Topics in Depth
The Theme of Basics of Reproduction in Animal Reproduction
Must Have Offspring…
As depressing as it seems, all animals will eventually die—unless we find a way to turn them into immortal robot creatures, that is. In order for a species to survive, it must replace these dying animals. The replacements are created through reproduction.
This need to reproduce is called a biological imperative. Biological means something related to life. Imperative means something that must be completed—no ifs, ands, or buts. A biological imperative is something that is essential for life to continue.
We do not fully understand this type of instinct, but almost every animal on the planet feels the urge to reproduce (at some point in their lives). It is hard-wired into their brain so strongly that many animals even sacrifice themselves in various ways to ensure that they have offspring AND that these offspring will survive. Some octopi have been known to give up texting for a whole week. Kidding. They always have a phone in one of their arms.
Since offspring inherit the genes of their parent or parents, this type of sacrifice doesn't just help the species to continue. It also allows the genes of the parents to continue on. Passing on genes lays the foundation for natural selection.
Parental sacrifices can occur before reproduction, during reproduction, or after reproduction. On a simple basis it might mean the physical sacrifice of being pregnant. Pregnancy requires lots of energy, and it can make animals slower and at risk to predators. At the extremes it might mean a fight to the death over a mate (or a whole beach full of mates). Other times, a male might sacrifice himself after mating. This provides the female with extra nutrition during pregnancy so that she produces the best possible offspring. Check out this video of a female preying mantis snacking on her mate.
Even more directly beneficial is the practice of matriphagy, formed by the root matr which is Latin for mother and phag which is Greek for to eat. But wait, that would mean…yep, you guessed it. The mother is eaten by her offspring. This is commonly seen in the Amaurobius ferox spider. Within a week of giving birth, this spider allows her newly hatched offspring to gobble her up. This hearty meal starts them off in great health.
Remember, the name of the game is survival of the species and survival of the family genes. They are the most valuable heirloom around.
Doesn’t this Amaurobius ferox look delicious? Maybe it's just a baby spider thing. Image from here.
Genetics and Reproduction Are Like Peas and Carrots
The key to understanding reproduction is basic genetics. All animals have a genome made of DNA which is like a blueprint of the animal. This genome is divided up into little wormlike segments called chromosomes.
These wormy little chromosomes contain all the blueprints to make someone just like you (if you are male).
Each chromosome contains smaller segments of DNA known as genes (not the denim kind). These genes contain specific bits of genetic information that come in different flavors, called alleles. Blue and brown are two alleles of the eye color gene. Different alleles are the cause of diversity among animal populations (including humans).
Different animals have varying numbers of chromosomes. Humans have 23, dogs have 39, cats have 19, and carp have 52. A cell which has one full set (one copy of each chromosome) is called haploid. A cell which has two full sets is called diploid.
Most animals have diploid cells, and some animals have even more copies than that. Humans have 23 different chromosomes. But, since they are diploid, they have two copies of each for a grand total of 46 chromosomes.
Anyhow, for successful reproduction, a new animal must be created with the correct number of chromosomes. Different animals have different numbers of chromosomes, but the goal is always the same: to produce an animal with the same number as the previous generation.
The Basics of Reproduction
There are many different types of reproduction in the world of living things. These types can be classified as either asexual reproduction or sexual reproduction. Most animals use sexual reproduction, and we promise that the bulk of this chapter will be very sexy. However, we will first discuss the basics of reproduction in general.
Basics of Asexual Reproduction
Asexual reproduction is a one-player game. There is only one parent. It passes on an identical copy of its entire genome. It is basically creating a "mini-me" or clone identical to itself (unless a mutation occurs).
Asexual reproduction is rare in animals. Some animals that use asexual reproduction can also use sexual reproduction. It just depends on the situation they are in.
It is fairly simple to get the right number of genes in asexual reproduction. Each cell in the parent has a full set of the genome. The only question is: which cells will become the offspring? Here are a few ways that offspring can be created:
Fission: This is where a parent splits into two (called binary fission) or more parts, and each part develops into a fully formed adult. Some types of sea anemones reproduce this way. Check out this video of a sea anemone splitting.
Budding: This is where a group of cells starts forming at the edge of the parent. When it is large enough, it separates (buds) into a new animal. Some types of sponges and hydras reproduce this way. Check out this video about the process of budding in a hydra.
Regeneration: This is what happens when a piece of an animal, like a starfish arm, is cut off and grows back. Sometimes, the separated piece will also grow a whole new body. Although starfish can re-grow limbs, they still practice sexual reproduction. There are some types of planaria flatworms that only reproduce through regeneration. If humans could reproduce this way, then the guillotine would be much less scary.
Parthenogenesis: This is when a female can lay eggs which develop into full individuals without being fertilized by a male. Basically, it is a virgin birth. Some animals reproduce this way all of the time (whiptail lizards), and some animals only reproduce this way occasionally (honeybees). Other animals only reproduce this way when they have no other choice (like a komodo dragon living in captivity).
Stay away male whiptail lizards, this female doesn’t need you at all. Image from here.
Basis of Sexual Reproduction
Sexual reproduction is a two-player game, and every animal needs a partner, or mate, to play with. The two parents will each pass on part of their genome to the offspring. It will then have a genome which is a combo of both parents.
As you can imagine, this makes it more difficult to produce offspring with the same number of chromosomes as the parents. If two diploid parental cells were to combine, then the offspring would have twice the regular number of chromosomes. Yikers.
Instead, the parents create special haploid sex cells, called gametes. The male gamete is a sperm. The female gamete is an egg.
When the haploid sperm and the haploid egg combine (at the moment called fertilization) they create a diploid offspring cell. Bingo. Problem solved. Sounds simple, right? Well, there is actually a LOT of work that has to happen to bring these gametes together.
First, production of the gametes is a complex process. Then, there is the well-known trouble of finding a partner to share gametes with. Finally, the act of mating and introducing the gametes together is a tricky process known as copulation (traditional internal fertilization)or spawning (if they are releasing gametes into the water). Since almost all animals reproduce sexually, we will discuss each of these processes in detail later.
Asexual vs. Sexual Reproduction
Each type of reproduction has advantages and disadvantages. We have streamlined this information into a table below. Compare these online dating profiles of asexual animals vs. sexual animals.
|Category||Asexual Reproducers||Sexual Reproducers||Winner|
|Courtship (or Dating, in some circles)||Don't have to waste time and energy courting||Courtship and mating require energy and really cut into their "me time"||Asexual|
|Spontaneity||They can reproduce whenever the mood strikes||They can't just wake up and reproduce (because they need to find a mate first)||Asexual|
|Adventure||They require a stable environment||They can adapt to changing conditions because of constant gene mixing||Sexual|
|Movement||Almost no movement required. Great for lazy animals like sponges and sea anemones||Requires movement (sometimes a LOT) to search for a mate||Asexual|
|Number of offspring||Only a small number of offspring per parent at one time||Ability to have many offspring at once||Sexual|
By a simple tally it looks like asexual is better. Why are there so many sexual reproducers? What makes sexual reproduction so great?
The advantage that sexual reproduction offers through gene mixing is HUGE. Combining genes in different ways allows organisms to use their strengths in all kinds of different environments. For those gamers our there, this is like choosing your class in World of Warcraft. Two parents can create a variety of different offspring that makes the population more diverse. Diversity can help protect the species from many things like environmental changes, diseases, and predators.
Having multiple offspring is also a major plus, but not all sexually reproducers have more than one offspring at once. An animal's size is often related to how many offspring it has. Larger animals usually have fewer offspring than smaller animals. This is because it takes more energy and more time to produce large offspring. We don't have exact numbers, but think about it this way: one baby elephant costs as much energy as about a gazillion cockroaches. Insects and other tiny things often have many offspring because they put less energy into each individual offspring.
You should now have a basic understanding of animal reproduction. In the rest of this module we will delve deeper into the crazy world of sexual reproduction since that is how most animals reproduce. We will discuss:
- Early development
The hydra is an animal known for its regeneration. In fact, it seems like it can regenerate forever. This is why scientists are using it to study the aging process.
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