Sexual Reproduction and the Many Eggs

So, mammals give birth to live young (with the exception of the monotremes!) and secrete milk through mammary glands.  What about everyone else?

Well, let’s start with fertilization first.  There are two types for sexual reproduction: internal and external fertilization.  Species that reproduce using internal fertilization would be all mammals, most reptiles and birds and some fish.  To make things interesting, many salamanders have internal fertilization, but the sperm is deposited externally and the female inserts it herself.

Mammals may be the group that holds the “live birth” flag, but they aren’t actually the only ones that do not lay eggs.  Some species of sharks (a fish) lay eggs (known as oviparity) while others (the majority) do something called ovoviviparious.  What this basically means is that the female’s eggs are fertilized internally, where they then develop on their own (there is no placenta for the female to transmit nutrients to the young) .  After the pups (baby sharks) reach a certain level of development they hatch out of their eggs, still inside the mother, then develop a little more, and then eventually the mother gives birth to them.

Just to make things a tiny bit more complicated, some sharks also have a placenta (viviparity).  I guess being around for 420 million years gives a group of species time to diversify.

Other internally fertilizing species are birds and most reptiles.  They lay hard or leathery shelled eggs that prevent evaporation from taking place.  How long the eggs stay inside the mother after fertilization varies.  Some species deposit them quickly while others retain them until shortly before hatching.  And, like the sharks, there are a few species of reptile that have placenta similar to that of the mammals and also give birth to live young.

Just to add to this live-birth thing, let’s add amphibians to the mix.  While most lay eggs, there are exceptions.  I guess what you could say is that while there are general “rules,” they aren’t set in stone.

Most frogs and toads fertilize their eggs externally.  To do this the male often holds the female in a position called amplexus.  As the female releases her eggs into the water the male releases his sperm over them.  Most of these frogs and toads then abandon the eggs to their own fates.  But, since we’ve established that diversity is the spice of life, some frogs/toads fertilize their eggs internally and some have developed ways to guard their eggs against threats.  The Surinam toad (Pipa pipa), for example, carries the fertilized eggs in a pouch on their back until they hatch.  The gastric brooder (Rheobatrachus silus) of Australia, on the other hand, has not developed a pouch despite being surrounded by pouch-carrying mammals.  In this case once the eggs are laid the mother then swallows them!  They develop in her stomach until such a time as her offspring are capable of crawling out of her mouth and venturing off on their own.

Many fish are similar to these amphibians in that they fertilize their eggs externally.  Some of them also then leave their eggs and swim off while others too have developed ways to defend their young.  Sea horses have developed a pouch much like the kangaroo, but in this case it is the male that carries them.  Other species of fish carry their offspring around in their mouths until they are capable of surviving on their own.

While birds and reptiles have more evaporation-proof egg shells, amphibians and fish lay eggs that are dependent on a moist environment (that should be obvious for fish, right?).  Most amphibians lay these eggs in or around standing water, where they eventually hatch into little tadpoles (baby frogs and toads) or larvae (salamanders/newts).  As you may have read in my update post about my herpetofaunal survey work, frogs/toad offspring transition from their tadpole state into their adult forms through a process called metamorphism.  Butterflies go through a similar process.  Some species, however, do not need standing water and may even skip the tadpole stage, hatching fully developed.

We can’t ignore the insect in this discussion.  They usually, but not always, reproduce with internal fertilization but some of them are also capable of asexual reproduction.  They are also similar to so many other groups in that while most of them lay eggs, not all do.  Some carry the fertilized eggs inside the body until they are about to hatch, and then laying them, while others give birth to live young.  Sometimes laid eggs are hidden on the underneath part of leaves while others are deposited into the flesh of other animals.  Some emerge from their eggs looking like miniature adults while others must  undergo metamorphosis to reach this stage.

Life is not black or white.  It is filled with such diversity that even species within the same group may reproduce using different methods.  Surrounding it all, whether there is  internal fertilization or external, hard shells or soft, live births or not, is the egg, in one form or another.