Adult tigers are solitary; each male has a large home range he roams while the females have a smaller one, relating to their motherhood duties. Each home range will overlap several others, both male and female. When a female goes into heat or estrous she announces this to any available male with pheromones in her urine and a distinctive coughing call or roar.
When a male arrives, they will spend some minutes getting comfortable with one another using scents, sounds and motions to communicate. When sufficiently comfortable with the situation the tigress will present herself to the tiger. This involves placing herself in some risk as her position is vulnerable if the male decides to attack instead; she is crouched facing away from him and once he mounts her, her neck and throat will be below his mouth.
The tiger mounts above the tigress and the copulation occurs quite rapidly, usually 20 to 30 seconds, then the tigress throws the tiger off and often takes a swipe at him. After another 15 to 20 minutes she will initiate another mating.
This can go on for up to 7 days with the occasional break to hunt, which they do separately. If another tiger should arrive, the weaker of the two will leave the field, so the tigress may mate with more than one tiger. At the end of this time the tiger returns to wandering his home range, having nothing more to do with the continuation of his species until he meets another tigress in heat.
The haploid sperm of the tiger is helped to travel from the top of the vagina to the oviduct by contractions of the uterine smooth muscle and the sweeping motions of cilia of the lining epithelial cells. In the oviducts the spermatozoa undergo capacitation, which is a series of changes that increases the likelihood of the sperm successfully fertilizing an ovum.
Towards the end of the estrous period ovulation occurs, releasing ova into the oviduct where the sperm are waiting. Many sperm will begin tunneling through the outer coverings of each ovum using digestive enzymes released as part of capacitation. Once one penetrates and introduces its genetic material into the ovum, a change in the cell membrane blocks any other penetrating.
The genetic material combines in the nucleus of the cell; the fertilized ovum is now a diploid zygote. Cleavage begins almost immediately as the zygote travels down the oviduct, this is mitotic cell division. The zygote becomes a morula, a solid clump of cells and then a blastocyst, a hollow sphere of cells, by the time it enters the uterine horn. A tigress will usually have two or three but may have as many as five blastocysts that will implant themselves, randomly spaced, in the walls of the uterine horns or the uterus. The blastocyst releases enzymes when it comes to rest against the uterine lining or endometrium, that dissolve a small pit the blastocyst then occupies.
During its journey it has obtained all its required resources by diffusion from the surrounding fluid. It has now grown too large for this to suffice, so some cells of the blastocyst differentiate into what’s called the trophoblast, which grows into the placenta while the rest, called the inner cell mass, grows into the embryo.
The embryo stays connected to the placenta by the umbilical cord. The outermost cells of the placenta, where it is attached to the uterine wall, receive oxygen and nutrients from capillaries of the mother while passing out carbon dioxide and other waste products. As the embryo grows and differentiates, the placenta grows with it, into multiple layers of fluid-filled membrane. The space containing the embryo is the amniotic sac; its membrane layer is the amnion. That is enclosed in the allantoic sac; its membrane layer is the allantois. The final membrane layer outside the allantois is called the chorion.
The embryo, then fetus, forms and grows for a gestation period of 102 to 107 days. The tigress then finds a safe location for the birthing. There she moistens her vulva with her tongue. When ready she stands or sometimes lifts her leg to maximize the dilation of the cervix between her uterus and vagina. Her epiglottis cartilage closes off her larynx to stabilize her thorax cavity for each contraction of her abdominal muscles to enable sufficient pressure to eject each cub and the remains of its placenta or afterbirth. Each newborn is licked clean to free it from its amniotic sac and stimulate it to begin breathing.
The cubs will stay with their mother for anything up to 2 years before striking out on their own. Initially fed milk by their mother, then brought meat, taken to the kills and finally at around 10 months encouraged to start hunting for themselves. Unlike lions, the tigress allows her kittens to eat their fill of her kills before she feeds herself (Sankhala, 1993; Colville & Bassert, 2002).
References:
Sankhala, K. (1993) Return Of The Tiger Revised Ed. Delhi: Lustre Press.
Colville, T. & Bassert, J. (2002) Clinical Anatomy & Physiology For Veterinary Technicians. St Louis: Mosby, Inc.
