1 · the Evolution of Alternative Reproductive Tactics: Concepts and Questions MICHAEL TABORSKY, RUI F

1 · The evolution of alternative reproductive tactics: concepts and questions MICHAEL TABORSKY, RUI F. OLIVEIRA, AND H. JANE BROCKMANN CHAPTER SUMMARY It is important to note here that in the study of allocation decisions in general, and ARTs in particular, any expression Here we outline the meaning of the term alternative of variation of traits is not regarded as reproductive tactics, or ARTs, and discuss why the existence continuous alternative tactics. Discontinuity in morphological and physiological of ARTs is so widespread in animals. We ask what we need to traits is often difficult to determine (Eberhard and know to understand the evolution of ARTs and the import- Gutie´rrez 1991, Emlen 1996, Kotiaho and Tomkins 2001). ance of general principles such as frequency dependence, In behavioral traits, in contrast, discontinuities may seem density dependence, and condition dependence, and what we easier to measure because of their visibility to observers. For need to know about proximate mechanisms involved in example, there may be overlap between male types of dung the regulation of ARTs to comprehend evolutionary patterns. beetles in their expression of horns and body sizes, but it is We discuss current issues in the study of ARTs and list 12 very clear-cut whether these male types fight for access to questions that we think need particular attention. Throughout females or copulate without investing in primary access to we shall provide representative examples of ARTs in animals mates (Kotiaho and Tomkins 2001; see also Hunt and to illustrate the ubiquitous nature of this phenomenon. Simmons 2000). However, subtle discontinuities might exist in any phenotype, including behavior (e.g., when the 1.1 WHAT IS THE MEANING OF performance of alternative tactics depends on condition or ALTERNATIVE REPRODUCTIVE situation: Brockmann and Penn 1992, Brockmann 2002). In TACTIC? a nutshell, in the context of ARTs, alternative refers to traits that show a discontinuous distribution. 1.1.1 Alternative The concept of ARTs refers to alternative ways to obtain 1.1.2 Reproductive fertilizations in both males and females. In its most common use, this term refers to traits selected to maximize fitness in We speak of alternative reproductive tactics when conspe- two or more alternative ways in the context of intraspecific cific, intrasexual competitors find different solutions to and intrasexual reproductive competition. In general, reproductive competition. It is irrelevant whether the alternative phenotypes are characterized by a discontinuous observed variation happens within or between individuals, distribution of traits evolved towards the same functional but reproductive discontinuity within one population at the end. Examples include size dimorphism, color poly- same time is of essence. In a general sense the concerned morphism, dimorphic morphological structures involved in traits are alternative responses to competition from mem- the monopolization of resources or mates, and various bers of the same sex. Examples are males either courting behavioral alternatives such as territoriality vs. floating, females or forcing copulations, as in guppies and other monopolization vs. scramble competition, or investment in poeciliid fishes (Bisazza 1993, Bisazza and Pilastro 1997), or primary access to a resource vs. social parasitism. Individ- females either digging burrows for their eggs or usurping uals allocate resources to either one or the other (mutually those dug by others, as in digger wasps (Brockmann and exclusive) way of achieving the same functional end using Dawkins 1979, Brockmann et al. 1979). It is irrelevant evolved decision-making rules (Brockmann 2001). whether adaptations to reproductive competition are mainly Alternative Reproductive Tactics, ed. Rui F. Oliveira, Michael Taborsky, and H. Jane Brockmann. Published by Cambridge University Press. ª Cambridge University Press 2008. 1 2 M. TABORSKY, R. F. OLIVEIRA, AND H. J. BROCKMANN Box 1.1 Examples of ARTs in animals Dominant breeders with helpers that share in reproduction in Reference to literature on ARTs in taxa mentioned here is scrubwrens, Campylorhynchus given in the text of this chapter and in other chapters of this wrens, and dunnocks book. Mammals Bourgeois males and satellites in ungulates such as waterbuck and Molluscs Phallic and aphallic males kob Horseshoe crabs Males attached to females and Displaying/defending males and satellites harassing interlopers in fallow deer Mites Fighter and scrambler males Harem owners and opportunistic, Crustacea Mate guarding vs. searching in submissive group males in many amphipods primates Three alternative male mating Flanged and unflanged males in types in isopods orang-utans Insects Calling and noncalling males in crickets What is not an ART? Winged and wingless male morphs in bladder grasshoppers Cooperative breeding, if helpers do not share in Single- and joint-nest foundresses reproduction (reproduction is a necessary component in social wasps of an alternative reproductive tactic) Color and horn polymorphisms in Interspecific brood parasitism, as heterospecifics are not male damselflies and beetles reproductive competitors Territorial vs. roaming males in Sex change, even though in species with alternative dragonflies tactics within one sex bourgeois and parasitic options in Fishes Bourgeois males and reproductive this phase may determine the threshold for the optimal parasites in sunfish, salmonids, timing of sex change (e.g., in wrasses with two or more wrasses, cichlids, blennies, and gobies male reproductive tactics: Munoz and Warner 2003) Bourgeois males and helpers or Simultaneous hermaphroditism, as shedding sperm is not satellites in ocellated wrasse, an alternative to shedding eggs among competitors for cooperative cichlids, and fertilizations anabantoids Infanticide, because it is not a reproductive tactic (i.e., to Courting and coercive males in obtain fertilizations or produce offspring, even though poeciliids it may indirectly contribute to this end) Amphibians Calling males and silent interlopers Pure scramble competition for reproduction without in frogs and toads discontinuous phenotypic variation Reptiles Differently colored males with Alternative phenotypes in nonreproductive contexts (e.g., different mating tactics in lizards foraging or trophic polymorphisms such as left- and Birds Courting males and satellites in lekking birds such as ruffs right-jawed fish, castes, and age polyethism in social Pair and extra-pair matings in insects when the different morphs do not engage in many monogamous species (e.g., reproductive competition; polymorphisms that involve red-winged blackbirds, blue tits) both males and females such as winged and wingless Single vs. joint courtship in man- forms in some insects, alternative migratory patterns akins and diapause patterns; seasonal polyphenism that does Nesting oneself or dumping eggs not involve reproductive characters or individuals; and elsewhere (i.e., intraspecific brood color polymorphisms caused by apostatic prey selec- parasitism) in many anatids tion or other anti-predator strategies) Concepts and questions 3 or partly resulting from intrasexual, intersexual, or natural (Shuster and Wade 2003). This may not be so bad in the end selection mechanisms. For example, the evolution of (see Grafen’s [1991] discussion on “the phenotypic courting and sneaking tactics in a species may be subject to gambit”). The difference made between phenotypic traits intrasexual rivalry, but it may also be influenced by mate produced by same or different genotypes has heuristic choice (intersexual selection) and by the tactic-specific importance for (game theory) evolutionary models, but it potential to evade predation (natural selection). Alterna- ignores the fact that virtually all phenotypic traits are the tively, there may be specialization of same-sex conspecifics product of genotypic and environmental influence (West- in exploiting different reproductive niches. Irrespective of Eberhard 1989, 2003, Scheiner 1993). Hence, in reality the the underlying selection mechanisms, ultimately the exist- borders between the terms “strategy” and “tactic” are vague ence of the two alternative tactics will be the expression of and flexible. The underlying mechanisms are usually different solutions to reproductive competition. Interspe- unknown (i.e., to which extent patterns are genetically cific brood parasitism, for example, is not an ART, because determined) at a point when we have not yet studied a it is not the result of reproductive competition; neither are phenomenon extensively but nonetheless wish to commu- phenomena like infanticide, sex change, or age polyethism nicate about it. Therefore, we prefer an operational use of in social insects (see Box 1.1). terms here instead of one encumbered with functional implications, just as in the sex-allocation literature (Char- nov 1982; see Brockmann 2001). In short, we regard “tactic” 1.1.3 Tactic and “strategy” as synonymous but prefer the use of “tactic” In a general sense tactic refers to a trait or set of traits serving because we mainly deal with phenotypes and because of the a particular function. In the context of ARTs, tactics usually connotations of the term strategy. involve behavioral traits, but the term is by no means In essence, “alternative reproductive tactics” refers to restricted to behavioral phenotypes. For instance, various discontinuous behavioral and other traits selected to maxi- types of horns in a male population of horned beetles

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