Reptile Reproduction - Anatomy & Physiology
Introduction
The evolutionary development of reptiles depended primarily on them being able to discard the attachment to a watery or humid environment for survival and breeding perposes. Of the numerous features exibited by reptilia as a group, the most important in an evolutionary context are:
- Almost all reptiles are covered by a protective layer of dry skin with almost no glands. The epidermis is raised to form scales or scutes. This prevents excessive water loss, thus is essential for terrestrial existence.
- They developed a large, yolked egg, allowing the embryo sufficient food reserves to develop fully before hatching. The yolked egg is surrounded by protective extra-embryonic membranes (the amnion and allantois), serving for respiration and the storage of waste. There is also a protective calciferous shell to prevent it from drying out.
The pineal gland and the hypothalamus/pituitary gland interpret environmental stimuli into hormonal change to regulate reproduction. In temperate species, rising temperatures and increased daylight stimulates the gonads, whereas in tropical species, food availability and rainfall are more important.
Hormones of Reproduction
- The main trigger of hormones involved in reproduction is increasing light.
- Melatonin, produced by the pineal gland, is only secreted at night. Thus, production declines when the days are longer, regulating circadian rhythm.
- Melatonin stimulates the hypothalamus to produce gonadotrophin releasing hormone (GnRH).
- GnRH stimulates the anterior pituitary gland to produce leutinizing hormone (LH) and follicle stimulating hormone (FSH).
Female
- FSH stimulates follicle growth.
- LH stimulates production of sex steroid hormones, ovulation and formation of the corpus luteum.
- Oestrogen stimulates vitellogenesis of the follicles and the LH surge, triggering ovulation.
- Post ovulation, the regressing follicle becomes a corpus luteum and produces progesterone.
- Progesterone maintains pregnancy by inhibiting argenine vasotocin and prostaglandin in the uterine smooth muscle.
- When the corpus luteum regresses, argenine vasotocin induces uterine smooth muscle contraction, which is then regulated by prostaglandins.
- Argenine vasotocin has the same function as Oxytocin in mammals.
Sex Determination
Two types of sexual determination can occur in reptiles, Gynotypic or Temperature Dependent Sex Determination (TDSD).
Gynotype Sex Determination
- Reptiles differ from mammals in that the female is heterogametic (ZW) and the male is homogemetic (ZZ).
Temperature Dependent Sex Determination (TDSD)
- Occurs in many lizards, some turtles and all crocodiles.
- Sex of the embryo is not determines by sex chromosomes.
- Sex of the embryo is determines by the incubation temperature during the early and middle incubation period.
- The time when the embryonic gonad develops into either testes or ovaries.
- Different temperatures act on the aromatase enzyme complex that converts testosterone to oestradiol.
- Oestrogen then binds the its receptor on the gonads to create females.
- To create males, enzymes convert testosterone to dihydrotestosterone, which binds to androgen receptors on the gonads.
Crocodiles
- Pivotal temperatures are important.
- Produce females at both high and low temperatures and males at intermediate temperatures.
- e.g. in alligators:
- Incubation at 30ºC produces all females which hatch at 74 days.
- Incubation at 33ºC produces all males which hatch at 62 days.
- Incubation at 34.5ºC produces all females that hatch faster.
Chelonia
- Produce females at high incubation temperatures.
- Produce males at low incubation temperatures.
Lizards
- Produce males at high incubation temperatures.
- Produce females at low incubation temperatures.
The Male
Testes
- Testes produce sperm and secrete the hormones responsible for mating behaviour and secondary sex characteristics.
- Size varies with season,temperature,light and food supply.
- Right testicle adjacent to the vena cava, connected to it by tiny vessels.
- Left testicle lies intimately associated with the left adrenal gland and has its own blood supply.
Hemipenes
- Lizards and snakes have two extracloacal hemipenes.
- Lie side by side, just caudal to the cloaca.
- Blind-ended organs containing walls of blood and lymph plus a seminal groove.
- Become engorged and evert from their cavity for mating.
Intracloacal Phallus
- Chelonia and crocodiles.
- Developed the ventral proctodeum of the cloaca into a single, unpaired intracloacal phallus.
- Protruded during copulation.