1. Introduction
Mate-choice copying is a type of non-independent mate choice [
1] whereby an individual uses public information, more specifically the observed mating behavior of another individual, in making their own mate-choice decisions [
2,
3]. This phenomenon has been observed in a wide range of taxa, including at least 26 species of invertebrates, fish, birds, and mammals [
4]. There is a growing body of literature showing that humans engage in mate-choice copying [reviewed by [
5]. The phenomenon has ramifications for sexual selection as it can lead to incredibly high variance in reproductive success [
6], and for illuminating the role of social learning—and, by extension, culture—in shaping mating behavior in humans and other animals [
7,
8,
9].
The potential of cultural evolution to shape population-level diversity in mating preferences turns on the distinction between “copying” and “cultural inheritance” of mate choice [
7]. Copying involves the immediate use of the socially transmitted information. Cultural inheritance requires
generalization—that the socially transmitted information is used to inform future mate choice decisions in addition to current ones. Bowers et al. [
10] make a slightly different distinction between “individual-based” copying, where the chances of mating with a particular individual is increased after having observed another mate with that individual, and “trait-based” copying, where the chance of mating with individual that have a particular characteristic is increased after having observed others mating with individuals possessing similar traits. These concepts are illustrated in
Figure 1A. With copying or individual-based copying, the socially transmitted information is unlikely to be transmitted beyond the model/copier dyad; with cultural inheritance or trait-based copying, the socially transmitted information becomes are preference that is likely to be transmitted to others, leading to the cultural evolution of mate preferences [
11]. Generalization of mate-choice preferences has been demonstrated in the species illustrated in
Figure 1B [
8,
10,
12,
13,
14,
15,
16,
17,
18].
Pruett Jones [
19] modeled mate-choice copying using evolutionary game theory, namely the logic of evolutionarily stable strategies (ESS). An ESS is a frequency-dependent strategy that affords the greatest net fitness benefit to the individual actors and, when adopted by members of a population, cannot be invaded by alternative strategies via the forces of natural selection [
20]. Discrete-strategy ESS models (e.g., Hawk-Dove [
21]) assess the proportion of individuals in the population that adopt each strategy. Pure ESSs are the extreme (100%), but intermediate solutions are possible also. In the focal model the strategies are copier and chooser, the ESS proportions of which depend on the costs and benefits of choosing. Copying is supported when the costs are high, or the benefits low, for choosing.
In this paper, I build extensions of the original model [
19] to include two important factors: (1) generalization; and (2) the potential for costs to accrue to the copier, the cost to immediate copying, that comes in the form of agonistic relations with the individual being copied and sperm depletion, among other things [
4]. The main aim of the paper is to assess whether these factors exert an influence on the adaptive value of mate-choice copying using the same, relatively simple, modeling strategy of the original model. By comparing the results of the original model and its extensions, I hope to illuminate potential evolutionary scenarios for the emergence and maintenance of mate-choice copying in species (such as humans) where there is a cost to immediate copying, such as in monogamous ones, and for explaining the emergence of true cultural inheritance in species with mate-choice copying. This has ramifications, albeit indirectly, for informing debates in the evolutionary behavioural sciences, such as the relative importance of domain specific and domain general mechanisms [
22].
3. Discussion
Pruett Jones’s [
19] model of mate-choice copying showed that, when there is a cost to choosing a mate by oneself, copying is selected for as a pure or mixed ESS, as shown in
Figure 2. Here, I have extended the original model in three ways by adding generalization (Extension 1), a cost to immediate copying (Extension 2), and both generalization and a cost to immediate copying (Extension 3). The results, outlined in
Figure 5, show that when either generalization or a cost to immediate copying is added by itself to the original model, except in a very limited range of circumstances, there is a decrease in the ESS proportion of copiers. Put another way, either factor, by itself, leads to a decrease in the adaptive value of mate-choice copying. When there is already a cost to immediate copying, however, generalization increases the adaptive value of copying. The converse fails to hold. When there is already generalisation, adding a cost to immediate copying decreases the adaptive value of mate-choice copying. These results suggest that a system marked by the “cultural inheritance” of mate choice (i.e., mate-choice copying with generalisation) would be more likely to have evolved from a system that had a cost to immediate copying. That is, a trajectory that went from individual-based copying with a cost to immediate copying → trait-based copying may be more likely than a trajectory that goes from individual-based copying → trait-based copying.
Figure 4.
Comparing model results: (A) comparing the model with cost to immediate copying (Extension 2) and the model with both extensions (Extension 3), and (B) comparing the model with generalisation (Extension 1) and the model with both extensions (Extension 3). In both, the lines are ESS proportions of copiers in the model with both extensions and the edge of the shaded area are the ESS proportions of copiers in the models with a single extension. As shown in (A), under several conditions, adding generalization to a mate-choice copying system where there is a cost to immediate copying leads to substantive increases in the ESS proportion of copiers (i.e., there is an increase in adaptive value). As shown in (B), under no conditions does adding a cost to immediate copying in a system with generalization increase the ESS proportion of copier. That is, there is a decrease in the adaptive value of copying, however small.
Figure 4.
Comparing model results: (A) comparing the model with cost to immediate copying (Extension 2) and the model with both extensions (Extension 3), and (B) comparing the model with generalisation (Extension 1) and the model with both extensions (Extension 3). In both, the lines are ESS proportions of copiers in the model with both extensions and the edge of the shaded area are the ESS proportions of copiers in the models with a single extension. As shown in (A), under several conditions, adding generalization to a mate-choice copying system where there is a cost to immediate copying leads to substantive increases in the ESS proportion of copiers (i.e., there is an increase in adaptive value). As shown in (B), under no conditions does adding a cost to immediate copying in a system with generalization increase the ESS proportion of copier. That is, there is a decrease in the adaptive value of copying, however small.
The distribution of mating systems among species that engage in mate-choice copying supports the evolutionary trajectory suggested by the models presented in this paper. Most species that engage in mate-choice copying engage in polygyny or promiscuity to some extent, and many of them are lekking species [
4,
23,
24]. There is only one obligatory-monogamous species with biparental care that engages in mate-choice copying, the zebra finch (
Taenipygia guttata)—a species that happens to engage in trait-based mate copying [
14,
16]. Many of the other species that generalize mate-choice copying have biparental care to some extent. As predicted by my model, generalization might be one way to avoid the costs of mating with an already-mated male, which Vakirtzis [
4,
25] outlines as follows: (i) increased competition with other females, (ii) diminishing paternal care, (iii) low male mating skew which renders the public information gathered related to mate choice less reliable, (iv) copying unattractive females who are paired with low-quality males, and (v) ambiguous relationship between male qualities and his number of mates. DuBois [
26] explores another possibility—that mate choice copying in monogamous species might be adaptive for extrapair copulations. The model results suggest an evolutionary trajectory for the emergence of key features of human mating strategies using a theory-based approach, rather than a purely speculative or induction-based approach.
Figure 5.
Summary of results, which suggests that generalisation of mate-choice copying (aka trait-based copying) would have been more likely to arise out of a system of mate-choice copying with a cost to immediate copying.
Figure 5.
Summary of results, which suggests that generalisation of mate-choice copying (aka trait-based copying) would have been more likely to arise out of a system of mate-choice copying with a cost to immediate copying.
The generalization of mate-choice copying has potential ramifications for shaping male displays. Several authors have shown that the intensity and nature of sexual selection on male traits can be influenced by how females use socially acquired information about mate choice [
27,
28,
29]. Wade and Pruett Jones [
6] showed that mate-choice copying could lead to increased variance in mating success accompanied by strong directional sexual selection, even traits that reduce male viability [
28] and runaway sexual selection in the absence of genetic correlations [
31,
32]. Others have argued that, while these effects might follow from trait-based copying, they are more likely to lead to stabilizing selection and the fixation of male displays [
10,
28,
33].
One simplification I employed was modelling the future contribution of the socially transmitted information on the copier’s fitness as the consequences of a single future mating event. It is unlikely that many, if any species, reproduce this way so it might be useful to at least consider how the results might change if generalization affected multiple future mating. Because generalization appears to counterbalance the negative effects of a cost to immediate copying, generalizing over multiple future mating would likely increase the fitness consequences of copying when the cost exists.
The approach I have adopted here models equilibrium states and may downplay constraints on the ability of frequency-dependent selection to lead to an ESS [
34,
35,
36]. Evolutionarily stable strategies are those that, when monomorphic in a population, resist the invasion of rare mutants [
20]. McNamara and Weissing [
35] clarify that although “the reasoning behind an ESS suggests a population that reaches an ESS will not evolve away from this strategy, it does not guarantee that evolution will actually lead to an ESS.” Constraints on their evolution include the specifics of the underlying genetics, the existence of additional equilibria, and the dynamic nature of the adaptive landscape under frequency dependence [
35]. Several authors [
34,
36] have argued for the use of a dynamic approach using the “replicator dynamics” [
37] over the sort of static one used here. I offer the following in defense of my choice to adopt a static approach. First, the original model [
19] used it. Second, each of the extensions of the original model has a single ESS and a simple strategy set. The dynamic approach would have presented unnecessary complexity. Third, my intention was to derive as-general-as-possible qualitative conclusions from the models. The static approach, it can be argued, is better suited for such a goal.
Of course, any modeling exercise will peel away some of the complexity of the real-world system under study [
38]. In the case of human mating (or the mating of most species where mate-choice copying occurs), where strategies are complex rather than binary, the results must always be interpreted with caution. For instance, DuVal et al. [
39] have modeled the evolutionary dynamics of preferences for specific traits. At the same time, whether this should be considered a shortcoming or feature of the model depends on one’s aims (as realism and generalizability of a model may trade-off against each other [
40]).
The generalization of mate-choice preferences, or trait-based copying, is necessary for the cultural evolution of inter- and intra-population diversity in mating preferences. Much is already known about this diversity in human mating [
39,
40]; less is known about other species that generalize their preferences (see
Figure 1B). One avenue for further research is the investigation of this diversity, including elucidating the cultural evolutionary dynamics for the origin and maintenance of mate-choice preferences. Part of this enterprise will be to systematically unravel the sometimes-intertwined influences of innate and learned factors on mate choice [
43,
44], which is easier to do in experimentation-amenable species, much more difficult for ethical reasons in humans In addition, future research should address the question of whether mate-choice copying is driven by domain-specific or domain-general mechanisms [
22,
45], and whether this differs for species with individual- versus trait-based copying.
Figure 1.
Mate-choice copying is normally used to refer to (A) individual-based copying, where an individual preferentially mates with an individual who they observed mating with somebody else. Mate-choice copying can lead to (B) “generalisation” or trait-based copying, where an individual preferentially mates with individuals that have the features of an individual that they observed mating with somebody else. The former can lead to the cultural evolution of mating preferences. Generalisation of mate-choice copying has been observed in several species, including: (C) fruit flies (Drosophila melanogaster), (D) sailfin mollies (Poecilia latipinna), (E) guppies (Poecilia reticulata), (F) zebra finches (Taenipygia guttata), (G) Japanese quail (Coturnix japonica), and (H) humans (Homo sapiens). Credits: fruit fly, CC Attribution 2.0 Generic, Paco Romero-Ferrero; sailfin mollie, CC Attribution-Share Alike 3.0 Unported, Bjoertvedt; guppy, CC Attribution-Share Alike 4.0, Marrabbio2; zebra finches, CC Attribution 4.0 International, Christoph_moning; Japanese quail, public domain; humans, CC Attribution 2.0 Generic, Scarleth White.
Figure 1.
Mate-choice copying is normally used to refer to (A) individual-based copying, where an individual preferentially mates with an individual who they observed mating with somebody else. Mate-choice copying can lead to (B) “generalisation” or trait-based copying, where an individual preferentially mates with individuals that have the features of an individual that they observed mating with somebody else. The former can lead to the cultural evolution of mating preferences. Generalisation of mate-choice copying has been observed in several species, including: (C) fruit flies (Drosophila melanogaster), (D) sailfin mollies (Poecilia latipinna), (E) guppies (Poecilia reticulata), (F) zebra finches (Taenipygia guttata), (G) Japanese quail (Coturnix japonica), and (H) humans (Homo sapiens). Credits: fruit fly, CC Attribution 2.0 Generic, Paco Romero-Ferrero; sailfin mollie, CC Attribution-Share Alike 3.0 Unported, Bjoertvedt; guppy, CC Attribution-Share Alike 4.0, Marrabbio2; zebra finches, CC Attribution 4.0 International, Christoph_moning; Japanese quail, public domain; humans, CC Attribution 2.0 Generic, Scarleth White.
Figure 2.
ESS proportions of choosers and copiers in the original model.
Figure 2.
ESS proportions of choosers and copiers in the original model.
Figure 3.
Model results for each of the two extensions: (A) In the original model plus generalisation (Extension 1), the ESS proportion of copiers decreases with decreasing values of m, as indicated by the arrows. The extended model’s results are identical to the original model when m=1, but otherwise leads to lower ESS proportion of copiers (i.e.,, adaptive value of copying decreases). At some levels of m, as illustrated, the ESS proportion of copiers is 0. That is, the chooser can be a pure ESS. (B) In the original model plus cost to immediate copying (Extension 2), the ESS proportion of copiers decreases with increasing c, as indicated by the arrows. The extended model’s results reduce to the original model when there is no cost to immediate copying. When the cost of immediate copying is greater than or equal to half the cost of mate choice, the chooser strategy is a pure ESS.
Figure 3.
Model results for each of the two extensions: (A) In the original model plus generalisation (Extension 1), the ESS proportion of copiers decreases with decreasing values of m, as indicated by the arrows. The extended model’s results are identical to the original model when m=1, but otherwise leads to lower ESS proportion of copiers (i.e.,, adaptive value of copying decreases). At some levels of m, as illustrated, the ESS proportion of copiers is 0. That is, the chooser can be a pure ESS. (B) In the original model plus cost to immediate copying (Extension 2), the ESS proportion of copiers decreases with increasing c, as indicated by the arrows. The extended model’s results reduce to the original model when there is no cost to immediate copying. When the cost of immediate copying is greater than or equal to half the cost of mate choice, the chooser strategy is a pure ESS.
Table 1.
Payoff matrix for the original model [
1].
Table 1.
Payoff matrix for the original model [
1].
|
|
When paired with: |
|
|
Chooser |
Copier |
Payoff to: |
Chooser |
W + f – k |
W + f – k |
|
Copier |
W + f |
W |
Table 2.
Payoff matrix for the original model plus generalisation (Extension 1).
Table 2.
Payoff matrix for the original model plus generalisation (Extension 1).
|
|
When paired with: |
|
|
Chooser |
|
Copier |
|
|
Immediate |
Future |
|
Immediate |
Future |
Payoff to: |
Chooser |
W + f – k |
W + f – k |
|
W + f – k |
W + f – k |
|
Copier |
W + f |
m (W + f) + (1 – m) W
|
|
W |
W |
Table 3.
Payoff matrix for the original model plus costs to immediate copying (Extension 2).
Table 3.
Payoff matrix for the original model plus costs to immediate copying (Extension 2).
|
|
When paired with: |
|
|
Chooser |
Copier |
Payoff to: |
Chooser |
W + f – k |
W + f – k |
|
Copier |
W + f – c |
W – c |
Table 4.
Payoff matrix for the original model plus both other extensions (Extension 3).
Table 4.
Payoff matrix for the original model plus both other extensions (Extension 3).
|
|
When paired with: |
|
|
Chooser |
|
Copier |
|
|
Immediate |
Future |
|
Immediate |
Future |
Payoff to: |
Chooser |
W + f – k |
W + f – k |
|
W + f – k |
W + f – k |
|
Copier |
W + f – c |
m (W + f) + (1 – m) W
|
|
W – c |
W |