MadSci Network: Zoology

Re: how does ratio of age of sexual maturity / lifespan vary across species?

Date: Tue Aug 1 10:49:44 2000
Posted By: Lyle Burgoon, Grad student, Pharmacology and Toxicology, College of Human Medicine, Michigan State University
Area of science: Zoology
ID: 963095668.Zo

Thank you for your question.

This is a very interesting question, and a very complex one. I believe the question you are asking is if you have a lifespan X and a lifespan Y (these would be in two different species of course), would the age of pubertal onset be different? Furthermore, you’d like to know, if one takes the age of pubertal onset and compares it to the lifespan, does this ratio ever change?

Well, I’m hoping that’s what you’re asking. My expertise does not lie much beyond animals, so I cannot really speak for plants. But what I can do is talk a little about humans (which is what I specialize in) and some rodents…for rodents let’s discuss rats (here again, something I’m familiar with).

So we have rats – a lifespan of maybe 3-6 years in the lab, and maybe 1- 2 years if they are lucky in the wild. Rats go into “heat” once every 5 days or so (we say they have a 5-day estrus cycle). Rats have a 21 day gestation period, and it takes them 21 days to before they can be weaned from their mother. From there it takes about another (you guessed it) 20- 21 days before they hit puberty (this is in the lab).

Humans are a drastically different story. Humans gestate for approximately 38 weeks (that is 38 weeks following conception). Girls tend to enter puberty before boys; the age of onset for girls is generally 10-12 years of age, boys is generally 13-15 (there is a great deal of variation). The lifespan for humans (depending on what country you live in) is approximately 68-70 years in the United States (possibly more). The average lifespan in a country such as Botswana is forecasted to be around 50 years old in the year 2020 (in 1998 it was 47), and that may be on the high side.

So why do I bring up Botswana? It’s a simple reason…Botswana is one of the countries hardest hit by the AIDS epidemic. For more information get a copy of the July 15th The Economist or goto An interesting hypothesis, human evolution at work, might be that one would expect to see, since the lifespan is lower there, advanced onset of puberty, compared to US children. However, according to UNAIDS, the average age of first sexual experience for girls is approximately 18 years of age (I was not able to find the information for US girls), and the growth rate for the population in urban centers is approximately 7%. Now then, well over half of the girls, in the year 2020 in Botswana will be infected with HIV. There is also a surprisingly high likelihood that, barring new medications, the children of these girls will be HIV+.

Thus, one could imagine a scenario where the population begins to hit a negative growth rate. Some might say that this may cause enough of a selection pressure to advance pubertal onset. Of course, the corollary to this is that the chances of the HIV+ children being born to pass on these genes, even if the mothers were adolescently advanced is very unlikely, here again, barring therapeutic intervention. Thus the lifespan of the population could be thought of as dropping, and therefore puberty should advance. It would be interesting to see, but personally I would rather not have that be the case. I would much prefer to have the HIV epidemic wiped out, as opposed to watching this horrible kind of experiment.

So, the case still remains, can we see within the species advanced pubertal onset? Yes we can, that’s not a big deal, we know it happens – pubertal onset is partially a cultural event. What about this puberty:lifespan ratio?

This is a little harder to tell. I mean, yes, there is definitely an advance of the age of pubertal onset when the lifespan of the organism is much shorter (rats vs humans). However, is the ratio different? Well, in rats and humans it’s drastically different!! Is the age of pubertal onset different between rats in the wild and those in the lab? No, and actually if it were different, the lab rats should go into puberty much later than wild rats.

So, the moral to the story is pubertal age and lifespan is related. If a species, in general, has a relatively longer life span, then yes, the pubertal onset can be delayed. If a species has a relatively short life span, then it needs to enter puberty sooner. Remember – the biological imperative that all animals must reproduce in order to be successful is always true, and reproduction is an absolute – it must occur for the propagation of the species (hence the imperative part).

I hope this answers your question. Thanks for your question.

Lyle D. Burgoon
Graduate Assistant
Department of Pharmacology and Toxicology
College of Human Medicine
National Food Safety and Toxicology Center
Michigan State University

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