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Science Notes

 

..

tapirs?

 

Reproduction
on Land

 

Reproduction on dry land presented problems for animals who were used to laying their eggs in the sea water. Over millions of years the animals developed a number of different reproductive strategies.

Reproductive Strategies

Animals that live in water release their eggs into the water, where the eggs are fertilized. The embryos develop in the eggs. Often the parents swim away, and the multitude of small creatures in the water provide food for the hatchlings. However, just as the plants had to find new ways to reproduce on land, animals, too, had to invent new structures and strategies.

Exothermic (cold-blooded) insects and spiders lay many small eggs. Some of these eggs hatch in a few days, some can lie dormant over the winter, allowing new insects to hatch in the spring when there is plenty of food. The organisms that hatch from the eggs generally do not look like their parents: the insect life cycle has four phases, egg, larva (caterpillar or grub), pupa, and adult.


Amphibians
 Cold-blooded amphibians, such as frogs, toads, and salamanders, live on land and breathe air but return to the water to lay their eggs. These eggs are enclosed in a membrane and contain stored food (the yolk) to fuel the embryo's development. These eggs start to develop right away. These animals begin life in the water as tadpoles (or newts if they will be salamanders) and metamorphosize into air-breathing adults.


Reptiles

lizard
 Cold-blooded reptiles, such as lizards, crocodiles, snakes, and turtles, developed eggs that could survive out of water. These eggs had a leathery outer covering that protected the embryo and the yolk sac from dehydration. Most of these animals lay many eggs, bury the eggs in warm sand, and walk away. The embryos develop, hatch, dig their way out of the sand, and start looking for food. An interesting fact about reptile eggs is that the gender of the hatchlings is determined by the temperature in which the embryos develop, not by X and Y chromosomes.


Birds

Warm-blooded birds developed the hard-shelled egg. This provided better protection from would-be egg-eaters and allowed the developing embryo to get oxygen from the air through pores in the shell. However, the embryo in this egg required higher temperatures in which to develop.

Birds became attentive parents, first keeping the eggs warm and then feeding the hatchlings. With parental care, demands on the parents increase and the chances of the young surviving also increase. Unlike reptiles and amphibians, birds lay only a few eggs, but they are more successful in raising their young to maturity than reptiles or amphibians.

Because birds fly, they cannot afford to carry much extra weight. Having the young birds in a nest allowed the adults to fly efficiently even as the young birds attained their mature size and weight in the nest.


Monotremes
   Warm-blooded mammals and monotremes developed a different strategy. They shelter the developing embryo inside their own bodies and then nourish the baby with milk. Monotremes, such as kangaroos and koala bears, allow a fertilized egg to begin development inside their bodies. The baby is not attached to the mother, so it cannot grow very big. It soon emerges, but in a very undeveloped condition.
 The baby clings to the mother's fur and climbs up into her pouch, where it attaches itself to a milk-giving nipple. It completes its development in the pouch, warmed and fed by the mother's body. When it is fully developed it begins to come out of the pouch and start to explore the world.


Placental Mammals
 Placental mammals have wombs. A womb is a special structure in the mother's body. Fertilized eggs can implant themselves in the womb and receive nutrients, oxygen, and antibodies from the mother's blood-stream through a special structure called a placenta. The placenta also allows the mother's blood to carry the embryo's waste products away.

  Being in a womb allows the young organism to develop in a safe environment over a pre-determined period of time.   It can be born when food is plentiful. Once born, the youngster is nourished with milk. This leads to bonding between mother and child, and makes it possible for the baby to learn from its mother.


Photograph from a Corel CD-ROM : for viewing only, not for downloading.  
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Copyright © 1999.   Elizabeth Anne Viau and her licensors.  All rights reserved. This material may be used by individuals for instructional purposes but not sold. Please inform the author if you use it at t eviau@earthlink.net