During the rut, rises in endogenous testosterone in entire male deer will directly affect behaviour (Lincoln et al. 1972; Chunwang et al. 2004). Testosterone rises will cause the cleaning of velvet leaving hard antler to be used as part of fighting behaviour and establishing social and sexual dominance (Lincoln et al. 1972; Muir et al. 1988; Bubenik et al. 1991). There is an association between combined body weight, animal size and testicular diameter, with overall antler size and antler score in red deer stags (Gaspar-López et al. 2010). Antler size and score appear to correlate with testicular size and seminal velocities (Malo et al. 2005), therefore likely fertility.

Red deer stags are capital breeders, losing significant condition during the rut (approximately 19.5%), due to fixation on mating and rutting behaviours over food intake. The activity of red stags increases, where males will extend their range and attempt to monopolise harems of hinds. Behaviours such as bellowing, wallowing, urinating on themselves and fighting with other males will be much more apparent (Stopher et al. 2011; Csányi et al. 2022). Aggression towards humans and other males will increase due to higher testosterone levels (Lincoln et al. 1972). In systems with fixed handling facilities, such as farms, males will often be routinely de-antlered once the antlers have cleaned but before overtly aggressive behaviour manifests. This reduces the risk of severe injury to other herd members or humans and also damage to fences and other infrastructure.

Sika stags behave similarly to red deer, marking territories and defending female groups (Miura 1983). Within their territories, sika stags may mark trees (bole scoring) and thrash trees and shrubs during the cleaning of their antlers.

Fallow deer may establish rutting stands as individuals, areas to which they are hefted, allowing females to seek them out when in oestrus. Alternatively, groups of males, with or without females, may form, sometimes referred to as a lek (Apollonio et al. 1992; Ciuti and Apollonio 2016). Behaviour, including vocalisation, parallel walking and fighting, is commonly observed in rutting males establishing hierarchy and dominance around females.

The usual oestrus cycle of deer is generally comparable to other domestic ruminants. Red deer undertake an oestrus cycle of approximately 18.5 days during the rut, if not pregnant. In non-artificially-induced oestrus cycles, typically a single ovulatory follicle is released in red deer with pre-ovulatory follicles measuring >7.5–8.9 mm in diameter (McLeod et al. 2001). A high proportion of farmed red deer will conceive within the first cycle when rutted with a fully reproductively competent stag (Audigé et al. 1999). Twins are considered rare in natural matings (<1%; Hudson et al. 1991) and higher twinning rates (approximately 5%) are observed in artificially synchronised female red deer when pregnant mare serum gonadotrophin (PMSG) is used, with higher PMSG doses increasing the twinning rate. Hinds in higher population densities, such as farmed systems, are more likely to synchronise oestrus activity within the group than those in lower population densities.

Reindeer have been observed to have variable oestrus cycles with an average length of approximately 24 days in adults (18–29 days range, Shipka et al. 2007). Primiparous animals display a shorter length of approximately 19 days, with greater individual variation (range: 13–33 days; Ropstad 2000).

Red deer have a gestation of approximately 235 days, with variability observed; the gestation length depends, in part, on the time of conception, with females conceiving later in the rut undergoing shorter gestation. A relationship of 10 days difference in conception date shortening gestation length by 1.9–4.9 days has been demonstrated (Scott et al. 2008).

The red deer placenta develops placentomes at 41 days of pregnancy. Calcification of ribs occurs at approximately 50 days, the skull at approximately 55 days and long bones at approximately 60 days following conception. By 75 days, the umbilicus is approximately 10 mm in diameter (Boyes 2021).

Roe deer undergo a diapause in embryo development following conception. The embryo will be arrested at the blastocyst stage for approximately four to five months, after which fetal development will occur resulting in an apparent gestation of approximately nine months (Aitken 1981; Chirichella et al. 2019). Reindeer will exhibit a variable gestation of 203–240 days, the date of conception being the primary factor in gestation length with later conceptions generating shorter gestation lengths (Rowell and Shipka 2009). Twinning appears more likely in reindeer, with twinning rates as high as 17.8% reported (Godkin 1986). Pseudohermaphroditism (freemartinism) has been reported in reindeer twins (Rowell and Blake 2019).

In red deer, parturition is generally a rapid process, with stage two parturition covering approximately 40 minutes (Marman 1973). Similar to other prey species, neonates are precocious and standing within 30–60 minutes. If required, synthetic prostaglandins appear to induce parturition in red deer (Fisher et al. 1994); however, this should be combined with a glucocorticoid to facilitate fetal lung maturation prior to parturition.

Deer generally require little intervention for dystocia during parturition compared to other farmed species. In red deer with dystocia due to fetomaternal mismatch, the calf is likely to be over 10 kg (D. DeBaedermaeker, personal communication). The commercial push for an increase in live weights in farmed red deer is likely to increase the risk of dystocia due to fetomaternal mismatch. Dystocia due to over condition of the hind is 2.7 times more likely in animals with body condition score (BCS) >3.5 (Audigé et al. 2001). An increase in the use of wapiti within commercial systems may increase the likelihood of dystocia if appropriate female selection is not conducted.

Pregnancy diagnosis of deer within farmed systems is routinely performed to determine reproductive performance and to identify barren deer, which may be candidates for culling. Flank scanning is rarely rewarding as a technique in deer due to the thickness of the coat and restraint being required.

Transrectal ultrasound can be performed using an introducer and a linear probe. Whilst commercially available cattle probes can be used, the length and leverage may increase the risk of rectal trauma in deer that are not adequately restrained. Specific deer probes can either be made or existing cattle products can be used in red deer with care.

Once the deer are restrained, the probe is introduced into the rectum facing ventrally. Often the first identified major structure is the bladder. The uterus of the hind is usually found as the first major structure after the bladder. Care should be taken not to mistake ultrasonographic reflections of the bladder wall as a separate structure. Visualisation of the fetus and cotyledons confirms the presence of pregnancy; cotyledons should not be used as a tool for ageing due to variations in size throughout pregnancy. In early pregnancies, the uterus may not be immediately obvious and may be found dorsally to the rectum. Visualisation of the non-gravid uterus is more difficult and identification of ovaries can be technically challenging for operators. In more advanced stages of pregnancy, the weight of the fetus may cause the uterine body to extend ventrally, making visualisation more difficult.

Fetal ageing in red deer can be estimated using the following chart (Table 11.2).

The measurement of elevated progesterone levels in reindeer is unreliable because the levels observed during oestrus are similar to those during pregnancy (Shipka et al. 2007). Rectal ultrasonography is reliable for pregnancy diagnosis in reindeer.