The most thorough recent theoretical study of the impact on density on the reaction norm for age and size at maturity is Marty, Dieckmann, Rochet, and Ernande. 2011. Impact of environmental covariation in growth and mortality on evolving maturation reaction norms. American Naturalist 177: 98-118. If you cannot measure mortality rates in the population, either change study systems or change questions.

I suppose it's basically a life control imposed by environment. Well, I tend do desagree with neodarwinism's theory that evolution is only or mainly the result of random mutations and natural selection. I think that environment controls almost everything and not only natural selection. I think there are sutil environmental mechanims that indirectely interpheres in genetics and that may be passed to gens primarily via hipotalamo. So, there is an autocontrol in populations that may explain your questions.

feed supply/nutrion is also has some effecyt on gonad maturity. High population density increase the intrespecific competition or decreasing the individual intake .

As density increases, I would expect a shift from from r to k selection. One component of this could be delayed maturity (along with lower fecundity, better quality offspring, longer life span). At a proximal level, yes, food supply is a good candidate

Maybe the reason why mortality rates among juveniles is higher than that in adults can give some indication.

The most thorough recent theoretical study of the impact on density on the reaction norm for age and size at maturity is Marty, Dieckmann, Rochet, and Ernande. 2011. Impact of environmental covariation in growth and mortality on evolving maturation reaction norms. American Naturalist 177: 98-118. If you cannot measure mortality rates in the population, either change study systems or change questions.

Haven't read the Am Nat paper, but I would imagine that the simple (evolutionary) answer to your first question is that producing offspring in a population which is at a higher than normal density would potentially lead to fewer offspring (resoource limitations), smaller offpsring (ditto), offspring experiencing higher than normal levels of competition for resources (including for mates), offspring having to feed on better defended food, etc etc. Depending on the costs of delaying, waiting to reproduce unitil the population density is significantly lower could pay off.

Thank you all for the valuable comments. They will be a great help!

Are there examples of organisms which preferentially reproduce at high densities? Perhaps species which are normally sparsely distributed, where locally high densities would alleviate mate-finding problems and reduce inbreeding...

I do not know if there is reference about it, Lawrence. But the problem is why delay maturity at higher densities. Literature shows that there is a relationship with growth rate and reproductive effort. The delay should be a consequence of the constraint effects of density dependence on these factors.

High competition for food and space can be cause of high mortality at juvenile stage in high density.

Do you have references for this comment, Win Win?

Nature would delay sexual maturity (i.e. reproductive capabilities) in a high desnity area so as to add "less density" to the herd. There are 2 other instances (among many) of Nature's presence in reproduction delay: (1) low body fat % among women is interpreted as a "starvation milieu" and pregnancy would be ill-advised; (2) during hibernation the high levels of melatonin inhibits ovulation for obvious reasons.

I think it's a consequence of abundat food and also of predators absence, the two main motors in evoluion. But, no matter you have to consider the above aspetcs of this question, it's very clear the fact that environment commands populations and its evolution.