Larger is not better: No mate preference by European Common Frog (Rana temporaria) males. Carolin Dittrich, Oliver Roumldel. bioRxiv, May 28 2021. https://doi.org/10.1101/2021.05.28.446140
Abstract: According to classical sexual selection theory, females are the choosy sex in most species. Choosiness is defined as the individual effort to invest energy and time to assess potential mates. In explosive breeding anurans, high intrasexual competition between males leads to a sexual coercion ruled mating system, where males could have evolved preferences for specific female traits. In the current study, we tested male mating preference in the explosive breeding European Common Frog without intrasexual competition. We hypothesized that males show preferences towards larger female body size in the absence of male competition. We conducted mate choice experiments, placing a male and two differently sized females in a box and recorded their mating behavior. Males did not show any preference considering female body size, neither in the attempt to grab a female nor during the formation of pairs. We witnessed a high failure rate of male mating attempts, which might make the evolution of mate choice too costly. However, small males are faster in attempting females, which could be an alternative strategy to get access to females, because their larger competitors have an advantage during scramble competition. Nonetheless, in successfully formed pairs, the females were on average larger than the males, an observation which deviated from our null-model where pairs should be of similar size if mating would be random. This indicates that selection takes place, independent from male mating preference or scramble competition.
26 Introduction
27 Research on sexual selection, exploring the mechanisms that lead to female/male mate choice and the evolution of
28 different mating systems that facilitate non-random mating, has increased considerably in recent years (Janetos 1980;
29 Ryan and Keddy-Hector 1992; Paul 2002; Edward and Chapman 2011). Studies addressing the theory of sexual
30 selection revealed that females are the choosy sex in most species. This is mainly based on one assumption, the
31 evolution of anisogamy, where males produce many small (cheap) gametes and females less but larger (expensive)
32 gametes. Thus, females invest more energy in the production of eggs than males invest in the production of sperm
33 (Trivers 1972). In consequence, reproduction is more costly to females and they should choose the 'fittest' male to
34 mate with. This includes those with the best possible genes to improve her offspring’s fitness and/or those who can
35 provide vital resources (e.g. territory, nesting place, food, parental care) to increase offspring survivability and
36 attractiveness, thereby increasing the female´s personal fitness (Fisher 1958; Hedrick 1988; Møller and Alatalo
37 1999). Here, choosiness is defined as an individual’s active effort to invest energy and time to assess potential mates,
38 whereas preference is defined as an intrinsic, passive attractiveness towards specific traits of the opposite sex
39 (Jennions and Petrie 1997; Cotton et al. 2006). However, female preferences can be overridden by dominant
40 intrasexual competition (Qvarnström and Forsgren 1998; Härdling and Kokko 2005; Formica et al. 2016).
41 Preferences can enhance the evolution of different mating strategies and tactics to increase reproductive output with
42 behavioral plasticity; depending on sex, age, physiological state or operational sex ratio (Parker 1982; Gross 1996).
43 Nevertheless, newer studies suggest that males can be choosy too, if mate availability is high and simultaneous
44 sampling possible (Barry and Kokko 2010), if there is variation in female quality/fecundity (Krupa 1995; Johnstone
45 et al. 1996), and if the benefits of choosing between females is higher then the costs associated with assessing
46 females (Edward and Chapmann 2011, and references therein). Some prerequisites are the presence of males’ ability
47 to detect differences and a preference for particular female traits. Body size can be such a trait, i.e. indicating
48 longevity based on good genes which could be heritable (Kokko and Lindström 1996; Møller and Alatalo 1999).
49 However, body size usually is based on a variety of genes and environmental processes, but might simply indicate
50 higher fecundity (Peters 1986; Shine 1988; Nali et al. 2014). Mating with a larger female thus may increase a male’s
51 individual fitness. A male’s choice however, should not only be based on such trivial correlation, it will be impacted
52 by trade-offs concerning its mating chances, and thus individual males indeed may follow very different strategies to
53 access females. Some examples of male tactics are satellite males, usually being smaller than their competitors (Arak
54 1983; Halliday and Tejedo 1995), mate-guarding (Parker 1974), prudent mate choice (Fawcett and Johnstone 2003;
55 Härdling and Kokko 2005), clutch piracy (Vieites et al. 2004) or even functional necrophilia (Izzo et al. 2012).
56 Mating systems in amphibians are diverse, and apart from environmental parameters, mostly depend on female
57 availability over time (Wells 2007). In frog and toad species (anurans) with long breeding periods (prolonged
58 breeders) female mate choice seems to be the rule (Wells 1977). At any given time, a few females actively choose
59 among many calling males, often based on call characteristics (Toledo et al. 2015; Ryan et al. 2019), the quality of
60 defended territories, or the availability of other resources to judge the males (Howard 1978; Kirkpatrick and Ryan
61 1991; Kokko and Jennions 2008; da Rocha et al. 2018). In lek-breeding anurans, the males aggregate in displaying
62 arenas that do not contain any resources required by females. Females visiting these arenas 'sample' several males
63 and choose a male to mate with (Bourne 1992). In lek-mating systems the operational sex ratio is highly skewed
64 towards males and individual males are not able to monopolize females, leading to higher intrasexual competition
65 (Emlen and Oring 1977). In contrast, in species with a short breeding period (explosive breeders) males are actively
66 searching for mates and engage in direct male-male competition over the arriving females. Explosive breeding is
67 characterized by an almost equal operational sex ratio, synchronized receptiveness of females and low sexual
68 selection (Emlen and Oring 1977). In theory all males are able to mate and reproduce, but larger/more dominant
69 males have an advantage to access and dominate receptive females during scramble competition leading to a
70 variation in male mating success (Berven 1981; Olson et al. 1986; Höglund 1989; Vagi and Hettyey 2016).
71 Therefore, some males are considered to sexually dominate the females in explosive breeding systems, leaving little
72 room for male and female mate choice if the cost for mate sampling are too high (Dechaume-Moncharmont et al.
73 2016). Nevertheless, male mate preferences could have evolved in explosive breeders, because female fecundity
74 highly dependents on female body size in most anuran species (Krupa 1995; Nali et al. 2014). Simultaneous
75 sampling of preferred females might be particular possible during the peak mating time because female availability
76 should then be highest (Arntzen 1999; Barry and Kokko 2006). All males should prefer larger females to increase
77 their own fitness according to adaptation theory, although preferences could be obscured by high intrasexual
78 competition. On the other hand, costs associated with mate choice depend on male density and the frequency of
79 different mating tactics within a breeding aggregation (Arak 1983; Höglund and Robertson 1988), as well as for
80 instance male’s individual predation risk (Magnhagen 1991; Bernal et al. 2007), all factors which may vary already
81 during a short breeding season (Olson et al. 1986; Vojar et al. 2015).
82 In this study, we investigate the mating preference of the European Common Frog (Rana temporaria) because it is an
83 excellent example of an explosive breeder with male-male competition. Although former studies suggest a lack of
84 male mate preferences in this species (Elmberg 1991), we observed non-random mating by body size and found
85 indications of male mate preference and different mating tactics in former experiments (Dittrich et al. 2018). Larger
86 females were paired more frequently than smaller ones and smaller sized males showed a different mating tactic to
87 get access to females (Dittrich et al. 2018). Here, we hypothesize that all males will prefer larger females
88 independent of their own body size, when intrasexual competition is absent and males are presented to differently
89 sized females. Additionally, we predict small males to be faster in attempting a female to increase their chances to
90 keep an exclusive access to the female during scramble competition
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