Overview
This lecture covers the biological process of birth across various animal groups, outlining mechanisms, stages, and adaptations in mammals, marsupials, reptiles, amphibians, fish, invertebrates, and some unique cases.
Birth in Mammals
- Birth (parturition) is the act of bringing forth offspring, triggered by hormones causing uterine contractions.
- Mammals can have precocial young (able to move soon after birth) or altricial young (dependent on parents).
- Marsupials give birth at a very immature stage; offspring continue developing in the mother's pouch.
- Large mammals like cattle, horses, and elephants generally have one offspring at a time, often precocial.
Human Birth Process
- Human birth is prepared by hormones from the pituitary gland, ovaries, and placenta, with a gestation period of about 38 weeks.
- Childbirth usually has three stages: dilation of the cervix, expulsion of the baby, and delivery of the placenta.
- Major circulatory changes at birth allow newborns to switch from placental oxygen to lung breathing.
Birth in Other Mammals
- Cattle birth involves three labor stages: finding a quiet place (stage 1), delivery of the calf (stage 2), and expulsion of the placenta (stage 3).
- In dogs (whelping), signs include restlessness, followed by passage of puppies and placentas, which are often eaten to avoid predator detection.
Birth in Non-Mammals
- Many reptiles, amphibians, fish, and invertebrates are oviparous, laying eggs with external or minimal internal development.
- Ovoviviparous animals hatch eggs internally and give birth to live young, nourished by yolk, not a placenta.
- Viviparous species, including some sharks, scorpions, and lizards, nourish young via a placenta-like structure.
Special Cases and Adaptations
- Whales, dolphins, and porpoises often give birth tail-first to prevent newborn drowning.
- Marsupial young crawl into the pouch immediately after birth and attach to a teat.
- Aphids may give live birth to clones through parthenogenesis, with continuous and rapid population growth.
Key Terms & Definitions
- Parturition — the process of giving birth.
- Precocial — young born relatively mature and mobile.
- Altricial — young born helpless and dependent.
- Gestation — the period of development from fertilization to birth.
- Ovoviviparous — animals whose eggs develop inside the mother and hatch at birth; no placenta.
- Viviparous — animals that give birth to live young, nourished via a placenta.
- Oviparous — animals that lay eggs, with embryos developing outside the mother's body.
Action Items / Next Steps
- Review animal reproductive strategies (precocial vs. altricial, oviparous, ovoviviparous, viviparous).
- Study the three stages of labor in humans and other mammals.
- Compare circulatory changes at birth in mammals.