The female donkey is often known as a mare or jenny, the male as a donkey stallion or jack.
Reproductive behaviours
Sexual behaviour is often more exaggerated in the donkey and stallion-like behaviour may persist in the male donkey after castration. It is recommended that, unless being used for breeding, all male donkeys are castrated between 6 and 18 months of age.
Females will start cycling regularly between 10 and 22 months old with a wide variation in oestrus duration of 2–10 days. Seasonality of ovarian activity is variable and likely to be influenced by photoperiod, nutrition and temperature. Older females will tend to show longer interovulatory intervals. Signs of oestrus observed in females include mounting (females will mount each other with the oestrous female on the bottom), mouth clapping, winking (eversion of the clitoris), raising the tail, urinating, posturing (abducted hindlegs, arched tail) and backing up towards the jack. During dioestrus, females will show a lack of interest in the male and will move away or kick if male interest is persistent.
Puberty in the male donkey occurs between 16 and 20 months, with sexual maturity at around 3 years of age; however, males may show mounting behaviours from young foals. Male sexual behaviour differs from horses in that jacks are often slow to achieve erection (10–40 min) [1] and may mount a jenny several times before becoming fully erect. Several periods of sexual interaction are usual, separated by periods where the jack will withdraw away from the jenny. Donkey stallions are territorial and can become very aggressive, especially in the presence of competing males and females in season.
Reproductive anatomy
Donkey reproductive organs are proportionally larger than horse reproductive organs and ligation of the testicular artery is mandatory when castrating donkey stallions [2]. Even slim donkeys can have large amounts of scrotal fat so care should be taken post castration that fat does not prolapse from the surgical site. Castration via the inguinal approach is recommended for large or mature male donkeys over 4 years of age. Donkeys castrated after 18 months of age are more likely to retain stallion-like behaviours.
Testing for anti-Müllerian hormone (AMH) has welfare and practical advantages over the hCG stimulation test for diagnosing cryptorchid donkeys. It only requires a single blood sample and following castration, AMH concentrations are undetectable within approximately 2 days, making AMH a useful test if there is any doubt over the success of a recent castration [3].
The jenny’s reproductive organ anatomy is generally similar to that of the horse mare although due to size, rectal examination of miniature donkeys may be challenging. AI techniques are similar to those described in the horse although the anatomy of the cervix, coning towards the caudal end [4], may make manipulation of the cervix for uterine flushing or other techniques requiring access to the uterine cavity during dioestrus difficult.
Pregnancy
Owner education is essential as many donkey owners are often inexperienced, unprepared or unaware that their donkey is in foal. Gestation is longer and has greater variability than horses and ponies; ranging from 11 to 14.5 months. The incidence of twin foaling at full gestation is reportedly higher than in horses and ponies.
Pregnancy diagnosis can include transrectal ultrasound; optimal time for early diagnosis is 14 days post ovulation, transrectal palpation from day 40 [2] and oestrone sulfate testing from day 120.
Pregnant females should be vaccinated following recommended equine guidelines, and prior to foaling, parasite prevention should be put into place including appropriate pasture management during and after pregnancy. Body condition should be regularly assessed and feed adjusted appropriately. Pregnancy and lactation are risk factors for hyperlipaemia.
Electrolyte levels in the mammary secretions can be used to predict foaling. A sodium:potassium ratio of <1 is indicative of foaling occurring in the next 24–48 h [2]. Calcium levels are less reliable but can also be used.
Jennies have a higher tendency to exhibit foal heat than horses and ponies, with higher pregnancy rates [2].
Foal management
The incidence of failure of passive transfer ranges from 3 to 40% [5]. Risk factors are similar to those found in the horse and the IgG level considered normal in horse foals is used. There is a problem with recognition of prematurity or dysmaturity when compared with horses. In horses, the covering date is usually known and the gestation period is more defined. In donkeys, especially in miniatures, the variation in gestation length can make it very hard to define a premature donkey foal [4]. Despite their thick fluffy coat (appearance of warmth and hardiness compared to the horse foal), donkey foals are not very hardy and require warmth and suitable shelter [5].
References
- S. Purdy. (2019) Small herd behaviour in domestic donkeys. Equine Veterinary Education 31, 199-202.
- The Donkey Sanctuary. (2018) The Reproductive System. In: The Clinical Companion of the Donkey, 1st edition, Matador, Leicestershire, pp 73-86.
- N. Matthews, T. Taylor, V. E. N. Blanchard South, A. E. Durham. (2017) Use of Anti-Müllerian hormone (AMH) for the diagnosis of cryptorchidism in donkeys. ECEIM Congress 2016 Abstracts. Journal of Veterinary International Medicine 31, 604-618.
- N. Matthews, T. Taylor, T. Blanchard. (2003) An overview of reproduction in donkeys. International Animal Health News: A publication of Christian Veterinary Mission 18.
- N. Aronoff. (2010) The donkey neonate. In: Veterinary Care of Donkeys, Editors: N. Matthews, T. Taylor, International Veterinary Information Service, Ithaca NY. Last updated: 29 March 2010.
