Guinea pigs are extremely precocious and prolific reproducers. Males begin mounting at 1 month old, even though ejaculation starts at 2 months; the females become sexually mature at 5-7 weeks of age. It is, however, sensible to wait until the males are 3-4 months old and females 2-3 months old before mating them.
The guinea pig is a polyoestrous species with spontaneous ovulation and an oestrous cycle that lasts 15-17 days; oestrus itself lasts 6-11 hours. The average gestation is 68 days (with a range of 59-72 days, depending on the litter size). By the end of the gestation, the guinea pig’s weight may have doubled and the animal should be handled with great care. Guinea pigs can enter oestrus 2-15 days after delivery and, if mated, have a 60-80% chance of becoming pregnant. A gravid guinea pig does not need to be separated from the other guinea pigs in the group, but the young of other guinea pigs could suckle the pregnant sow’s milk, thereby depriving her own neonates.
Towards the end of the gestation, the fibro-cartilaginous pubic symphysis progressively separates by as much as 2.5 cm the day before delivery (Fig. 1) A guinea pig mated for the first time after the age of 7 months will not be able to deliver her young naturally, since ossification of the pubic symphysis will have already occurred: in this case, a Caesarean section will be necessary to deliver the litter.
Delivery takes about 30 minutes and usually occurs during the night. The average number of piglets is three (but a litter can range from 1 to 6 piglets) (Fig. 2). The higher the number of young in the litter, the smaller each animal is; a newly born guinea pig weighs between 45 and 115 g. Piglets that are too small have very little chance of surviving, whereas if they are too large they can cause dystocia; the ideal number of piglets in a litter is three. After delivery the mother usually eats the placentas. The piglets are very precocious; they are born covered with hair, with open eyes and can walk; they start eating solid food within a few days. The first feed does not occur for 12-24 hours. Piglets can be touched within a few hours after birth without this causing the mother to reject them. Being touched helps the youngsters to socialise with humans.
As their first food, moistened pellets and grated carrots can be left available to the piglets. Maternal milk production lasts for about 18-30 days and the piglets can be weaned after 2-4 weeks.
CARE OF ORPHANS
Being very well developed at birth, the possibility of survival of very young guinea pigs is better than that of other rodents. Solid food can usually be eaten already from the second day of life and it is known that guinea pigs orphaned on the third day of life have survived without any help. However, the possibility of survival is higher if the young animals receive milk for the first week. A nursing guinea pig will sometimes accept piglets that are not her own quite easily. In any case, it is useful to keep piglets with an adult guinea pig because they learn to eat by copying the latter.
Guinea pig milk contains about 4% fat, 8% protein and 3% lactose. Cow’s milk or artificial milk for puppies can be used as a substitute, although these products should only be used for a short time and young guinea pigs reared on milk other than their own natural milk tend to develop cataracts. Milk must not be forced into a piglet’s mouth, because doing so could cause aspiration pneumonia.