Question

Births, deaths, and emigration/immigration define life history

1a. What are the different strategies for reproduction of semelparous and iteroparous pecies? What environmental strategies favor each one and why? (Inlude the trade-off strategy in discussion) (Explain with examples)

1b. Describe 2 organisms that have very different iteroparous life histories. Describe the environment the organisms grow in and their strategies for growth and reproduction. Relate answer to life history for evolution.

(K-strategists are organisms that have pretty low offspring production but high parental investment. K-strategists are most likely operating in a place where the population is at equilbrium in terms of envrionmental pressures on population size, or a slow life cycle. R-strategists are organisms that have many offspring but have small parental investment, r is a value of reproduction, or fast life cycle.)

Answer 1

Semelparous species are those in which the reproduction or mating takes place only once in their life. Whole of the resources and energy available with such an organism is used up in mating.

Iteroparous species are those which are capable of mating/reproducing a number of times in their life. They do not invest whole of their energy into mating.

Answer to Q1a)

As per the information about the strategies provided in the question, K-strategists have lesser offsprings in one mating i.e. they have comparably lower gamete production per mating cycle but have more parental care which means the organisms following this would have offsprings that are weak on birth and cannot survive on their own, they need parental care which is possible only in case of Iteroparous species as the parents need to be present after the birth of offspring.

Eg. Human beings can produce at maximum triplets and that too is rare, most commonly they have one or two offsprings in one mating. The offsprings produced are incapable of survival without parents and so parental care is mandatory.  

In case of semelparous species, the R-strategy is favoured because the offsprings produced are capable of surviving on their own so parental care is not required (even the parents are not there after the birth of offspring in semelparous species). The offspring number is higher in these so as to continue the population. Also the large number of offsprings in them are required as these only when mature will be able to continue the population.

For example: Pacific Salmon which lays maximum of 7500 eggs in a single batch and on hatching the young ones can survive on their own and hence can grow witout parents.

The trade-off strategy involves factors like fertility, parental care and time of reproducing event (i.e. in early age or later age). More fertility, later age of mating and low parental care favour semelparous mode while the other ones favour iteroparous mode.

Answer to Q1b)

Organisms with different Iteroparous life histories could be the Humans (Iteroparous but capable of mating all through the year and have menstrual cycle) and Red fox (Iteroparous but can mate only once in year and have oestrous cycle).

Humans usually have less (maximum three) number of offsprings in a batch in comparison to fox which have more (4-5) offsprings in a batch. Both of these species show parental care but to different extents. Because parental care is qualitatively better in humans than dogs, the number outweighs the survival rate in the two species.

Humans have longer gestation period than foxes and so even after having menstrual cycle i.e. capability of mating all around the year will have lower offsprings than dogs with smaller gestation period.

The cycles are under control of environment as the organisms with oestrous are capable of reproducing only when the resources are available in ample amount while in case of humans such a thing does not affects as they can have the resources all the year around. For example Foxes are in oestrous in spring season so that the offspring produced can grow easily in the upcoming summers and is fully grown till winters and can survive the winters easily.

As far as evolution is concerned the mammals with oestrous cycle are on the lower branch of evolutionary tree than those with menstrual cycle as the placenta in latter has direct approach to maternal blood and the fetus can get the required nutrients providing better growth and hence more chances of survival. In contrast the organisms with oestrous cycle has such kind of placenta in which mother's body can channelize the nutrition supply to fetus in accordance with its needs so the fetus may or may not get required nutrition. In this way, the oestrous period occurs only when the nutrients are ample in environment while the menstrual cycle can occur all around year without affecting nutrition of the offspring.