Thursday, April 16, 2009


In a couple of weeks I'll be attending the conference of the Population Association of America. It will be a bunch of demographers. Most of the conference is talks divided into topic-specific sessions. They don't have a session for biodemography, or really anything related to my research, but the poster-session is open to whatever topic, so I'm presenting a poster.

My title is, "Post-Reproductive Lifespan in Humans: Cultural Artifact, Widespread Primate Trait or Unique Adaptation?"

I've had fun making my poster, mostly because it is an excuse to play with Photoshop and Powerpoint instead of writing my thesis. For my poster I needed to find a compact easy way to display how the fertility and survival of a population changes with age, and simultaneously explain my methods. And I needed to be able to do that for several populations side by side in a small space. Now this is all tailored to make sense to the demographer, so it may not be that intuitive to anyone else, but I like what I've got. To clarify, in demographerese, lx means what portion of the individuals that survive to each age and mx means how fertile are individuals at that age.

By plotting mx and lx on the same graph I make it visually clear (to a demographer) that the population nears an endpoint to fertility (age M) long before it nears an endpoint of survival (age Z). I then go on to use some math and demographic methods to define good ways to measure post-fertile survival in ways that allow for straightforward comparisons between species. I call the two measurements G and S. But the fun part comes in when I use the graphical format established in Figure 1 to compare populations in Figure 2:

My hope is that having labeled and explained the parts of the graph in Fig. 1, the meaning of these graphs in Fig. 2 will be quickly obvious to the demography crowd. I'll leave you to interpret what these graphs say about post-fertile survival in humans and chimps in different environments. I can't give away everything.


jte said...

Can you give anything away? Like: remind us what "fertile" means. If my mate and I produce triplets in year 9, does that mean that we contribute "1.5" to the value of mx in that year? What I'm getting at is, how does mx ever rise above lx?

Also, why establish ages B, M, and Z to exclude the 5% tails? Is this some kind of confidence interval thing?

Dan Levitis said...

mx and lx have different units.

In fact they don't have units at all, but if they did, they would have different units. So to day that one is above the other isn't really meaningful. Also, this isn't individual fertility, it is mean offspring per female-year lived at that age. So if 44 females live to age 35 and between them they have 8 offspring between their 35th and 36th birthdays, then m35=8/44=0.18