Basic reproduction strategies of vertebrates


Fishes:
(1) Fishes have high potential fecundity with most species releasing thousands to millions of eggs and sperms annually.
(2) Fishes have reproductive methods, structures and an attendant physiology that have allowed them to great variety of aquatic conditions.

Amphibia:
Each of the three living orders of Amphibia (caecilians, salamanders and Anurans) have evolutionary trends towards terrestrial environments.
(1) The evolution of direct development of terrestrial eggs.
(2) Ovoviviparity.
(3) Viviparity that have been important in the successful invasion of mountainous environments by amphibians.
(4) They are the first land vertebrates.

Other vertebrates:

The reproductive adaptations of reptiles, birds and early mammals and later mammals including human are similar. The reptilian system includes shelled, desiccation – resistance eggs. These eggs had three basic embryonic membranes that still characterize fiz and mammalian embryo. A flat embryo is developed and under went gastrulation atop a huge yolk mass. The same process of gastrulation is still seen in mammalian embryos, even though the massive yolk mass has been lost.

Mammals:
(1) The mechanisms for maintaining the developing embryo within the female for long period of time evolved in early mammals.
(2) During gestation the embryo was nourished with nutrients and oxygen yet it was protected from attack by female’s immune system.
(3) After birth first mammals nourished their young with milk from the mammals glands just as primates do today.
(4) Female apes and monkeys are asynchronous breeders. Mating and births can take place over much of the year.
(5) Female mate only when in estrus, increasing the probability of fertilization. Human females show less distinctive estrus phase and can reproduce throughout the year. They can also engage in sexual activity without reproductive purpose; no longer is sexual behaviour tied to ovulation. The source of this reproductive adaptation may be physiological or a result of evolution of the brain, a process that gave humans some conscious control over their emotions and behaviours that hormones, instinct and the environment control in other animals.
(6) The separation of sex from purely reproductive function has evolved into long lasting pair bonds between human males and females (e.g. marriage) that further support the offspring. This type of behaviour has also resulted in the transmission of culture, a key to the evolution and success of human species.
(7) Humans have highest degree of parental care.

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