The draft I am currently writing is about the fifth attempt to write this particular paper. The initial idea yielded a basically uninteresting paper, but I had all these compiled data and lots of carefully crafted paragraphs, so I rewrote it with the same concept, but a different focus. That didn't work either, so I changed my question, did a bunch of new analyses, got some interesting results and wrote a really mushy, mealy mouthed paper (because I hadn't done enough background research no my new topic to know exactly what my point was). I sent that to one of my advisers to ask him what I should do to make it less ugly. He sent some comments, which required rewriting. I sent that to another adviser who totally eviscerated my introduction and discussion (her list of comments, suggestions and objections was as long as the paper itself), and had some problems with my methods. Her feedback is tremendously useful, and appreciated, despite being painful. So I spent this weekend doing more background reading and yesterday rewrote my introduction. Today I'm fixing the methods and results, and hopefully will get a start on discussion.
For anyone who is interested, here is the deeply flawed and poorly written introduction I just threw out:
Trivers (1972) set the direction of much of the modern study of sexual selection by stating that, "what governs the operation of sexual selection is the relative parental investment of the sexes in their offspring. Competition for mates usually characterizes males because males usually invest almost nothing in their offspring (pg 141)." In this view, female parental investment is a resource for which males are expected to compete, and the less investment males make in offspring, the more valuable the investment of females becomes. The relative lack of paternal investment drives male-male competition, which in turn influences a wide range of other morphological and life history variables.
Trivers' statement then implies that interspecies variation in paternal investment is a key to understanding the evolution of life histories, socioecology and sexually selected traits. While a large and fruitful comparative literature on primate life-histories has matured over the last few decades (e.g. Gage 1998; Lee 1999; Nunn 1999; Ravosa and Dagosto 2007), few of these studies have considered paternal investment as a variable (but see Geary 2000; Mitchell and Brandt 1972; Smuts and Gubernick 1992; Wright 1990). Those that have done so have incorporated data on relatively few species or not coded paternal care in a way that allows for quantitative analysis.
One need not subscribe to Trivers' (1972) view to see paternal care as an important life history variable. Interspecific variation in paternal care may be caused by, rather than causing, male-male competition: because males compete for mates, male variance in reproductive success is relatively high, increasing the opportunity cost to males who provide care (Queller 1997). In this model, male-male competition is predicted to determine both paternal investment and sexual dimorphism. The prediction that intensity of sexual selection on males should be correlated with paternal care is common to both mechanisms. This prediction has been addressed using data from cichlid fishes (Gonzalez-Voyer et al. 2008) and shorebirds (Thomas and Székely 2005). These papers attempt to establish the direction of causality through a series of phylogenetic analyses (Maddison 1990; Pagel 1994) that attempt to describe the coevolution of traits. This type of analysis requires two types of assumptions, each of which is questionable. First, each trait is treated as binary (paternal care or none, sexual selection or none). Second, one must assume that the details of the inferred pattern of past change (or the probability distributions thereof, (Maddison 1990) are correct. There is reason to doubt the validity of these assumptions when studying a group of inter-correlated, continuously varying traits. Furthermore, while these mechanisms are positioned as alternatives, it is also plausible that sexual selection and level of paternal care may influence each other reciprocally.
Sexually dimorphic traits are useful exemplars of sexually selected traits. Observations of sexual dimorphisms were the impetus that caused Charles Darwin to introduce (Darwin 1872), and explore (Darwin 1882) the concept of sexual selection. Where natural selection is expected to act similarly on the two sexes so long as they differ only in gametes, sexual selection may apply quite distinct pressures to each, causing them to diverge. Indeed, primates, particularly Haplorhines (apes and monkeys) are known to display greater dimorphism in species with more intense male-male competition (Mitani et al. 1996; Plavcan and van Schaik 1992; Plavcan and van Schaik 1997; Thoren et al. 2006). The present study presents a comparative dataset, with each species assigned to one of four levels based on intensity of paternal investment. We use these data to test the prediction that paternal care is closely correlated with male-male competition, and to examine the utility of paternal care as a variable in models explaining sexual dimorphism. We specifically predict that if the linkage between dimorphism and paternal care is through male-male competition, models containing both paternal care and male-male competition should have little more explanatory power in describing patterns of dimorphism than do models excluding paternal care. A further prediction is that measures of dimorphism should be more closely correlated with male-male competition than with paternal care, and that this inequality should be robust to phylogenetically controlled analyses.
Primates are an ideal taxon in which to make this type of comparison, as their breeding systems are highly varied and they are well studied. The large number of studies of individual primate species allows a rich field of comparative studies compiling single variables for many primate species (Lee 1999). From this literature we draw comparative data on intersexual canine tooth dimorphism, mass dimorphism, and sex-differential longevity (longevity dimorphism). Primates employ their canine teeth in threat displays, and as weapons, and as such males are expected to invest in disproportionately larger canines when fighting between males is common (Plavcan and van Schaik 1992). Canine dimorphism incorporates information on both mass dimorphism (assuming allometric scaling) and disproportionate investment in canines by one sex. Mass dimorphism, has also been suggested to reflect intensity of intrasexual conflict, but to a lesser extent than canine dimorphism (Plavcan and van Schaik 1997).
Our final measure is sex-differential longevity, which by analogy we refer to as longevity dimorphism. The sex experiencing more intense sexual selection is predicted to live less long (Promislow 1992). This prediction arises both because of direct mortality associated with intrasexual conflict, and because of physiological and developmental tradeoffs between competitive ability and longevity. Allman et al. (1998) compared longevity dimorphism and paternal care in a group of ape species and New World monkeys (Platyrrhini), and suggested that the correlation they found was attributable to sexual selection. However their analysis was not phylogenetically controlled. New World monkeys are unusual among primates in several life history variables (Ross 1991; Wright 1990), including their tendency to have caring and long-lived males. In this light, the correlation between paternal care and longevity dimorphism observed by Allman et al may be attributable to a phylogenetic correlation. As most life history variables are correlated with each other, most are also correlated with phylogeny (Leigh and Blomquist 2007); we examine their correlations in this light.
A more direct link between longevity dimorphism and differential care by the two sexes is proposed based on the value of caregivers. As the selective disadvantage caused by death before a particular age is related to the residual expectation of reproduction at that age (Charlesworth 2000; Hamilton 1966), and as care is a form of reproduction which on the average is expected to occur later in life than fertility, a population that provides care to descendents may experience selection for greater longevity (Chu and Lee 2006). Males who provide care may tend to live as long or longer than conspecific females because of this selective pressure. This hypothesis does not depend upon male-male competition, and therefore predicts that paternal care is an directly important in the evolution of longevity dimorphism.