We have called the two extremes in reproductive behavior, Brooders and Spawners. This lesson is intended to show the validity of each strategy and their relative advantages and disadvantages.
Brooders - "K-selection", have fewer offspring but invest more time and resources in insuring survival, both in gestation and rearing.
Spawners - "r-selection", have numerous offspring to overwhelm the odds. This means that few resources and minimal energy are invested in the survival of each individual.
- about 20 minutes
Humans have a reproductive strategy we will refer to as "brooders", referred to as "K-selection" in population biology terms. We have relatively few offspring that we care for, for a long period of time. Sea urchins use a strategy we will refer to as "spawners", referred to as "r-selection" in population biology terms. Large numbers of offspring are produced and receive little parental care. We depend on a variety of species for our survival and those species have developed different survival strategies. If we can not "put ourselves in their shoes" we may incidentally contribute to the extinction of an important species. Consider how many fish we can safely take and insure that there will still be enough fish for the next year.
Consider human reproduction. Consider the adaptive advantages and disadvantages of few or many children. What happens when the number exceeds the resources available?
In later lessons we will use a "spawner", the sea urchin. What we will learn will still have implications for the human population.
The whole gambit from the most prolific spawners to the most protective brooders are found and are successful. This is best represented as a spectrum rather than a two column list.
- Brooders – care for young, generally few offspring at once, internal fertilization - shelled eggs or live birth.
- Spawners – little or no care for young, many offspring, external fertilization and external development, young dispersed widely, can repopulate a niche quickly.
You might expand this list as a class activity. There is no "hard and fast" rule where along the line to place each species. It all depends how heavily you weigh parental care and numbers of offspring. For example: The octopus can have up to 150,000 young, but the female does guard and take care of the eggs for 50 days.
- Overhead projector or handout
- Clear and empty space in the center of your room
Activity: Have the all of your class but one or two line up on one side of the wall. The chosen ones are the predators. At the signal the rest of the class has to get from one side of the room to the other without being tagged by a "predator". What happened? How many got through? Why?
Activity: Have the largest person and the smallest person in your class pair up. The larger is the "protector parent" and the smaller is the offspring. Use only one predator. Have the protector escort the offspring across the room, preventing the predator from getting close. What happened? Why?
The sea urchin egg is the same size as the human egg. The female sea urchin produces several million eggs per year, but the adult is much smaller than we are. The "gonad" of the sea urchin can comprise up to 80% if the mass of the urchin during breeding season. Because the sea urchin egg is fertilized outside the body of the sea urchin, not all of the eggs will get fertilized. From that point on the young embryo is totally at the mercy of the sea. There are many organisms that will eat the young sea urchin embryo and later the young sea urchin.
The human female produces roughly one egg per month and can have roughly one offspring per year. The gonads are very small in relation to body size. Because fertilization and development occur inside the body of the female, there is a much greater likelihood of the young embryo developing into a human child. From birth to adulthood the young human is cared for and protected, increasing its chances for survival.
Even as brooders, we humans are doubling the world population every 35 years (currently about 6,000,000,000 people: see Paul Erlich's Population Bomb).
Another example of exponential growth is the Chinese story about a monk that only asked that the King pay him back by placing one grain of rice on the 1st square of a chess board and 2 on the next, 4 on the next, etc. There being 64 squares this means the final answer is (2^65) -1 grains of rice or approximately 37,000,000,000,000,000,000 grains of rice! This works out to be app 1.6 x 10^15 lbs of rice or 800,000,000,000 tons (that's 800 billion tons!). And this is assuming that the population only doubles each generation. What if they tripled or more? In asexual reproduction a bacteria can double every 20 minutes. How many would you have in one day, one month, one year?
Imagine we were a squid with 300 babies each, or an octopus with 150,000 babies, or a mushroom with millions of spores. How long would it take before we were covered in mushrooms? Fortunately, most of the offspring will die in some way, but this does show how quickly an organism can take advantage of new resources.
see this table of some calculated values, or try the Generation Calculator:
No organism can continue to reproduce unchecked, even humans. We have seen two general strategies for dealing with sexual reproduction. An organism can depend on numbers of young to overwhelm predators and "beat the odds" or, with extensive care and protection of the young, the species will survive even though it produces a limited number of offspring.
In later lessons we will be studying sea urchin fertilization and development. Sea urchin development is very similar to our own at the earliest stages. We will be taking advantage of the fact that the sea urchin is a "spawner" and produces many eggs that are released into the environment to be on their own. We can use these many embryos to study fertilization, development and environmental effects. (And humans are increasingly responsible for changes in that environment.)
- What do you see as the advantages and disadvantages of each contrasting strategy?
(Spawners must devote a very large amount of energy and body mass to the production of gametes. Few will survive. Brooders must expend a lot of energy in care and feeding of offspring after fertilization and after birth. Most survive, but very few to begin with.)
- Which would you choose and why?
- Why are there in-betweens?
- Which environmental factors influence the success of each strategy? (hint: look at where brooders and spawners are found?)
(many spawners in the sea, most are smaller organisms in harsh environments, lower on the food chain)
- What are the relative costs per surviving offspring for each group?
(about the same, otherwise we would be overrun with one type or the other)
- History shows a shift in reproductive strategy in human society. 200 years ago it was not uncommon for a couple to have 10-15 children. Now, the average number of children per family in the United States is close to 2. Why? What was the leading cause of death for women 200 years ago and what is it today?
(Fewer children survived 200 years ago. A couple needed to have more children in the hope that some would survive. The leading cause of death in women 200 years ago was death in childbirth, and now it is heart disease and cancer. Life expectancy has grown from 30 years at the beginning of human existence to 70+ years today.)
(button back to "generations" program)