Archive for August, 2010

I’ve touched upon life history theory earlier, in an oblique fashion, while discussing evolutionary perspectives on personality.

Life History theory posits that an individual’s life efforts can be subsumed under two headings- somatic life efforts
and reproductive life efforts. The latter relates to selection due to being able to successfully replicate one-self; the former relates to the ability of an organism to survive and thus act as a vehicle for genes that can be replicated at a later date.  To elaborate more on the life history theory I quote:

Life History Theory is a mid-level theory from evolutionary biology that describes the strategic allocation of bioenergetic and material resources among different components of fitness (e.g., calories and nutrients devoted to growth vs. reproduction). Somatic Effort anchors one end of the first dimension of this trade-off whereas Reproductive Effort anchors the other. Somatic Effort refers to resources devoted to continued survival of the individual organism whereas Reproductive Effort refers to resources devoted to production of new organisms as vehicles for survival of the individual’s genes. The second dimension of this trade-oV further partitions Reproductive Effort. Mating Effort anchors one end of this continuum whereas Parental Effort and Nepotistic Effort jointly anchor the other. Mating Effort refers to resources devoted to obtaining and retaining sexual partners whereas Parental/Nepotistic Effort refers to resources devoted to enhancing the survival of existing offspring and other genetic relatives. Thus, a life-history strategy allocates an individual’s bioenergetic and material resources among the competing demands of survival and reproduction.

I break the somatic effort into five different functions:

1. Survival- each organism needs to survive and maintain its body integrity
2. Growth – each organism needs to consume energy and grow
3. Maintenance- each organism needs to maintain the body it accumulates as a result of previous efforts
4. Development- there is developmental unfolding and stages where different needs are met at different times
5. Differentiation/specialization- the organism makes efforts to create an environmental niche for itself

I agree with the authors that reproductive effort can be broken into 3 parts:

1. Mating effort- effort to find and retain a mate.
2. Parental effort- efforts devoted to nurture offspring
3. Nepotistic efforts- efforts devoted to helping close genetic kins.

The first three stages/efforts are related to energy balance while the next two are timing related. Finally the final three are efforts proper, thus completing the eight stage model.

The life history theory says that one has limited energy, time and efforts and needs to invest these limited resources wisely- between current and future reproduction; between quantity and quality of offspring; and between self and offspring.

Let me elaborate.

Current and future reproductive trade-offs can be conceptualized as a giant trade-off between somatic effort and reproductive effort. If I reproduce now, I do not invest in my growth, maintenance, survival etc and thus have lowered future chances of being able to survive and reproduce.  On the other hand if I do not reproduce now, I can invest the excess energy in somatic efforts- granting me future reproductive benefits, but incurring the opportunity cost of not reproducing now.

Quality and quantity of offspring trade-off can be conceptualized as how best to allocate resources, once I have decided to reproduce,  amongst a litter- the size of litter (no. of offsprings), the size of individual baby at birth and the quality or survivability of offspring at birth. All these are orthogonal to each other and involve trade-offs.

The final traditional trade-off is between self and offspring- the parent-offspring conflict. Given that I am reproducing some  offsprings now, what resources should I invest in them now,  such that my future reproductive costs are not sky-high and I can survive and reproduce later and invest in later offsprings too.  Here the two sexes have different incentives when a baby is gestating- for mother later offsprings will also be hers; for father later offsprings may not be his; thus father wants the baby to consume as many resources now as possible; while mother wants to spread resources more evenly- the genomic imprinting conflict theory.

Here it is instructive to pause and note the cost of reproductive (to mother)-

1. future survival (death in childbirth – for mammals important)
2. future energy capture (pregnancy, lactation and nursing resulting in less  loco mobility and foraging)
3. future reproduction (lowered fecundity as a result of prior child birth)

The avid reader will note that the three trade-offs refrred to above relate to the three stages of reproductiuve effort – mating (now/future), parenting (one/many child) and nepotism (future related babies/unrelated babies)

There do exist of course tradeoff between somatic and reproductive efforts and it has been shown time and gain that they are inversely related; however more interesting is the fact that somatic efforts themselves are multi-dimensional; thus forgoing reproduction may not simply lead to longer lifespan; it may just result in more growth and heavier bodies.

Also important to note the concept of r and K strategies.

Let us now look at some of the life history variables and again present them is a stage fashion, with special focus on r-K strategy and how these have antagonistic and opposite effects on variables under consideration:

1. Mortality rate (survival)- low in r , high in K
2. Body size (growth) -low in r, high in K
3. Metabolic activity related lifespan (maintenance)- low in r , high in K
4. Age of sexual maturity (developmental) – early in r, later in K
5. Age of first reproduction (differentiation/specialization)- early in r, later in K
6. duration of gestation/pregnancy (mating) – lesser in r, greater in K
7. no. and size of offspring in a litter (parenting)- more number and lesser size in r, smaller no. and bigger size in K
8. no. of reproductive events (nepotism)- semelparous or one reproductive event only in many r; iteraparous or multiple reproductive events in most K

Another way of parsing the data is to link these to developmental and evolutionary tasks and personality traits, as I had done earlier:

1. Foes: Survival
2. Food: Growth
3. Friends: Maintenance
4. Maturity: Development
5. Mate selection: specialization/differentiations (displays of creativity etc)
6. Mate retention: mating long term for future reproductive benefits
7. Parenting: parental efforts for ensuring well being of offsprings
8. Nepotism /altruism: favoring kin/ non-kin to ensure inclusive fitness

The last three are generally referred to as dilemmas of parental investment- balancing reproduction, well being of offsprings and inclusive fitness via nepotism with somatic or self fitness.

Its also instructive to note that many ecological variables affect the somatic as well as reproductive effort distribution (the r and K strategy are under ecological constraints to an extent).

For ex, changes in ecology like Mortality hazard (foe related) , food availability (food related) , density of con-specifics (friends related) , shortage of food during critical developmental window( maturity related) or differential habitats leading to different genetic variants (niche related) all lead to changes in these life history variables sometime affecting growth, sometimes reproduction, sometime lifespan etc etc. For eg caloric restriction may lead to increased lifespan etc etc

The above life history analysis was applied to organisms, but the same can be extended to brains and neurons-the following processes are involved in neuronal life history (neural Darwinism)

1. Neuronal survival
2. Neuronal growth
3. Neuronal maintenance
4. Neuronal developmental and
5. Neuronal specialization/differentiation.

In all of the above neurotropins or growth factors are instrumental and drive these processes. Whether or not this applies to neuronal level, but to organism level life history theory makes a lot of sense. What do you make of it and its fit with the 8 stage evolutionary theory?

Graf, M., Cellerino, A., & Englert, C. (2010). Gender Separation Increases Somatic Growth in Females but Does Not Affect Lifespan in Nothobranchius furzeri PLoS ONE, 5 (8) DOI: 10.1371/journal.pone.0011958
FIGUEREDO, A., VASQUEZ, G., BRUMBACH, B., SCHNEIDER, S., SEFCEK, J., TAL, I., HILL, D., WENNER, C., & JACOBS, W. (2006). Consilience and Life History Theory: From genes to brain to reproductive strategy Developmental Review, 26 (2), 243-275 DOI: 10.1016/j.dr.2006.02.002

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