Introduction

The dromedary, also known as Arabian camel or Camelus dromedarius,distinguishes itself from the Bactrian camel by its single hump. The hump is composed of fat bound by fibrous tissue, acting as food and water storage in times of need and its size varies with the nutritional status of the camel. It becomes smaller and leans to one side during times of starvation. The hump gives camels their legendary ability to travel up to 100 desert miles (161 kilometers) without water so they can walk five, seven days and can lose a quarter of its body weight. When water is available, dromedaries can drink 200 litres in 3 minutes.

Geographical location

This species is able to survive in hot temperatures (49°C) that is normally lethal to other species so it is located in arid regions of the Middle East through northern India and in Africa, especially in the Sahara Desert. It has also been introduced to arid regions of central Australia. It is only found in areas with long dry season and short rainy period.

Economic and social relevance

Dromedaries have an important social and economic role. They are potentially a useful and economically important addition to food supply chain, in terms of milk, meat and other products. (Ahmad et al. 2010:191). They are also used as transportation for humans and cargo and in competitive sport racing.

Importance for research

Due to their increased economic importance, studies into the camel are becoming ever more important. If the camel is to become a significant and viable source of income for certain countries, a greater understanding of their complex reproductive physiology is required. Understanding reproduction is essential in maintaining herd numbers, breeding for certain qualities and milk production; making the commodity financially successful and sustainable.

Male dromedary reproduction

Camels are seasonal breeders, which results in seasonal changes in hormone levels and morphology.

Sexual behavior and maturity

The male puberty is not well defined and they become sexually mature/active between the age of 3-5 years and can continue until 20 years of age. The breeding season of camels can vary but is generally during the period of low humidity, low temperature, and increased rainfall. These seasons are best known as rut. The main characteristics of this season are the increase in androgen levels in addition to a strong sexual desire. Moreover, the male dromedary acts aggressively towards other males, or even towards people, during the breeding season. Due to this continuous stress the camel can face a reduction in food intake and frequent diarrhea resulting in a loss in weight - sometimes up to 35 %. Sexual behaviour is also characterized by exteriorization of the soft palate, known as the dulah, occuring every 15 - 30 minutes alongside loud gurgling and roaring sounds becoming more frequent as they get more and more excited In addition we can notice urine spraying and smudging of poll gland secretions as well.

Hormones of male camel reproduction

Testosterone and LH

Testosterone plays an important role in the seasonality of reproductive physiology of the male camel. Endocrinological evidence shows an increased secretion of testosterone during certain seasons of the year. In Morocco, for example, we can observe an increase from 2 ng/ml in October to 24 ng/ml in January and return to 4 ng/ml in May concerning testosterone levels.

Testosterone is released by Leydig cells in the testis of the camel as a result of pituitary secretions of LH (luteinizing hormone). Higher testosterone levels in the rutting season is due to increased synthesis and release of testosterone either, through an increased sensitivity of Leydig cells to LH, or an enhanced secretion of LH from the pituitary gland or both.

FSH

Pituitary and circulating FSH (follicle stimulating hormones) levels are affected by season too. FSH is an important hormone for the male in terms of spermatocytes in the Sertoli cells of the testis to undergo the first meiotic division. It also plays a role in the creation of ABP and is critical in spermatogenesis. The FSH increases during winter to their maximum then drop to a minimum during summer before increasing again in autumn.

Prolactin

Serum prolactin (PRL) levels are high during the non-breeding season and decrease significantly in the rutting season. For the male the deep understanding and rigorous researches concerning the reproductive system of the dromedary are crucial because of infertility caused by the hyperprolactinemia in male during the non-breeding season.

Sexual morphology

Testis

Seasonal morphological changes are marked in the increase in size of the testes, due to the increase in size of Leydig cells and diameter of the seminiferous tubules. Spermatogenesis, is at the highest activity during the rut season, although it never stops during the year. The testes of the camel are located in the perineal area close to the body.

Accessory glands

Camels do not have vesicular glands, however, fructose and citric acid concentrations in the seminal plasma are fairly high. The seasonal changes in weight of the accessory genital glands are correlated with testicular activity. However, the most characteristic change in the dromedary during the mating season is the increased in the secretory activity of the poll glands which gives a special smell to the rutting male camel. Poll gland weight can increase from 40 - 100 g in the non-breeding season to 200 - 240 g in the rutting season.

Female reproduction

Camels are seasonal reproductive species, sexual activity is during a short period know as the wet season.The most important aspect of female reproduction in the dromedary is that they are induced ovulators.

Female dromedaries have their sexual maturity at 2-3 year old, whereas in males it is older. However even though the maturity is young, females are not bred before 4 years. This could be because fecundity increases with age even after sexual maturity. They give birth to young once every two years. Breeding is during winter or rainy season corresponding to the availability of food. Once the season has started, they come into heat every 20-25 days. Outside of this (breeding season) mating activity stops so ovaries are inactivated or have small follicles.(Mukasa-Mugerwa, 1981)

During the estrous cycle the females tend to becomerestless, seeking the company of males and bleat continuously. There is no luteal phase in dromedaries, ovarian activity is mainly follicular. The corpus luteum is thus only observed during pregnancy. One or three CL may be found on the same ovary (Shalash).Two researchers found that camel’s pregnancy is almost exclusively left-sided (99%) because there is more ovulation from the left ovary than the right one. (Shalash, Musa and Sineina) So the left-horn of the uterus is larger.

Induced ovulation

The most important and significant aspect of the female camel reproduction is that they have an induced ovulation (evidence= corpora luteum formation) so can not be ovulated without coitus (Mukasa-Megerwa, 1981). It makes artificial insemination of the camel difficult and research into this aspect of camel reproduction is economically very important. Ovulation has been seen to be influenced by the release of factors from the male semen (seminal fluids)(Ratto et al., 2012). Ovulation occurs 24-48 hours following coitus as a result of the release of LH. If there is no fertilization the follicles will regress after a period of time.(Mukasa-Megerwa, 1981). The process of copulation does not only influence ovulation and a protein called the “nerve growth factor” has been established as the most important influence on this.

Nerve Growth Hormone factor (NGF)

Scientists have recently identified a substance in semen of certain species, such as the camel and llama, which elicits spontaneous ovulation in the female, it is called nerve growth factor beta (NGF-β). NGF-β is neurotrophic protein, which is related to nerve development in mammals. NGF-β has been observed to have a similar chemical structure to that of the previously identified ovulation inducting factor (OIF). (Richards, 2012). OIF had formerly been isolated from semen fractions as stimulating the release of luteinizing hormone (LH) and eliciting ovulation (Ratto et al., 2012). The existence of OIF in seminal plasma was confirmed in experiments by Adams et al., 2005.

Recent studies into NGF

Recent investigations carried out by Ratto et al, 2012 into OIF of spontaneous ovulators revealed through MALDI-TOF (Matrix-assisted laser desorption/ionization, used in mass spectrometry to identify chemical structures), that the molecular mass of OIF was 13,211Da and it has a 12-23 amino acid sequence. OIF was shown to be a homolog to human, bovine, murine and porcine sequences of NGF- β. Further analysis, using x-ray diffraction to discover the entire sequence and structure revealed OIF as NGF-β. Blot analysis of seminal plasma further corroborated the findings through immunorecognition of OIF by using mouse anti-NGF. Also, when NGF- β from mice was administered to induced ovulators it led to ovulation. They concluded that the OIF was in fact NGF- β and that it was highly conserved.

Role of NGF-β in induced ovulation

The effects of NGF-β found in the seminal plasma is believed to work on ovary function through endocrine action. Previous studies proposed the notion that ovulation worked through mechanical (process of copulation) and paracrine way, with specific target organs. However, more current research points towards the endocrine pathway, which has been supported by many experiments. Intramuscular administration of seminal plasma has shown a high rate of ovulation compared with a lesser rate after intrauterine treatment with curettage of the endometrium (mimicking the effects of endometrial inflammation following copulation) and no ovulation at all after intrauterine administration with saline (this shows that mechanical stimulus of the female reproductive tracts does not elicit the ovulation). Intermuscular or intertrauterine administration of seminal plasma results in a dramatic increase in LH leading to ovulation around 30 hours later (Ratto et al., 2005).

Process of NGF-β entering the blood steam

NGF-β enters the blood stream through excoriated endometrium as a result of mechanical damage during copulation. It is believed to be facilitated further by post-coital hyperemia (increased blood flow) of these tissues (Ratto et al., 2005).

Endocrine effects on the hypothalamus and hypophysis

It has been proposed that NGF-β works through hypothalamic (indirectly stimulating LH release) and hypophyseal stimulation (direct). The NGF-β stimulates the release of GnRH (gonadotropin-releasing hormone) in the hypothalamus, which stimulates the release of LH in the hypophysis. NGF-β passes the blood-brain barrier to stimulate the hypothalamus. However, the process is unknown about the responsiveness of the GnRH neurons to NGF-β. The hypohysis is not protected by the blood-brain barrier, so there is a direct effect of NGF-β on the pituitary gland. NGF-β works through binding to tkrA receptors found on LH-containing gonadotrope cells of the anterior pituitary gland (Ratto et al., 2012).

The process by which the male stimulates the hypothalamo-hypophyseal system of the female through NGF is somewhat unique to induced ovulators such as the camel and llama.

Pregnancy

Pregnancy in the camel lasts around a year between, 11 and 13 months. A behavioral sign of pregnancy is that a female will curve their tail (tail “cocking”) when approached by a male.A female with mammary gland hypertrophy means that she is pregnant. Presence of one or more well-developed CL is suggestive of pregnancy.

Pregnancy hormones

Progesterone and estradiol are 2 sex steroids hormones important in female.

Progesterone There is a rise of progesterone around 5 days after mating, and drops suddenly before parturition. The same time as progesterone increases there is an increase of LH and PGF2 alpha (immediately after copulation).

Estrogen

Plasma 17-beta estradiol is very low during the second day after ovulation,it starts to increase by day 290 of pregnancy and reaches a maximum value around day 22 before parturition. Afterwards it decreases gradually on the day of parturition. During the first week post-partum there is a slight increase of this hormone but not very significant.

Parturition

Most important sign is the relaxation of the sacrosciastic ligaments. Between 3 and 5 hours before deliver the female will show an agitated behavior. Generally, delivery takes about 24 minutes (in lying or standing position). The vulva becomes swollen and she will seek solitude from other camels.

Stages:

1. Appearance of water bag
2. Front of legs will appear
3. Then the head will be passed
4. During the last stage the placenta will be shed with the newborn. In other animals the placenta would usually be passed after birth.

If the female is in lying position after delivery, she will stand and the umbilical cord will be ruptured. Camels are the only ungulates which do not lick their newborn but will help newborn find the teat. They will dry the newborn with a sack or straw. Camels generally never have more than one offspring.

Conclusion and future research opportunities

There is potential for further study into the complex processes by which NGF-β works in the induction of ovulation in the camel. For future agricultural management of breeding and reproduction of the camel there are potentialities that the NGF-β can be utilized for artificial insemination; if indeed the mechanical process of copulation is not responsible for ovulation. Camel farmers have the opportunity to breed at any time of the year, as long as correct shelter and food is supplied to the animals. In this way, the phenomenon of induced ovulation can be exploited and perhaps improve the productivity, genetic health and sustainability of using camels as a viable economic resources for countries today and for future generations to come.

References

1. Adams G.P, Ratto, M.H., Huanca W. and Singh J. 2005. Ovulation-inducing factor in the seminal plasma of alpacas and llamas. Biol reprod. 73: 452-7.

2. Ratto, M.H., Huanca, W., Singh, J. and Adams., G.P. 2005. Local versus systemic effect of ovulation-inducing factor of the seminal plasma of alpacas. Reproductive Biology and Endocrinology. 3: 29.

3. Ratto, M. H., Leduc, Y. A., Valderrama, X. P., van Straaten, K. E., Delbaere, L. T. J., Pierson, R. A. and Adams, G. P. 2012. The nerve of ovulation-inducing factor in semen. Proc Natl Acad Sci. doi: 10.1073/pnas.1206273109, 2012.

4. Richards, S. 2012. Ovulation Induced by Nerve Growth Factor. Accessed online: http://www.the-scientist.com/?articles.view/articleNo/32506/title/Ovulation-Induced-by-a-Nerve-Growth-Factor/

5. http://www.vets-net.com/Default.aspx?page=pages/news/NewsItem.aspx&query=QMitemEQ146

6. http://animaldiversity.ummz.umich.edu/accounts/Camelus_dromedarius/

7. http://biomedicum.ut.ee/sjlas/36_1_19-29.pdf

8. http://placentation.ucsd.edu/camfs.htm

9. http://www.ilri.org/InfoServ/Webpub/fulldocs/MONONO5/Reproduc.htm

10. http://www.lrrd.org/lrrd23/8/abdu23178.htm