This week we are reading a few papers that establish links between life history theory and body size evolution in general and also serve to better acquaint us with the empirical patterns of body size and growth in living humans. (note: these papers are slightly different from what's on the syllabus! We substituted Crimmins and Finch with Palkovacs)
The assigned papers are as follows:
Walker, R., M. Gurven, K. Hill, A. Migliano, N. Chagnon, R. D. Souza, G. Djurovic, R. Hames, A. M. Hurtado, H. Kaplan, K. Kramer, W. J. Oliver, C. Valeggia and T. Yamauchi
2006 Growth rates and life histories in twenty-two small-scale societies. American Journal of Human Biology 18(3):295-311.
Palkovacs, E. P.
2003 Explaining adaptive shifts in body size on islands: a life history approach. Oikos 103:37 - 44.
Stiner, M. C., N. D. Munro, T. A. Surovell and E. Tchernov, Bar-Yosef, Ofer
1999 Paleolithic Population Growth Pulses Evidenced by Small Animal Exploitation. Science 283:190 - 194.
The Walker et al paper shows empirical patterns in growth and body size among contemporary hunter-gatherer populations. Pay close attention to the general relationships that this paper demonstrates. In parts of the paper there are a lot of statistics that you may or may not be familiar with. As always, do not get bogged down in the technical language. Do your best and maybe google some or all of the terms you aren't familiar with (if there are any). The discussion section of the paper is quite lucid and does a good job of clarifying the implications of the statistics in the paper.
The Palkovacs paper takes a life history approach to explaining the patterned changes in body size that are described by the Island Rule. To the best of my knowledge this is the only life history approach to the problem that has been published. What do you think of the approach taken in this paper? Is it more complicated than it needs to be? Is there anything missing or is there any extraneous detail? Remember the definition of a reaction norm that we mentioned briefly in class on Thursday. A given gene doesn't necessary code for a single static phenotype. Phenotypes can change depending on the conditions of the external environment. This capacity for change is a reaction norm. Some examples include changes in skin color in response to sun exposure or changes in growth rate in response to energy availability.
The paper by Stiner and colleagues is an archaeological analysis of data that might seem removed from what we've been talking about. But when do we see changes in human diet and what types of changes occur? What might cause these changes? What was going on with human body size at this time? Do any of the patterns we see in Stiner et al. relate to what we are learning about body size change in Walker et al. and from Palkovacs?
We have also posted a few optional papers showing that some of the same tradeoffs we've been discussing in the contexts of the Island Rule and among hunter-gatherer growth patterns are present in industrial economies. These papers are really exciting and I hope you can check some of them out, especially now that you're armed with some theoretical tools to put these patterns into a larger context. (For readers outside the class - note that you can access these papers by following the directions and hotlink on the sidebar to the right where it says 'ereserves').
Please let me know or post a comment as soon as you can if there are questions about terms and/or concepts in these papers.
I hope you all have great weekends.
Oskar
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25 comments:
Why does growth happen in 'spurts' in favorable environments? If the resources were available, shouldn't there be a more linear, consistent growth over time?
That's a good question. Another way to think about it is, why do humans wait so long to start the spurt? That's why the spurt seems odd, right (?), because it is preceded by a fairly lengthy period of slower growth (see Figure 1 in Walker et al). Growth rate actually slows down for a few years after birth. It could be that slow growth during this time before the spurt is optimal because it lowers the energetic demands on the mother, who can put the energy into older siblings. Or it may be that a lot of energy is required for brain growth and investment in immune function in the early few years of life and hence fast growth can't be afforded until these other components of development reach a certain level. Growth during the spurt is often fairly linear. Growth data for the Ache are fit very well by the mammalian growth law, for example (See Hill and Hurtado, 1996).
best,
O
The Walker article refers to larger body size in males as potential evidence for male competition for access to females, a common explanation. Is there any evidence that this is actually the case or is there possibly an alternate explanation for this?
In Stiner's paper, table 1 and 2 shows Italy population have totally different food structure during the time series. Is it caused by different environment or different geographical time period? Population of Isreal did not have shellfish in the food list. Is it caused by their location that is far from the beach?
In the Stiner article, the authors use a sudden decrease in body size of tortoises during the late Middle Paleolithic as evidence that the animals were overhunted (pg 192.) They state in the caption of Fig. 4 that the change doesn't coincide with climate change, based on Shackleton and Opdyke's oxygen isotope stages, which they include in the figure. However, based on what they've included, it looks like these might coincide pretty closely. Can we discuss this index or other methods of measuring climate change in relation to the timing of the shift in tortoise size?
Walker and collogues suggest that faster/earlier development of offspring can help bolster a better immune system earlier in order to help combat parasites and infectious diseases. They also suggest in passing that this may also be a result of size-dependent mortality, citing infanticide. This idea seems to be the inverse of what’s expected. Are there any examples similar to this where cultural practices sway biological evolution?
In table2 stiner says "†Counting unit is always number of identified skeletal specimens" for column 4 "total small game".
Is this a reliable indicator of how much small game was hunted? I know that fossils can be incomplete, and I suspect it is possible for a community of hunter gathers to eat much more small game than surviving skeletons; as long as this is proportional to the actual number then it is ok. Is this a reasonable assumption?
In the Palkovacs (2003:42) paper, he refers to a study by Case (1978) where the insular Anolis species was larger than the mainland species, even though the insular population was experiencing slower growth due to reduced resources. They attribute this to a combination of both reduced resources and to reduced predation rates. It would seem obvious that these two pressures would both change in the movement to a new area, so how can the effects of one be separated from the effects of the other?
How suitable a proxy for overall nutrient availability is adult body size, ala Walker et al? Does using a life history variable as a proxy for an energetic constraint really allow the model to address both the traditional bioanthropological and life history perspectives?
Since adult body size correlates (negatively) with adult mortality, the paper seems seems to really be looking at the interplay between juvenile and adult mortality on growth schedules, and thus growth in their model is driven entirely by risk, not energetic constraints.
Second that, Justin. In particular, both perspectives (bioanthropological and life history ideas discussed in Walker's paper,) would result in smaller adult body size, as shown in the figure in his blog entry.
The West et al. (2001) ontogenetic growth model (OGM) suggests that the organism’s growth rate (really, its growth curve) is determined by its asymptotic mass (i.e. adult body size), which they take as exogenous.
Palkovacs (going on Stearns’ 1992 book The Evolution of Life Histories) says that growth rate will be determined by resource availability, and the ‘stopping point’ reaction norm will be determined by mortality. (This reaction norm, if we hold it still, says that faster growers will end up bigger, and will achieve that adult size at an earlier age.) Put these together and you have equilibrium adult body size.
I'm trying to think through whether/where these models connect.
I think the OGM is taking the ability to process energy (dependent on current body size) as the only constraint on growth at any one point in time. If we were to give the animal less food than this, that food limitation would become the binding constraint, and the organism's growth curve would look different (right?).
So presumably, if you allow selection to act on this animal, and it's consistently starved, it would decrease its asymptotic mass (adult body size), so that its metabolic system was ready to deal with the amount of resources it actually receives, rather than a greater amount. (It might do this by turning up (!) the in-vivo cellular metabolic rate, Bc in the OGS?)
So I think that the imaginary creature in the OGM would behave as Palkovacs/Stearn expect in response to resource limitation.
...The difficulty is that the models take different things as exogenous, and see how the other things are determined...
Now what about the Palkovacs prediction of [decreased mortality --> age at 1st reproduction shifts later --> bigger adult body size]?
Ok, it's interesting to note that he doesn't say that growth rate will adjust in response to decreased mortality. Only the stopping-point does. But the OGS suggests that if something shifts optimal adult body size, growth rate will shift. Bigger adult size --> faster (absolute) growth. (I wasn't sure if this would be true; but if you take the derivative of the growth rate with respect to asymptotic mass, it's indeed positive...) So shouldn't Palkovacs' growth curve get steeper in response to decreased mortality?
In Walker's et al.'s article as possible causal agents for body size they elude to selection pressures, but do not go into detail about what these may be. Could you elaborate?
Is there any evidence of animals getting smaller on continents because of limitation of resources? If an animal has a very particular feeding niche and starts to become limited, might it not be easier to just stay and eat less than risk migrating and dying? Humans were significantly smaller during the Upper Middle Paleolithic, can that be attributed to sexual selection or resource pressures? After all in the paper they say even though turtles were over hunted, humans environment "allowed" them to move on to faster game.
As the OGM model showing, if we have limited food the growth rate will slow down. However, the expected adult size will not change except the time to achive adult will become longer. As the growth time become longer, it is possible that most of them will die before get the expected adult size.
In Walker's paper, Table 4 shows that larger body size relates to faster growth rate and lower survivalship to 15 relates to faster growth rate. However, what is the relationship between body size and survivalship to 15? Are they independent to each other?
other than body size, are there other traits that change as an organism's life history is changed by life on an island?
Is there an assumption that high mortality rate is due to predation? If not, isn't there a risk that by focusing energy on growth, immunity is compromised, contributing to the higher mortality rate?
In the Stiner et al. paper, the patterns in size and number tortoise and shellfish exploitation are used to state that Mediterranean populations in the Upper and Epi-Paleolithic were not technologically advanced enough to exploit quicker game effectly. Do the patterns they see in these species transfer to the quicker game exploited by later more technically inclined populations of the area? Are the quicker game responding with changes in body size due to over harvesting as well or is there a different reaction? Does this response pattern of body size in slow moving animals also appear in other areas, such as further inland, higher elevation, lake settings, etc.?
Hey!
Deepta, good question. There is exactly that assumption. In fact, the notion that a change in predation pressure can be reworded as a change in extrinsic mortality is perhaps THE LINK between the traditional community ecology approach to the island rule and the life history approach. And yes also - if energy is diverted toward faster growth then the energy available for maintenance (including immune function) must be less. It is possible that this could raise mortality but the life history papers tend to have the causal arrow going in the other direction as they generally think that its a raise in mortality that causes the faster growth. There is reason to doubt this. Walker et al argue that its unlikely that the faster growth causes higher mortality because they doubt that an organism would pay the high costs of growth only to experience higher mortality as well. But - from their regressions you could easily make either argument.
Traci,
I think that the fast growing game are less likely to have to adjust to predation pressure by changing their body size because they are more resilient to predation. Also, since they are the lower ranked prey in the diet, any increase in abundance in any game species that is easier to take will give them a small break from predation. (i.e., if ungulates are suddenly very available for a season or two, the hunting pressure on the fast species would likely relax for some time).
See you all tomorrow
oskar
Why is it that this island dwarfism and gigantism is one of the quickest evolutionary changes? One might think it's because it only requires changes in timing of developmental patterns...yet evo-devo folks will tell you that's how all evolutionary change.
Is it because it requires only changes in pre-existing developmental patterns, whereas other evolutionary changes may require novel mutations?
As a comment to Crimmins and Finch, and their attempt to link increases in height to a reduction in exposure to infectious and inflammatory diseases, I would think such conditions who act to stunt growth, but dont see how their absence would stimulate it.
When resources are more readily available or predation released, does body size increase or does population increase first? Is it one before the other, or does one cause the other? I'm sure the answer is probably in one of the papers, but all of this info is kind of becoming blurry....
Would it be possible to apply the optimal hunting theory of Stiner et al. to a wider range environment e.g. an environment where the “slow” species also pose a mortality threat?
How Price did the survey for the Atlas of World Cultures? Compare to other continents Asia seems to have less cultures overall.
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