|Abstract or Summary
- Six hundred and forty nutrias, Myocastor coypus (Molina), were collected near Corvallis, Oregon, and examined to determine criteria of their growth and reproduction. Two hundred and eighty-nine
were tagged, measured, and released to determine growth under natural conditions. Nutria weights were calculated to the nearest 0.1 pound by weighing the live-trapped animals in the trap arid subtracting the weight of the trap following their release. Total lengths, right hind foot lengths, and body lengths were measured in inches while the nutrias were restrained within a holding chute. Three hundred and fifty-one nutrias were sacrificed during the investigation
and examined internally for criteria of growth, reproductive status and presence of diseases. In addition, the weights of the adrenals, kidneys, liver, heart, and air-dried skulls, were recorded in grams. Smears taken from some of the testes were stained and examined for the presence of spermatozoa. Ovaries were examined for corpora lutea and uterine tracts for pregnancies. Mean weight at birth for the nutria kits was 217 grams. No appreciable differences were determined in body measurements between the sexes. Body weights of maximal sized males and nulliparous females were 24.0 and 16.0 pounds respectively. Sexual maturity was attained in Oregon nutria from 6 to 9 months of age in males and from 4 to 9 months of age in females. Mean body weights at the onset of sexual maturity for males and females were 6.1 and 4.1 pounds respectively. Peak birth periods occurred in January, March, and May. A lesser peak occurred in October. Litters averaged 5.0 kits at birth with a 1:1 sex ratio. Prenatal mortality was observed in 24.6 percent of the 60 pregnancies examined. Gravida after resorption averaged 4.9 fetuses per female. Based upon 55 pairs of ovaries, 7.6 corpora lutea were observed per ovary. Differences between corpora lutea
and the fetuses that survived prenatal mortality indicated a total embryonic loss of about 33 to 35 percent. Weaning occurred in feral kits 7 weeks after birth. Kits forcibly weaned when 2 to 8 weeks old survived hut grew at somewhat slower rates than normal kits. Nine kits, less than 1 week old when forcibly weaned, suffered a depleted condition and six of them died.