On Human fertility: Individual or group benefit?

pre-printCaldwell et al. (CA 28:25-43) have pointed to the pervasive influence of Carr-Saunders's (1922) concept of population regulation throughout two-thirds of a century of anthropology and demography

Human hunting mortality threshold rules for extinction in mammals (and fish)

Question: Are there general life-history rules for exploitation-caused extinction of mammal populations? Mathematical methods: A population of size N faced with the added mortality of human exploitation will deterministically go extinct if its per-capita birth rate can no longer match its per-capita mortality rate as N approaches zero. We develop exploitation-extinction theory for a mammal life history using R₀ < 1 as N goes to zero, and combine the criterion with several facts about mammal life histories. Conclusions: Extinction results if the ratio of the instantaneous mortality rate caused by hunting (F) divided by the adult instantaneous mortality rate (M, for the unexploited population) exceeds a critical value (F/M > C). The C value is determined mostly by the level of recruitment compensation as N declines, and C is likely very similar for different sized mammals. We use existing mammal life-history data to estimate C (~0.5). We then estimate the threshold of instantaneous mortality rate, F, as a function of adult body mass, W; it's a -0.25 power allometry. Finally, we extend the model to fish. C is expected to vary a lot between fish species, mostly because fish are expected to have much larger recruitment compensation than mammals, the recruitment may correlate with body size, and immature fish are often not exploited. We show how to combine these to predict C.Keywords: exploitation-caused extinction, fisheries-extinction, life span allometry, mammals, population recruitmen

Shrimp adjust their sex ratio to fluctuating age distributions

Long-term data sets that quantitatively confirm basic ecological theory are rare for field populations. Highly variable recruitment often causes wide temporal variation in population age distribution and basic theory for adaptive sex ratio often predicts ‘sex ratio tracking’ to match the fluctuating age distribution. Using sex-changing shrimp as a model system, we test this in a new data set of 20 years duration. The new data support the theory, despite intense fishery exploitation that itself has greatly altered the age distribution in recent years.Keywords: evolutionarily stable strategy, frequency dependence, recruitment variation, protandry, fisheriesKeywords: evolutionarily stable strategy, frequency dependence, recruitment variation, protandry, fisherie

Lifetime Reproductive Effort

In a 1966 American Naturalist article, G. C. Williams initiated the study of reproductive effort (RE) with the prediction that longer-lived organisms ought to expend less in reproduction per unit of time. We can multiply RE, often measured in fractions of adult body mass committed to reproduction per unit time, by the average adult life span to get lifetime reproductive effort (LRE). Williams’s hypothesis (across species, RE decreases as life span increases) can then be refined to read “LRE will be approximately constant for similar organisms.” Here we show that LRE is a key component of fitness in nongrowing populations, and thus its value is central to understanding life-history evolution. We then develop metabolic life-history theory to predict that LRE ought to be approximately 1.4 across organisms despite extreme differences in production and growth rates. We estimate LRE for mammals and lizards that differ in growth and production by five- to tenfold. The distributions are approximately normal with means of 1.43 and 1.41 for lizards and mammals, respectively (95% confidence intervals: 1.3– 1.5 and 1.2–1.6). Ultimately, therefore, a female can only produce a mass of offspring approximately equal to 1.4 times her own body mass during the course of her life.This is the publisher’s final pdf. The published article is copyrighted by University of Chicago Press and can be found at: http://www.press.uchicago.edu/ucp/journals/journal/an.html.Keywords: Williams’s hypothesis, lizards, dimensionless, life-history optimization, mammalsKeywords: Williams’s hypothesis, lizards, dimensionless, life-history optimization, mammal

A Dimensionless Invariant for Relative Size at Sex Change in Animals: Explanation and Implications

Recent comparative studies across sex-changing animals have found that the relative size and age at sex change are strikingly invariant. In particular, 91%–97% of the variation in size at sex change across species can be explained by the simple rule that individuals change sex when they reach 72% of their maximum body size. However, this degree of invariance is surprising and has proved controversial. In particular, it is not clear why this result should hold, given that there is considerable biological variation across species in factors that can influence the evolutionarily stable timing of sex change. Our overall aim here is to explain this result and determine the implications for other life-history variables. Specifically, we use a combination of approaches to formalize and make explicit previous analytical theory in this area, examine the robustness of the empirical invariance result, and carry out sensitivity analyses to determine what the empirical data imply about the mean value and variation in several key life-history variables.This is the publisher’s final pdf. The published article is copyrighted by University of Chicago Press and can be found at: http://www.press.uchicago.edu/ucp/journals/journal/an.html.Keywords: protogyny, sequential hermaphroditism, sex allocation, life history, protandry, Buckingham’s π theoremKeywords: protogyny, sequential hermaphroditism, sex allocation, life history, protandry, Buckingham’s π theore

Simultaneous hermaphroditism and sexual selection

Theory about the evolution of sexual behavior in dioecious species is based on the general assumption that egg production is limited by a female's ability to garner resources to make eggs, not by a lack of sperm to fertilize them. Reproductive success for males is thus limited by access to females (and their eggs). I suggest that egg production by simultaneous hermaphrodites also obeys this principle—that fertilized egg production by an individual is not limited by sperm availability, but by resources allocated to eggs. If true, this suggests that sperm competition (reproduction success through male function) and a form of male—female conflict have played important roles in the evolution of hermaphroditism