Reproductive parameters of donkey jacks undergoing puberty
Introduction
In Europe the donkey has been the livestock species most affected by industrialization, depopulation of rural districts and mechanization of agriculture; this has led to a population decline of up to 80% in the twentieth century. Today, most of world donkey breeds either lack a population census, or are endangered or critical (Food and Agricultural Organization, 2014). Interest in donkey breeding, however, is now increasing both in an attempt to preserve biodiversity and for novel uses in the fields of milk production, social activities (onotherapy) and tourism (trekking). Semen preservation and artificial insemination may be used both to preserve precious genetic material and as an aid to breeders, in order to reduce inbreeding and improve selection of desired traits. For these reasons knowledge of donkey jacks’ reproductive physiology, including age at which they reach puberty and changes occurring during that time are of importance.
Puberty is the transition period following an increase in frequency and amplitude of GnRH secretion. It is completed when animals are able to release gametes, to show sexual behavior, and to successfully reproduce (Hafez, 1993; Senger, 2003). Spermatogenesis is dependent on the presence of testosterone and other hormones. In stallions, puberty was considered attained either when testosterone significantly increased (Brown-Douglas et al., 2004; Dhakal et al., 2012) or, as in bulls (Wolf et al., 1965), used the criteria 50 × 106 spermatozoa with a motility ≥10% in the ejaculate (Naden et al., 1990). The first statistically significant rise in testosterone appeared in spring, at the age of 7–8 or 11–12 months in autumn or spring-born colts, respectively (Brown-Douglas et al., 2004; Dhakal et al., 2012). Semen collections, however, were not attempted in these studies. Naden et al. (1990) described two serum testosterone rises, a small one at 40–50 weeks and a dramatic one at 75–80 weeks, during the spring of the animal’s second year, and 50 × 106 spermatozoa with a motility ≥10% were present in the ejaculates at 83 ± 2.9 weeks (range: 13–26 months) (Naden et al., 1990). This was in agreement with observations in Quarter Horses (12–23.6 months), although the mean age described in this breed was lower (55 weeks) (Cornwell, 1972). To our knowledge there are no studies on testosterone concentration and ejaculate characteristics during development and puberty of donkey foals, thus it is not known when they may first be employed as studs.
Testicular growth is also dependent on the endocrinological changes occurring during the pubertal period. In stallions, this growth has been described to occur from 42 weeks of age and to continue linearly thereafter, up to 96 weeks (Naden et al., 1990). In donkeys, the average volume of testicular parenchyma increased three folds from 1.5 to 5 years of age (Nipken and Wrobel, 1996), however nothing is knows about earlier changes. B-mode ultrasonography allows examination of the scrotal content, it can also be used to take precise measurements of the testes and evaluate testes structure and presence of abnormalities (Love 1992). The association of color and pulsed Doppler ultrasound enables evaluation of vascularization and blood flow. Physiological factors affecting testicular artery blood flow, including age have been evaluated in stallions (for a review see Ortega-Ferrusola et al., 2014). In humans it has been shown that resistivity index is lower in adults compared to pre-pubertal children (Paltiel et al., 1994). Whether this is true for equids, and thus whether Doppler could be used to evaluate peri-pubertal events, is not known, as no such studies have been performed in pre and post-pubertal colts or donkey foals.
The aim of this study was to evaluate the age at which male donkeys reach puberty, and to characterize age associated changes in testicular size, testicular blood flow, serum testosterone concentration and semen quality.
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Animals
The study included five Amiata male donkeys, born between May and July 2012 and kept from birth and for the period of the study at the Department of Veterinary Sciences, Pisa University. During the period of the study the foals were maintained all together in a paddock, under natural weather and light conditions, and fed with hay ad libitum and horse food (Equifioc, Molitoria Val di Serchio, Lucca, Italy). The study was conducted between November 2012 and June 2014. At the end of the study, the
Ultrasound examinations
Testicular width was significantly affected by age (P < 0.0001); after an initial plateau (up to 10 months), it increased progressively through the study. Statistically significant changes were observed between 6–10 and 22–24 months (P < 0.05; Fig. 1).
In three of the young donkeys’ testes no abnormalities were observed during the ultrasound examinations. In one animal a small anechoic structure (a testicular cyst or a tubular ectasia of the rete testis), localized at the cranial pole of the
Discussion
This is the first comprehensive study on male donkey puberty, which was attained between 19 and 20 months of age, with a mean age of 19.5 months. The donkeys included in this study were born between May 29th and July 4th 2012, and all became pubertal between January and February 2014, thus after their second winter solstice and when daylight length was increasing. Four donkeys showed normal mounting behavior and ejaculated in the artificial vagina after their first winter solstice (July–August
Conflicts of interest
None.
Acknowledgements
The study was funded by University of Pisa (Fondi di Ateneo), Regione Toscana and Ente Terre Regionali Toscane. We are grateful to Regione Toscana and Ente Terre Regionali Toscane for allowing us to employ the animals for this study, and to Corinne Laclie for hormonal assays.
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