56. Reproduction of the Lesser Shrew (Sorex minutus Linnæus).



The material consisted of 277 Lesser Shrews, of which 156 were males, 120 females, and one intersex (described elsewhere, 1936). The sex-ratio of the whole sample was 56–52±2.01 males per cent. The percentage of males obtained in April and May was higher than during the rest of the year. The proportion of Lesser to Common Shrews obtained was 16±6 per cent., but in one small wood Lesser Shrews were found to be the commonest species.

Young Lesser Shrews when they first appear in the traps in summer weigh 2–75 to 4 gm. They do not breed in their first season, and remain at approximately the same weight until the end of the following March, when they grow to the adult size of 4 to 6 gm., the heaviest obtained being a female weighing 7.3 gm. Adult animals die at the end of the breeding-season.

The breeding-season in North Wales begins in mid-April, reaches its height in June, and ends in October.

Since no non-pregnant females with recent corpora lutea in the ovaries were obtained, ovulation appears to result in pregnancy in 100 per cent, of cases. The mean number of follicles ovulated at œstrus is 6.8, and the mean number of implanted embryos in utero is 6.2. The litter size is therefore 0.5 smaller than in the Common Shrew.

The œstrous cycle closely resembles that of the Common Shrew. The first œstrus is preceded by prolonged vaginal cornification and gradual hypertrophy of the reproductive organs. The largest follicles observed were 328 μ. in diameter. There is a post-partum œstrus at which the majority of females become pregnant, gestating and lactating simultaneously. Animals that do not become pregnant at the post-partum œstrus exhibit an anœstrous period during lactation.

Forty-seven pregnant females were obtained, 4 with tubal ova, 12 with free uterine blastocysts, and 31 with implanted embryos. Migration of blastocysts from one uterine cornu to the other takes place freely, as in the Common Shrew. The histological changes in the uterus and vagina during pregnancy closely resemble those in the Common Shrew.

The data regarding the relation of the weight of the testes to body-weight are given, and the relative sizes of the various male reproductive organs are compared with those of the Common Shrew.

The authors' thanks are due to Dr. A. S. Parkes, F.R.S., and Mrs. Parkes, for their advice and for collecting part of the material in Kent, and to Messrs. L. H. Jackson and I. W. Rowlands for assistance in the routine trapping in North Wales. The authors are indebted to the Rt. Hon. Lord Penrhyn for permission to trap on his estates, where many of the animals were obtained.

The expenses of this research were defrayed in part by grants from the Government Grant Committee of the Royal Society to one of us (F. W. R. B.), for which we wish to express our thanks.