mean weight at first capture was 228.5 g and mean estimated age at first capture was 29 days. Mean increase in mass approximately after weaning (14 days old, 110 g) was 8 g/day. Genotyping using 6 polymorphic microsatellites (2 others were non-variable in this population) demonstrated that all 6 rabbits contributed to the first cohorts consisting of 15 young in 2002. In 2003, we recorded a total of 275 offspring from the 3 pens. Data on reproduction for 2003 still are being analyzed and also will be reported.

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