Hartman (1928:154) stated that there were at least two litters of young per year in the southern states with a small percentage of unusually fecund females producing a third litter. Lay, in eastern Texas, concluded (1942:155) that "The present investigation substantiates Hartman's deduction of two litters being normal, but fails to disclose any evidence of a third litter." He found females carrying young in the pouch only within the seven-months period January to July with definite peaks in February and June, and stated that second litters appear in the pouch from early April to as late as May 20 to 23. Reynolds (1945:362) found that the breeding season in central Missouri in 1941 and 1942 began about the first of February, with known or calculated birth dates of 42 litters rather evenly distributed throughout the periods February 12 to April 2, and May 16 to June 4. Eight of these females had given birth to young between March 16 and April 2, approximately six to nine weeks after the beginning of the breeding season. Reynolds assumed that these were individuals that had failed to find mates during the first oestrus of the season and that after completing the regular dioestrus of about 28 days they had then mated and borne young. Wiseman and Hendrickson (1950:333) in southeastern Iowa recorded a female with a litter no more than two days old on February 23, and several other females with young were estimated to have borne litters at approximately this same date, while still others bore litters as late as early March. Two lots of small young found in early June may have been second litters.
For the region represented by the present study, the data indicate a breeding season with later onset and sharply circumscribed limits as compared with an earlier onset and less circumscribed limits in Texas, central Missouri, and even southeastern Iowa, which is a little farther north. The available data indicate that there are two distinct and well-defined breeding seasons in the course of the annual cycle on the University of Kansas Natural History area. The whole population, including young of the preceding year, some still far below average adult size, breeds from about the middle of February into early March, and first litters are born mainly in early March. Individual females may vary as much as two to three weeks in the time of breeding, and varying weather conditions from year to year may hasten or delay onset of the breeding season. Data are recorded below for all females caught in March that were carrying litters.
| Date | Weight of female in grams | Number of young | Development of young |
|---|---|---|---|
| March 1, 1952 | 2000 | 9 | Newborn |
| March 2, 1952 | 1450 | 6 | Newborn |
| March 2, 1952 | 1230 | 7 | Newborn |
| March 5, 1950 | 1200 | 10 | About 16 mm. snout to vent |
| March 5, 1950 | 1300 | 1 | About 14 mm. snout to vent |
| March 6, 1951 | 1110 | 4 | Newborn |
| March 18, 1952 | 1930 | 8 | Not present when female was trapped on March 1 |
| March 18, 1952 | 1520 | 6 | |
| March 18, 1952 | 1230 | 12 | About 40 mm. snout to vent |
| March 19, 1951 | 1000 | 8 | Estimated 1 week old |
| March 22, 1950 | 1040 | 9 | About 34 mm. snout to vent |
| March 24, 1950 | 1280 | 10 | 74 mm. snout to vent |
| March 24, 1950 | 1480 | 8 | |
| March 27, 1950 | 965 | 8 | Total length 26 mm., weight .8 g. |
| March 28, 1951 | 820 | 7 | 20 mm. crown to rump; born since previous capture of female on March 7 |
| March 30, 1950 | 1325 | 9 | Total length 33 mm. |
| March 31, 1952 | 1930 | 8 | |
| March 31, 1952 | 1630 | 5 | Total length 73 mm. |
None of the females trapped in February was carrying young in the pouch, but probably some early litters are born in the last week of February or even earlier. By late March most of the females are carrying young in their pouches, and those which do not have young, have their pouches enlarged and vascularized for accommodation of the young. Presumably such females have already borne young and then lost them. Nearly all the litters seen in the latter half of March had young that were much larger than at birth.
Of 13 females examined in April, 12 were carrying young, and the remaining one was known to have been carrying a single young on March 1, but had lost it. Eleven females were examined in May, four of which were the same ones examined in April. Eight of the eleven females were carrying young; of the remaining three, one had lost the litter of young that it had been carrying when trapped in April. Two had empty pouches on May 19 and 20, but probably had successfully reared the litters of young which they had been carrying when trapped in April. The young of all those females trapped on different dates in April and May were in stages of growth indicative of birth about the first week in March. The latest date on which a female was recorded with first-litter young in the pouch was May 22, 1951, and these were the largest pouch young observed. Their eyes were recently opened, they were estimated to weigh 60 grams each with hind feet 20 mm. long. Young continue to grow rapidly after leaving the female's pouch. A young female caught on June 16, 1949, weighed 126 grams. For seven young caught on July 5 and 6, 1952, weights and hind-foot measurements were, for males: 660 grams, 52 mm.; 560 grams, 46 mm.; 550 grams, 48 mm.; 450 grams, 44 mm.; 370 grams, 44 mm.; 330 grams, 37 mm.; and for the one female: 430 grams, 46 mm.
The wide variation in size in this small group of young of nearly the same age is noteworthy. Size and condition of the females carrying them, number of competing litter mates, and early success or handicap in independent life causes so much divergence in size that at the age of four months some young are twice as large as others.
By late fall the young grow to small-adult size. For example, the female that weighed 126 grams when first caught on June 16, 1949, was recaptured on November 29, 1949, and on that date weighed 1710 grams.
A second breeding season ensues soon after the young of the first litter leave the pouch, and these young probably soon learn to shift for themselves. Second litters are usually born in early June. On June 14, 1952, a female was taken with young only a few days old in her pouch. On July 5, 1952, two females last taken on May 19 and May 20, with their pouches recently vacated by first litters, were found to have young the size of half-grown mice, evidently two to three weeks old. In the months of October, November, December and January, a total of 11 young, thought to represent second litters, were taken. Dates of capture, weights in grams and sexes were as follows:
| Oct. 3, 1950 | 400 grams | male |
| Oct. 6, 1950 | 510 grams | female |
| Oct. 8, 1950 | 260 grams | female |
| Oct. 8, 1950 | 350 grams | female |
| Oct. 18, 1950 | 350 grams[A] | female |
| Dec. 5, 1951 | 630 grams | female |
| Dec. 30, 1950 | 710 grams | female |
| Jan. 1, 1951 | 660 grams | female |
| Jan. 1, 1950 | 700 grams[A] | male |
| Jan. 9, 1950 | 550 grams | male |
| Jan. 11, 1950 | 550 grams | male |