The act of copulation has rarely been observed in the wild and there are some conflicting reports of how the female accommodates the male; this may indicate a response to the presence of an observer. Nonetheless, more recent observations on captive animals and video footage suggest copulation generally involves the female flattening her spines and pressing her stomach to the ground, thereby arching her back, while raising her rear end to present her genitalia to the male. The male responds by mounting her, often gaining purchase by gripping the female’s shoulder spines in his mouth.
Males have a bulbospongiosus–type penis, which means it has a muscle, the bulbospongiosus, covering the bulb of the penis that contributes towards erection and ejaculation. Erection is achieved by engorgement of the corpus cavernosum (spongy tissue) and muscles running down its length. I’ve not come across any measurements of the erect penis length, although, in their Natural Hedgehog, Lenni Sykes and Jane Durrant wrote that the penis extends from the middle of the abdomen to beyond the nose, which suggests the erect penis is several centimetres long. This would seem to be supported by a trailcam video captured by Paula Felischmann in 2014 that shows a hedgehog in a nest box cleaning its penis – the full penis is obscured by the bedding material, but it appears to be several centimetres in length.
Weights from autopsied animals suggest it increases with age and body size, from one or two grams in immature specimens up to six grams (about one-fifth of an ounce) in adults. Indeed, during the peak of the rut, Reeve notes that the male reproductive tract may account for 10% of the animal’s body weight and the seminal vesicles alone can have increased by ten times (up to 30g/1 oz.) from that of their regressed hibernation state.
Mating may involve the female being mounted for between three and twenty minutes, with three to six copulations consisting of ten or eleven rapid thrusts. Once mating has taken place, the male dismounts and the pair separate; there are no records of mate guarding in this species, and males don’t play any role in provisioning for the female or her offspring.
Nigel Reeve notes that hedgehogs are polyoestrous (i.e. experience a succession of oestrous cycles) and can, under favourable conditions, produce two litters in a year. Indeed, several authors have suggested that females found pregnant in September or October are probably on their second litter. Working on the Uists in the Outer Hebrides, for example, Digger Jackson found that 80% of females in his sample produced a second litter and all nine adult females that failed early in the breeding season made a second attempt. Second litters are, however, generally considered rare outside of New Zealand, where the climate is much milder than in Europe and three litters in a year have been recorded.
Late litters are relatively common in Britain and it is often assumed this is a second brood, but it is difficult to be certain. In Britain, hedgehogs rarely seem to conceive at their first oestrous and there is some evidence to suggest that they may suffer two or three pseudopregnancies (infertile copulations lead to a swelling and vascularization of corpus lutea for up to two weeks, before oestrous recurs) before they successfully conceive. Similarly, death of her young early in the year will bring the female back into oestrous and observations on captive animals have shown lactating females whose litters have been removed during the first week of suckling are highly fertile and usually become pregnant shortly afterwards. Consequently, late litters are not necessarily second litters.
In The Hedgehog, Pat Morris notes that most females will fail to breed in their year of birth, but many will breed into their sixth year. There may also be a weight contingent affecting likelihood of pregnancy and B. Morris reported that, among his captive animals at Nottingham University, it was rare for females weighing less than 600g (1.3 lbs) to fall pregnant, with the majority of females becoming pregnant at weights exceeding 700g (1.5 lbs). This may be an artefact of captivity, however, and Morris noted that the seven females in his collection that were pregnant at weights between 550g and 600g were animals brought in pregnant from the wild.
The conception rate in hedgehogs is apparently low and, in Hedgehogs, Reeve describes them as “rather inefficient at getting pregnant”. Once conception has occurred, however, it seems that pre-natal mortality (i.e. embryos resorbed in the uterus) is low; around 3%. The gestation process takes around 35 days, although brief periods of torpor may extend this and there are records of gestations lasting more than 40 days.
Following successful gestation, litter sizes range between two and eleven young (“piglets” or “hoglets”); the average in the UK is four and a litter of 15 recorded in 1997 was probably the combination of the litters of two females. Hedgehogs have five pairs of nipples, suggesting they would struggle to raise litters of more than 10 hoglets. In The Natural Hedgehog, founder of the Welsh Wildlife Hospital in Llanddeiniol, Jane Durrant, wrote that she has observed larger average litter sizes in her local town hedgehogs when compared to their rural conspecifics, which is presumably a reflection of a better habitat (higher density of food, water and suitable shelter).
The litter size can also vary on a wide geographic scale. A study of 85 hedgehog litters from across Sweden by Hans Kristiansson, for example, found the average litter size to be about five hoglets, with litters of three to six accounting for 72% of the reports. He also noted that the breeding season in Scania, southern Sweden is only about two months, compared to four months in Britain, which should favour a phenotype of females that can produce a single large litter and may explain the larger litter sizes observed here.