In this essay we will discuss about the reproductive cycle of mammals.
Reproductive Cycles of Mammals
- Essay on the Reproductive Cycle in Chiroptera
- Essay on the Reproductive Cycle in Insectivora
- Essay on the Reproductive Cycle in Rodentia
- Essay on the Reproductive Cycle in Primates
Essay # 1. Reproduction in Chiroptera:
Patterns of Reproduction:
Majority of the bats in temperate zone are monoestrous and the annual reproductive cycle spreads throughout the entire year. In tropical bats, the patterns of reproduction fall into four types seasonal monoestry, bimodal polyestry, seasonal polyestry and aseasonal polyestry.
Seasonal monoestry exhibits two variations:
(a) Copulation late in the autumn is immediately followed by fertilization and pregnancy with the delivery of young in the following spring, Ex. Megaderma lyra lyra, and
(b) Copulation in the spring is immediately followed by pregnancy as in Miniopterus australis.
Bats exhibiting bimodal polyestry show a restricted breeding season with two pregnancies in quick succession. Such condition is observed in small pteropids such as Rousettus aegyptiacus and phyllostomids. Seasonal polyestry is continuous, breeding during most of the year with a brief reproductive quiscence. Such condition is observed in Pipistrellus mimus. A seasonal polyestry is continuous asynchronous breeding as observed in tropical bats.
Mechanics of Reproduction:
Reproduction in Male:
The position of testes in bats varies between species and with the reproductive stage. Testes may descend from the abdomen to the inguinal canal during the breeding season as in Nycteris hispida or they may descend into the postanal scrotal sacs in the breeding season.
The unpaired prostrate gland and paired Cowper’s gland occur in all species. Seminal vesicles are present only in some bats while ampullary glands and urethral glands occur in almost all species. Several species of bats have no fixed breeding season and spermatogenesis occurs throughout the year.
The males of some seasonally breeding bat species store spermatozoa in the epididymis for considerable time; however, in other seasonal breeders spermatozoa are stored in the epididymis only temporarily. In some Indian bats, spermatozoa are stored in the epididimys throughout the year.
Reproduction in Female:
In all chiropterans ovaries are enclosed in an ovarian bursa. Ovulation occurs from both ovaries and individuals show corpora lutea in both ovaries. Two corpora lutea may be present in one ovary occasionally while the other ovary may contain none. In such females, each horn of the uterus may contain a fetus indicating that the ovum before or after fertilization had moved into the other cornu for implantation.
Similar migration of the ovum from the ovary to the opposite uterine horn for implantation is a common process in certain bats. In most of the Indian bats only one ovum is released per cycle. In temperate zone bats, which undergo hibernation, copulation occurs in the autumn and ovulation in the spring.
During the winter the ovary contains only one ripe follicle, which is characterized by the hypertrophy and hyperplasia of the granulosa cells, especially the cells of the discus proligerus resulting in the near obliteration of the antrum. These hypertrophied granulosa cells show heavy accumulation of glycogen, which is transferred to the oocyte by follicle cell processes that pass through the zona and penetrate the oocyte surface. Such modifications are adaptations to meet the energy requirements of the ovarian-follicle complex during the dormant period of winter. Such conditions also occur in non- hibernating tropical bats also.
The pattern of growth and survival of the corpus luteum of pregnancy varies widely in bats. In the African bats, Nycterus and Triaenops the corpus luteum disappears very early in the pregnancy. The corpus luteum in Dermodus rotundus and other bats is large in size, develops rapidly and persists till parturition.
Progesterone is secreted throughout the period of gestation. In some bats the corpus luteum persists for several months after parturition. The variation in the life span of the corpora lutea in bats is related to the changing levels of progesterone secretion by the ovary and the placenta.
Sperm Storage in the Female Genital Tract:
In temperate zone bats, spermatozoa are stored in the genital tract of the female for prolonged periods of time. Decreasing ambient temperatures in these areas results in decrease in the population of insects fed by birds after copulation. This leads to prolonged hibernation. In tropical bats also sperm storage occurs in some species, which do not hibernate for prolonged periods. These bats store spermatozoa for much shorter periods as compared to hibernating temperate bats.
The stored spermatozoa are in intimate contact with the epithelial lining, which provides protection to the spermatozoa. Secretory products of the epithelium of the storage organs, such as glycogen in the uterine epithelium of Pipistrellus abramus and fructose in the uterine plasma of Pistrellus pipistrellus facilitate the survival of stored spermatozoa.
Gestation Period and Fetal Growth Rate:
The period of gestation in bats varies from 44 days to 7 months as in the vampire bat. In the African bat Eiolon helvum the gestation period is as long as 9 – 11 months due to delayed implantation of the blastocyst. Some species of bats show large variation in the length of the gestation period. They breed twice in quick succession.
The first pregnancy in the year starts in mid January and early February, and extends to 150 days. The second pregnancy commences in May and lasts for 80 days only. The slow growth rate and prolonged gestation period in the first pregnancy is due to the low temperature in the winter and the torpor period during January and February.
During the second pregnancy, the fast growth and short gestation period are due to the warmer climatic conditions. Differential growth rate of the fetus is also found in the fruit bat, which breeds two times in a year. The first pregnancy is for 145 to 155 days and the second extends to only 115 to 125 days. The increase or decrease in the growth rate of the fetus found in these bats is an unique feature among mammals. In some bats, the length of gestation varies with the temperature and food supply during pregnancy.
Breeding Cycle and Environmental Factors:
Tropical bats show much variation in the breeding pattern as compared to temperate bats. Majority of the tropical bats breed only once in a year during a sharply defined season, either in the autumn season or spring. Some breed during the rainy season. Geographical variation in the timing of reproductive events is observed in many bats. Only a few species reveal entirely different breeding patterns in different areas.
In bats which breed once or twice in a year, gestation period is completed either from mid March to early May or from late June to August. Parturition in these bats is correlated with the seasonal availability of food supply. Abundant food supply ensures survival of the young as well as the adults.
In insectivorous bats seasonal availability of the insects is a limiting factor. In frugivorous species seasonal availability of fruits and flowers is the limiting factor for breeding. Thus, it can be concluded that the breeding patterns of bats have evolved to take advantage of the availability of food material.
Essay # 2. Reproduction in Insectivora:
The musk shrew can be considered as a generalized representative of the insectivores. In musk shrew scrotal sacs are absent. Testes lie in shallow coelomic evaginations called cremaster sacs. Spermatozoa are produced throughout the year in the seminiferous tubules and they are characterized by the presence of a large shield shaped acrosome. Spermatogenesis occurs in two cycles.
In the first phase, the tubules contain spermatogonia and spermatocytes while the second phase is marked by the presence of spermatids. The male accessory reproductive glands consist of bilobed prostate and paired ampullary and Cowper’s glands. The prostrate is a tubulo-alveolar gland with a tall columnar secretory epithelium. Its secretion is discharged into the lumen by a process from the acinar tip of the columnar cells.
The ampullary glands are pear shaped enlargements of the vas deferens. Cowper’s glands are minute and open in front of the bulboischiocavernous muscles. Prostrate secretes fructose and citric acid. The ampullary glands secrete fructose in large quantity but citric acid in small quantity.
The reproductive system of the female musk shrew presents certain interesting features. The ovaries are enclosed in an ovarian bursa and the fallopian tubes are highly convoluted. In the non-pregnant female, the uterine horns are cylindrical and unite posteriorly before opening into the relatively short cervical canal. The distal end of the vagina exhibits a flexure, especially in the non-pregnant female. The proximal part of the vagina is dorso-ventrally flattened.
Sexual Maturity and Breeding Season:
Musk shrew attains sexual maturity by 30 days in females and by 51 days in males. Pregnant and lactating females are found throughout the year. Pregnant females are more frequently noted from February to September months. Spermatozoa are also found in the testes and epididymis through out the year. Females breed continuously while males show breeding activity from July to December.
The peak lies in August and September. Regression of the testes starts from January. Mature spermatozoa occur in the seminiferous tubules from March to May, and from September to November. During these periods there is a preponderance of the pregnant females. Females show estrous cycle throughout the year indicating that the onset and termination of the breeding season is determined by the male.
Ovarian Follicles and Corpora Lutea:
The left ovary of the musk shrew contains larger number of follicles than the right ovary but the percentage of the atretic follicles is higher in the right ovary. During mid and late pregnancy the corpora lutea retract into the ovary. Frequently, composite corpora lutea are also met with. Formation of corpora lutea by eversion also occurs in a number of mammals.
The corpora lutea of the musk shrew remain active through out the pregnancy. The corpora lutea attain maximum size after about 10 days and show little change until after parturition when regression is rapid in non-suckling females. In suckling females, moderate luteal activity remains for some time.
Musk shrews are able to maintain pregnancy without hormonal supplementation even after removal of the ovary after fifth day of pregnancy. Progesterone level in plasma does not change upto 15 days of pregnancy but on 20th day drastic increase is observed. This is followed by decrease until parturition.
Changes in Female Genital Tract:
Vaginal smears of musk shrew reveal six types of cells.
(i) Elongated cells with elliptical or oval nuclei,
(ii) Small round epithelial cells with hyperchromatic nuclei,
(iii) Large oval or polygonal nucleated epithelial cells,
(iv) Cornified epithelial cells,
(v) Non nucleated cornified cells with folded edges, and
(vi) Small round leukocytes with polymorphic nuclei.
No cyclic changes occur in the cell types of the vaginal smears and all cell types occur through out the year. However, nucleated epithelial cells occur in larger number. Normal females do not show large number of leukocytes in the vaginal smears.
Sexual Receptivity and Mating Behaviour:
Musk shrews show elaborate mating behaviour spread over a period of 3 to 4 days. This is necessary for inducing ovulation. Females continue to receive males even after removal of the ovary. Pregnant females also continue to mate with the males. Male aggressiveness plays an important role is successful coitus. Female musk shrews remain in a constant state of sexual receptivity for several weeks.
Ovulation is induced by copulation in musk shrew. Ovulation occurs after 18 hours of copulation. Embryos are equally distributed in the two horns of the uterus indicating that blastocysts migrate from one horn to the other. Gestation period is very long, probably due to delayed implantation. Occurrence of pregnant and lactating females shows the post partum estrous.
Role of Hypothalamo-Hypophyseal Complex:
Musk shrews are not seasonal breeders. Therefore, the gonadotrophic cells do not show any fluctuation in structure during the different months of the year. However, conspicuous hypertrophy, hyperplasia and intense secretory activity occur in the prolactin cells during pregnancy and lactation. In pregnant and lactating females the neurosecretory cells show normal appearance.
Essay # 3. Reproductive Cycles in Rodentia:
Annual Breeding Cycles:
Based on the breeding activities, rodents can be divided into two broad categories. The first type breed only during a specific and restricted period while the second type breed through out the year. However, the second category shows one or two seasonal peaks of breeding activity.
Most of the animals living in varied environmental conditions show restricted breeding periods. Rodents in high altitude breed only during spring and summer months when environmental conditions are most favorable for the survival of the offspring. In Kashmir, Marmota candata hibernates during the winter months and breeds immediately after hibernation in May to July.
In Himalayan forests many rodents breed in spring and summer months only when environmental conditions are optimal. During the breeding season, they breed more than once. In low mountain regions, due to less severe environmental conditions, the breeding periods are extended. Unlike cold climate, hot weather in desert areas does not restrict breeding of desert rodents.
M6st of the rodents being nocturnal and fossorial are not exposed to severe weather conditions in deserts and plain as in high altitude areas. In India, most of the rodents breed throughout the year with distinct breeding peaks. Favorable environmental conditions, abundant availability of food material, increased vegetation due to high rainfall during monsoon; favorable temperature and light exert a positive selection pressure leading to the evolution of distinct breeding peaks in most of the continuously breeding species.
Most of the Indian rodents like rats breed throughout the year with peak in monsoon season. The second breeding peak is in the months of February and March. During winter months, the rodents remain in a state of torpidity. As soon as the winter period is over, the animals awake from the torpidity and sexual activity increases leading to a peak in breeding period.
In plains and deserts availability of adequate food to young ones and the adults is the most important regulating factor or seasonal reproduction in rodents. However, reproductive periodicity in high altitude species of rodents is determined not only by the availability of food but also other factors like temperature and day length.
The fecundity of rodents is very high. Most of the species produce large number of young ones. However, the annual productivity varies within and in between different species. Rattus meltada shows higher productivity in Rajasthan deserts as compared to other areas. This difference in the productivity of individuals of the same species at two different locations is due to variable ecological conditions of the region. In deserts high productivity has adaptive value.
Development of Testes:
The testes are paired well-organized structures enclosed in scrotal sacs. Each testis is covered over by the fibrous connective tissue sheath, the tunica albuginea. Seminiferous tubules are lined by germinal epithelium consisting of small sized Sertoli cells. The interstitium is made of mesenchymal cells. The seminiferous tubules in 17 – 19 days old rat embryo consists of large gonocytes with large, round nuclei containing 1 – 4 nucleoli.
The Sertoli cells are small with irregular nuclei and coarse chromatin. Large gonocytes and small Sertoli cells continue their existence upto three days of post-natal life. By 8th day after birth, spermatogonia and primary spermatocytes appear. Tubular lumen becomes conspicuous on 10th day and by 25th day secondary spermatocytes and spermatids make their appearance. Leydig cells are formed by hypertrophy of undifferentiated mesenchymal cells and they become fully formed on day 17. In neonatal rat the number of Leydig cells abruptly declines followed by decrease in testosterone production.
Annual Reproductive Cycles and Changes in Testes:
The weight of testes undergoes changes during different months of the year and different species show changes during different seasons. Testes weight is maximum during June to October and lowest during April and May. The relation between increase in testicular weight and male fecundity is well known. Increase in testicular weight during the peak period of breeding activity of June to October is correlated to the repeated spermatogenesis and increase in sperm cell number in the seminiferous tubules.
Seminiferous tubules in relation to the reproductive activity undergo seasonal variation in their diameter. Minimum diameter of the seminiferous tubules during winter months and maximum during summer and monsoon is related to the periods of reproductive quiescence and activity respectively.
In Rattus meltada the adult males attain maximum diameter of seminiferous tubules in autumn while Mus platythrix shows maximum diameter of tubules during July to October, and minimum during March and April, which corresponds to peaks of reproductive activity and quiescence respectively.
Essay # 4. Reproductive Cycles in Primates:
Animals come into breeding condition at different times of the year depending on the stimuli they receive from the external and internal environmental conditions. If these cues or signals are appropriate and processed correctly, then breeding will take place. This process involves a complex series of changes in the body mediated by altering the concentrations of hormones in blood.
The sperm and ova then mature for fertilization. Gonadal activity cycles are simpler in the male than in the female. Males may produce the sperm through out the year but females produce ova periodically and if not fertilized, the ova survive only for less than a day. The occurrence of ovulation is conveyed to the male, the female sends out some external signals and seeks the company of the male. In mammals this period of sexual activity by the female is called as ‘heat’ or “estrus”.
The preparatory period for this process is “proestrus”. If the animal is in a quiescent state and ova are not produced for fertilization, the stage is called as “anestrus”. The period of time during which the ova are being specially prepared for fertilization is called as the estrous cycle. This period varies from four days in the rat to 27 days in kangaroos and 28 days in women.
Many animals have only one estrous cycle in a year (monoestrous) while others have several such cycles of production of ova spread out over several months of the breeding season or the entire year (polyestrous). If fertilization takes place, there will not be any more estrous cycle to follow but if fertilization does not occur there may be more estrous cycles in the breeding season.
Ovarian Cycles in Women:
The ovarian or menstrual cycle lasts for 28 days and is quite distinct from that in non-primate mammals. The same type of cycle occurs in monkeys and apes. The corpus luteum persists for a more prolonged period of the cycle than in the rat. Details of the menstrual cycle are discussed in the chapter on Reproductive strategies in vertebrates.