reproductive strategies
Resource allocation in parental care is constrained by fitness trade-offs occurring between current and future reproduction (Stearns 1992). For example, investment in offspring may divert resources away from investment in survival or mating opportunities (Alonzo-Alvarez and Velando 2012). As a consequence parents must weigh up the costs and benefits to overall fitness that each reproductive event entails. Reproductive strategies become more complex in systems with bi-parental care, as sexual conflict may arise in the amount of care that each parent is willing to invest (Parker et al. 2002; Johnstone and Hinde 2006; Lessells 2006).
Bateman’s principle predicts that males should endeavour to maximise reproductive success by seeking additional matings, and hence social polygyny would appear to be the optimal male reproductive strategy. Yet in contrast, 90% of all bird species are socially monogamous (Clutton-Brock 1991). Monogamy is hence purported to have originated from the necessity of bi-parental care in raising offspring (Lack 1968), which is adaptive for males when the fitness benefits of investing in offspring outweigh the costs of reduced mating opportunities (Emlen and Oring 1977; Székely and Cuthill 1999). Conversely, the optimisation of offspring fitness should play a major role in female reproductive strategy. Females may achieve this either through mate choice (see above), or via investment in offspring.
Differential allocation occurs when reproductive investment is influenced by mate attractiveness (Sheldon 2000). This hypothesis predicts that females mated to high
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quality males should invest relatively more in these offspring in order to gain more attractive offspring (Burley 1986). As a consequence, males are predicted to provide relatively less care and instead invest in future reproductive success (Burley 1986). There is substantial empirical support for this hypothesis (e.g. de Lope and Moller 1993; Petrie and Williams 1993; Kolm 2001; Limbourg et al. 2004; Bonato et al. 2009; McFarlane et al. 2010; Robart 2012).
Negative associations between male ornamentation and paternal effort may also arise due to evolutionary trade-offs between current investment in offspring and either investment in male-male competition or mating effort (Magrath and Komdeur 2003). Under this hypothesis high quality males are predicted to provide less parental care because the fitness benefits from the alternative strategy outweigh those acquired through parental investment. However, such mechanisms may often be more complex than theory predicts. For example, high quality males may experience less of a trade-off as they may have more resources to invest in both behaviours (Magrath and Komdeur 2003). In addition, although male-male competition is likely to be costly and thus representative of a trade-off (Pryke and Griffith 2009), it has been argued that acquiring additional matings requires little effort on the part of the male (Stiver and Alonzo 2009), and thus a trade-off may not always occur.
In contrast, alternative theoretical models predict a positive relationship between male advertisement and paternal care, when male advertisement is an honest indicator of direct benefits (i.e. the good parent hypothesis, see above). Thus it is not easy to predict the relationship between the expression of male traits and parental investment. However, the benefit to males of each strategy depends on
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the marginal fitness gains from multiple mating. Increasing marginal gains are predicted in systems where genetic quality has a significant influence on offspring fitness, and thus male advertisement should be negatively related to paternal care in species with extra-pair mating (Kokko 1998).
Sexual selection may favour the existence of multiple alternative male reproductive strategies within populations depending on a male’s residual reproductive value (Gross 1996; Badyaev and Hill 2002; Kelly and Alonzo 2009). However, the benefit of certain reproductive strategies is complicated in species engaging in EPP by a reduced average certainty of paternity. Socially monogamous species are characterised by generally high rates of paternal care (Clutton-Brock 1991), thus raising the question of how a lowered average paternity of a brood should affect male parental investment. As paternity decreases, the benefit to males of providing care decreases (Fig. 1.2). Trivers (1972) stated that a male should be selected to reduce investment in offspring that are not his own. Indeed, socially monogamous species exhibit a large variance in rates of paternal care between species (Clutton-Brock 1991), and negative interspecies correlations found between the rate of EPP and paternal effort in comparative studies of socially monogamous birds suggest that reduced paternal care may be an adaptive response to a male’s greater uncertainty of paternity (Møller and Birkhead 1993; Møller and Cuervo 2000; Sheldon 2002; Matysiokova and Remes 2013).
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Figure 1.2: Visual representation of Queller’s (1997) relationship between paternity, the fitness benefits of parental care, and the outcomes of selection on parental care, from Alonzo & Klug (2012). Lines show the boundaries at which selection favours changes in male or female parental care. Overall multiple paternity reduces the benefits to males of providing care. Note, however, males are still predicted to care even at low levels of paternity (<0.2) if the benefits of care are high.
The relationship between mating patterns and parental care is likely to be complex and may be influenced by multiple factors such as sexual conflict, male quality, paternity, as well as the probability of future reproductive events (Westneat and Sherman 1993; Freeman-Gallant 1997; Neff and Sherman 2002; Alonzo 2012). For example, a recent meta-analysis found that, in order to avoid the adverse dilemma of reducing care to potentially related offspring, a reduction in paternal investment is predicted only when EPP is frequent in the population, and when care is costly to a male’s future reproductive success (Griffin et al. 2013). Furthermore, there
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may be an inverse relationship between paternity and paternal effort if cuckolded males are of low quality but actually are better parents in compensation (Eliassen and Kokko 2008). These aspects make it difficult to draw definite conclusions about the causality of detected links between male quality and reproductive strategies. This thesis attempts to contribute further knowledge of these systems in a species which is characterised by both a high variation in male parental effort and high rates of infidelity, thus potentially providing an important addition to the parental care and paternity debate. In Chapter 5 I draw on conclusions from other chapters to address the question of whether males exhibit alternative reproductive strategies that influence their parental investment in relation to their quality and parentage.