Mate choice

Make Him a Monogamy Junkie

The Monogamy Method

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Many studies have now used molecular data to conduct parentage analyses, and perhaps the most general conclusion that we can reach is that even in socially monogamous species both males and females will often mate with multiple partners. However, not all individuals are equally successful at attracting mates, and this leads us to the question of what makes a mate particularly attractive to a member of the opposite sex. Mate choice may be exercised by both males and females. Female blue-footed boobies (Sula nebouxii; Figure 6.6), for example, experienced a greater degree of intra- and extra-pair courtship if their feet were particularly colourful, suggesting that this is a trait that promotes male mate choice (Torres and Velando, 2005). Generally speaking, however, females are choosier than males because usually they invest more in eggs than males do in sperm. Understanding why individuals choose particular mates -- both social and extra-pair -- and not others is necessary before we can understand the evolution of mating systems.

Studies on mate choice, which have been accumulating rapidly in recent years, have been based on a combination of field, experimental and molecular work. In this section we will concentrate on two hypotheses that may explain mate choice, and that have benefited particularly from molecular data: the good genes hypothesis and the genetic compatibility hypothesis. While reading this section, bear in mind that forced copulations, mate guarding and intrasexual competition may mean that females do not always mate with their male of choice. Nevertheless, the relative ease with which we can determine the genetic parentage of offspring has provided us with some interesting data on why females (and sometimes males) choose particular mates with which to copulate.

The good genes hypothesis states that mates will be chosen on the basis of some characteristic that will always confer high fitness values on offspring. In Atlantic salmon, for example, individuals with an MHC e allele have the highest survivorship in populations that are infected by Aeromonas salmonicida bacteria, and therefore must be regarded as good gene donors (Lohm et al., 2002). The good gene hypothesis can provide a plausible explanation for EPFs if a female's extra-pair male has one or more beneficial genes that are lacking in her social partner. Female great reed warblers (Acrocephalus arundinaceus), for example, obtained EPFs from neighbouring males that had larger song repertoires than the female's social mate. Because the survival of offspring was positively correlated with the size of their genetic father's song repertoire, females appeared to be selecting males with good genes (Hasselquist, Bensch and von Schantz, 1996).

The genetic compatibility hypothesis is based on the idea that a particular paternal allele will increase the fitness of offspring only when it is partnered with specific maternal alleles. In other words, genes are not universally good but, instead, each is more compatible with some genotypes than with others. Under this hypothesis, an individual will choose his or her mate on the basis of their combined genotypes. Female mice (Mus musculus) and female sand lizards (Lacerta agilis), for example, tend to choose mates whose MHC loci are as dissimilar to theirs as possible (Jordan and Bruford, 1998; Olsson et al., 2003), a tactic that may be designed either to increase heterozygosity at the MHC in particular or to decrease inbreeding in general. Under some circumstances the genetic compatibility hypothesis seems to be the most plausible explanation for EPFs, for example one study found that in three different species of shorebird, the females were more likely to engage in EPFs when they were socially partnered with genetically similar males (Blomquist et al., 2002). Interestingly, this study also found that males were more likely to fertilize quasi-parasitic females (Box 6.1) when they had a genetically similar social mate. The most likely explanation here seems to be inbreeding avoidance.

Post-copulatory mate choice

In females, mate choice is not limited to pre-copulatory behaviour. After copulation, cryptic female mate choice may occur through the selection of sperm genotypes. In the flour beetle (Callosobruchus maculatus), unrelated sperm had a higher fertiliza tion success rate than related sperm, suggesting cryptic female choice that was being driven by genetic compatibility in an attempt to decrease inbreeding and maximize the genetic diversity of offspring (Wilson et al., 1997). In the marsupial Antechinus agilis, fertilization success was inversely correlated with the number of alleles that were shared by copulating males and females, once again suggesting post-copulatory mate choice based on genetic compatibility (Kraaijeveld-Smit et al., 2002). There is also evidence to suggest that in mice, sperm are at least partially selected on the basis of their MHC haplotypes (Rulicke et al., 1998). Similarly, although invertebrates lack MHC, fertilization in the colonial tunicate Botryllus is influenced by a polymorphic histocompatibility locus that controls allorecognition (Scofield et al., 1982).

Somewhat surprisingly, even when fertilization is external it may be influenced by female choice. This is true of the ascidian Ciona intestinalis, in which external fertilization is partially regulated by maternal cells. Broods that were of mixed male parentage showed a relatively high proportion of fertilizations by males that were distantly related to the female compared with more closely related males (Olsson et al., 1996). Finally, post-copulatory mate choice may sometimes be based on good genes, the quality of which may vary depending on environmental conditions. Female yellow dung flies (Scathophaga stercoraria) have three spermathecae (sperm-storage organs) in which they can partition sperm. In one study, the genotypes of offspring varied depending on whether the eggs were laid in the sun or in the shade, and this suggested that the females of this species use the egg-laying environment as a cue for choosing different sperm genotypes (Ward, 1998).

So far in our discussion of mating systems we have been looking at how parentage analyses based on molecular data have highlighted some of the differences between social and genetic mating systems (see also Box 6.2), and have also provided insight into several aspects of mate choice. Ultimately, parentage analysis has enabled us to quantify more accurately the fitness of individuals. However, not all reproductive success is achieved through the direct production of offspring, and in the following section we will take a look at how fitness can be enhanced through social breeding.

6.2 Extra-pair fertilizations and Ne

We know from Chapter 3 that variation in reproductive success (VRS) can influence the effective size of a population (Ne). In species such as the elephant seal, in which a few males with harems achieve most of the reproductive success, we expect to find a high male VRS and hence a low Ne/Nc, but how do EPFs affect the VRS, and hence the Ne, of other species with less extreme mating systems? In theory, EPFs may either decrease VRS by enabling unpaired males to reproduce, or increase VRS by allowing a handful of males to father a disproportionately high number of offspring.

Representatives of the endangered hihi bird (Notiomystis cincta) were translocated to several islands off the coast of New Zealand in an attempt to establish new populations. Because these were small populations there was a concern that genetic diversity would be low, and researchers therefore investigated the possibility that Ne would be reduced further by VRS. The hihi is predominantly socially monogamous, although will sometimes form polygamous units. Parentage analysis of 56 clutches from one island over the course of 4 years revealed that 46 per cent of all chicks were fathered by extra-pair males. From one year to the next, the effects of EPFs on VRS were varied; in some years EPFs increased VRS but in other years they decreased it (Figure 6.7). However, although fluctuations in VRS were fairly pronounced, mortality rates were high, which meant that the net effect of VRS was to cause relatively modest fluctuations in the Ne/Nc ratio from one year to the next, ranging from a 4 per cent decrease to an 8 per cent increase (Castro et al., 2004). These results are similar to those of another study that found an EPF-driven decrease in Ne/Nc of approximately 2 per cent in purple martins (Progne subis) and 8 per cent in blue tits (Parus caeruleus), two other socially monogamous bird species. In contrast, two socially breeding bird species, strip-backed wrens (Campylorhynchus nuchalis) and Arabian babblers (Turdoides squamiceps), had estimated increases in Ne/Nc of 5 and 15 per cent respectively, that were attributable to EPFs (Parker and Waite, 1997).

1994 1995 1996 1997

Figure 6.7 Effects of EPFs on the variation in reproductive success (VRS) in hihi birds, with reproductive success calculated as the number of young that fledged from each nest. The VRS of putative fathers (i.e. without the effects of EPFs) may be either higher or lower than that of genetic fathers (i.e. with the effects of EPFs). The VRS of mothers is included for comparison. Adapted from Castro et al. (2004)

1994 1995 1996 1997

Figure 6.7 Effects of EPFs on the variation in reproductive success (VRS) in hihi birds, with reproductive success calculated as the number of young that fledged from each nest. The VRS of putative fathers (i.e. without the effects of EPFs) may be either higher or lower than that of genetic fathers (i.e. with the effects of EPFs). The VRS of mothers is included for comparison. Adapted from Castro et al. (2004)

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