Squid male alternative reproductive tactics are determined by birth date.

Alternative reproductive tactics (ARTs) are discontinuous phenotypes associated with reproduction, observed in males of many species. Typically, large males adopt a tactic of competing with rivals for mating, while small males adopt a tactic of stealing fertilization opportunities from the large males. The 'birth date hypothesis', proposing that the date of birth influences the determination of each male's reproductive tactic, has been tested only in teleost fish to date. Here, the birth date hypothesis was tested in ARTs of Japanese spear squid Heterololigo bleekeri (consort/sneaker) by analysing statolith growth increments. The birth date significantly differed between consorts (early-hatched) and sneakers (late-hatched). However, no differences were detected in growth history up to 100 days from hatching. Most immature males caught during the reproductive season were larger than sneakers, and their hatch date was similar to that of consorts, suggesting that these immature males had already been following a life-history pathway as a consort. These results indicate that ARTs of H. bleekeri are determined based on their hatch date in early life. This study firstly suggests that the birth date hypothesis applies to aquatic invertebrates, suggesting that the mechanism by which birth date determines the individual phenotype is a phenomenon more common than previously believed.

A supergene affects androgen concentrations during early development in a bird with alternative reproductive morphs.

Species with alternative reproductive tactics typically show pronounced phenotypic variation between and within sexes. In some species, this variation culminates in discrete reproductive morphs that are genetically determined, facilitating studies on how genetic variation translates into phenotypic variation. In ruffs (Calidris pugnax), an autosomal inversion polymorphism underlies three reproductive morphs (Independents, Satellites and Faeders), which differ in circulating steroid concentrations in adults. Yet, it remains unknown whether morph differences in steroid concentrations already arise before adulthood. We examined variation in circulating testosterone, androstenedione and progesterone concentrations between morphs and sexes in ruff chicks and juveniles and compared the differences to those in adults. Since measured hormone concentrations only provide momentary states and show high within- and between-individual variation, we took repeated measurements to compare means, variances and skewness between groups. We found clear differences between morphs but not the sexes in early life. Between morphs, androgen concentrations in young ruffs differed in variance and skewness, but not in their means. For testosterone, Independents had a higher variance than Satellites/Faeders, whereas for androstenedione, we observed the opposite pattern. For progesterone, we did not detect clear differences between groups. Skewness values mirrored differences in morph variances. Compared to adults, premature ruffs had lower androgen concentrations. In both life stages, we detected morph-specific associations between androgen concentrations: androstenedione concentrations increased with testosterone concentrations more in Satellites/Faeders than in Independents. These observed morph differences during early life are consistent with a supergene-mediated regulation of androgen variation that underlies the diversification of adult behavioural phenotypes.

Genomic basis of Y-linked dwarfism in cichlids pursuing alternative reproductive tactics.

Sexually antagonistic selection, which favours different optima in males and females, is predicted to play an important role in the evolution of sex chromosomes. Body size is a sexually antagonistic trait in the shell-brooding cichlid fish Lamprologous callipterus, as "bourgeois" males must be large enough to carry empty snail shells to build nests whereas females must be small enough to fit into shells for breeding. In this species, there is also a second male morph: smaller "dwarf" males employ an alternative reproductive strategy by wriggling past spawning females into shells to fertilize eggs. L. callipterus male morphology is passed strictly from father to son, suggesting Y-linkage. However, sex chromosomes had not been previously identified in this species, and the genomic basis of size dimorphism was unknown. Here we used whole-genome sequencing to identify a 2.4-Mb sex-linked region on scaffold_23 with reduced coverage and single nucleotide polymorphism density in both male morphs compared to females. Within this sex region, distinct Y-haplotypes delineate the two male morphs, and candidate genes for body size (GHRHR, a known dwarfism gene) and sex determination (ADCYAP1R1) are in high linkage disequilibrium. Because differences in body size between females and males are under strong selection in L. callipterus, we hypothesize that sexual antagonism over body size initiated early events in sex chromosome evolution, followed by Y divergence to give rise to bourgeois and dwarf male reproductive strategies. Our results are consistent with the hypothesis that sexually antagonistic traits should be linked to young sex chromosomes.

Age-specific survival of territorial and non-territorial male chamois.

How alternative reproductive tactics (ARTs) are maintained in wildlife populations is one of the major questions in evolutionary biology. As a dominant status, territoriality is typically linked to increased mating opportunities, and one explanation why this behaviour coexists with other tactics is that dominance implies survival costs. Such a trade-off may occur in the Northern chamois Rupicapra rupicapra, as reproductive advantages of territorial males over non-territorial males could be counterbalanced by a reduction in survival mediated through energy expenditure, stress and parasitic infections, ultimately favouring ART coexistence. Here, we analysed age-dependent survival probabilities of territorial (n = 15) and non-territorial (n = 16) adult chamois using information collected over 12 years between 2010 and 2021 in the Gran Paradiso National Park (Western Italian Alps). Survival rates were estimated with a CMR approach using Burnham's joint modelling of live encounter and dead recovery data. The model selection procedure, based on AICc value minimisation, supported a linear decrease of survival with age but the results did not match our predictions, as territorial chamois did not have lower survival rates than non-territorial chamois. In contrast, territorial males appeared to enjoy reproductive success at lower survival costs. This, in turn, supports the role of other factors, such as snow-dependent environmental stochasticity, in the maintenance of ARTs in chamois populations. The limited sample size, however, calls for caution in interpretation, and long-term studies of lifetime reproductive success and survival are necessary to clarify the mechanisms underlying the expression and coexistence of different reproductive behaviours in this species.

Female reproductive fluid concentrations affect sperm performance of alternative male phenotypes in an external fertilizer.

There is growing evidence that the female reproductive fluid (FRF) plays an important role in cryptic female choice through its differential effect on the performance of sperm from different males. In a natural spawning event, the male(s) may release ejaculate closer or further away from the spawning female. If the relative spatial proximity of competing males reflects the female pre-mating preference towards those males, then favoured males will encounter higher concentrations of FRF than unpreferred males. Despite this being a common situation in many external fertilizers, whether different concentrations of FRF can differentially influence the sperm performance of distinct male phenotypes (favoured and unfavoured by the female) remains to be elucidated. Here, we tested this hypothesis using the grass goby (Zosterisessor ophiocephalus), a fish with distinct territorial-sneaker reproductive tactics and female pre-mating preference towards territorial males, that consequently mate in an advantaged position and whose sperm experience higher concentrations of FRF. Our findings revealed a differential concentration-dependent effect of FRF over sneaker and territorial sperm motility only at low concentrations (i.e. at the distance where sneakers typically ejaculate), with increasing FRF concentrations (i.e. close to the eggs) similarly boosting the sperm performance of both sneaker and territorial males. The ability to release sperm close to the eggs is a prerogative of territorials, but FRF can likewise advantage the sperm of those sneakers that are able to get closer, allowing flexibility in the direction of female post-mating choice.

Mating Behavioral Function of Preoptic Galanin Neurons Is Shared between Fish with Alternative Male Reproductive Tactics and Tetrapods.

Understanding the contribution of neuropeptide-containing neurons to variation in social behavior remains critically important. Galanin has gained increased attention because of the demonstration that galanin neurons in the preoptic area (POA) promote mating and parental care in mammals. How widespread these mechanisms are among vertebrates essentially remains unexplored, especially among teleost fishes, which comprise nearly one-half of living vertebrate species. Teleosts with alternative reproductive tactics exhibit stereotyped patterns of social behavior that diverge widely between individuals within a sex. This includes midshipman that have two male morphs. Type I males mate using either acoustic courtship to attract females to enter a nest they guard or cuckoldry during which they steal fertilizations from a nest-holding male using a sneak or satellite spawning tactic, whereas type II males only cuckold. Using the neural activity marker phospho-S6, we show increased galanin neuron activation in courting type I males during mating that is not explained by their courtship vocalizations, parental care of eggs, or nest defense against cuckolders. This increase is not observed during mating in cuckolders of either morph or females (none of which show parental care). Together with their role in mating in male mammals, the results demonstrate an unexpectedly specific and deep-rooted, phylogenetically shared behavioral function for POA galanin neurons. The results also point to galanin-dependent circuitry as a potential substrate for the evolution of divergent phenotypes within one sex and provide new functional insights into how POA populations in teleosts compare to the POA and anterior hypothalamus of tetrapods.SIGNIFICANCE STATEMENT Studies of neuropeptide regulation of vertebrate social behavior have mainly focused on the vasopressin-oxytocin family. Recently, galanin has received attention as a regulator of social behavior largely because of studies demonstrating that galanin neurons in the preoptic area (POA) promote mating and parental care in mammals. Species with alternative reproductive tactics (ARTs) exhibit robust, consistent differences in behavioral phenotypes between individuals within a sex. Taking advantage of this trait, we show POA galanin neurons are specifically active during mating in one of two male reproductive tactics, but not other mating-related behaviors in a fish with ARTs. The results demonstrate a deep, phylogenetically shared role for POA galanin neurons in reproductive-related social behaviors with implications for the evolution of ARTs.

Kin-mediated plasticity in alternative reproductive tactics.

Conditional strategies occur when the relative fitness pay-off from expressing a given phenotype is contingent upon environmental circumstances. This conditional strategy model underlies cases of alternative reproductive tactics, in which individuals of one sex employ different means to obtain reproduction. How kin structure affects the expression of alternative reproductive tactics remains unexplored. We address this using the mite Rhizoglyphus echinopus, in which large males develop into aggressive 'fighters' and small males develop into non-aggressive 'scramblers.' Because only fighters kill their rivals, they should incur a greater indirect fitness cost when competing with their relatives, and thus fighter expression could be reduced in the presence of relatives. We raised mites in full-sibling or mixed-sibship groups and found that fighters were more common at higher body weights in full-sibling groups, not less common as we predicted (small individuals were almost exclusively scramblers in both treatments). This result could be explained if relatedness and cue variability are interpreted signals of population density, since fighters are more common at low densities in this species. Alternatively, our results may indicate that males compete more intensely with relatives in this species. We provide the first evidence of kin-mediated plasticity in the expression of alternative reproductive tactics.

Tactic-specific antimicrobial activity suggests a parental care function for accessory glands in a marine toadfish.

Males of some species possess extra reproductive organs called accessory glands which are outgrowths of the testes or sperm duct. These organs have a well-established role in reproduction; however, they also appear to have other important functions that are less understood. Here, we investigate the function of the highly complex accessory glands of a marine toadfish, Porichthys notatus, a fish with two reproductive male types: large care-providing 'guarder' males and small non-caring 'sneaker' males. While both male types have accessory glands, guarder male accessory glands are much larger relative to their body size. We show that accessory gland fluids strongly inhibit the growth of bacterial genera associated with unhealthy eggs and have no effect on the growth of strains isolated from healthy eggs. This antibacterial effect was particularly pronounced for extracts from guarder males. Furthermore, we demonstrate that both healthy and unhealthy plainfin midshipman eggs have diverse but distinct microbial communities that differ in their composition and abundance. The highly specific inhibitory capacity of accessory gland fluid on bacteria from unhealthy eggs was robust across a wide range of ecologically relevant temperatures and salinities. Collectively, these ecological and molecular observations suggest a care function for the accessory gland mediated by antimicrobial agents.

Coevolution of female fidelity and male help in populations with alternative reproductive tactics.

In socially monogamous species, pair-bonded males often continue to provide care to all offspring in their nests despite some degree of paternity loss due to female extra-pair copulation. Previous theoretical models suggested that females can use their within-pair offspring as 'hostages' to blackmail their social mates, so that they continue to provide care to the brood at low levels of cuckoldry. These models, however, rely on the assumption of sufficiently accurate male detection of cuckoldry and the reduction of parental effort in case of suspicion. Therefore, they cannot explain the abundant cases where cuckolded males continue to provide extensive care to the brood. Here we use an analytical population genetics model and an individual-based simulation model to explore the coevolution of female fidelity and male help in populations with two genetically determined alternative reproductive tactics (ARTs): sneakers that achieve paternity solely via extra-pair copulations and bourgeois that form a mating pair and spend some efforts in brood care. We show that when the efficiency of mate guarding is intermediate, the bourgeois males can evolve to 'specialize' in providing care by spending more than 90% of time in helping their females while guarding them as much as possible, despite frequent cuckoldry by the sneakers. We also show that when sneakers have tactic-specific adaptations and thus are more competitive than the bourgeois in gaining extra-pair fertilizations, the frequency of sneakers and the degrees of female fidelity and male help can fluctuate in evolutionary cycles. Our theoretical predictions highlight the need for further empirical tests in species with ARTs.

Sperm competition risk affects ejaculate strategy in terms of sperm number but not sperm size in squid.

In polygamous species, the mode of sperm storage in females influences evolution of sperm quantitative and qualitative traits because it provides the arena for sperm competition, cryptic female choice and fertilization processes. In this study, we compared ejaculate traits of two squid species, Heterololigo bleekeri and Loligo reynaudii. Both species show dimorphic sperm traits associated with alternative reproductive tactics where consort and sneaker males transfer sperm to different storage sites within a female (on the oviduct and near the mouth, respectively). Due to differences in reproductive behaviours and sperm placement, sperm competition risk is expected to be higher in sneakers than in consorts of both species and higher overall in L. reynaudii. Our results demonstrate that the instantaneous number of released sperm is adjusted to the expected sperm competition risk via an elaborate sperm package. Consort sperm are similar in size; however, sneaker sperm have a significantly longer flagellum in H. bleekeri than in L. reynaudii, most likely due to intra-tactic conflicts associated with sperm storage conditions. From consideration of the different mating tactics, we suggest that while levels of sperm competition determine quantitative traits, sperm quality traits are determined more by the mode of sperm storage and fertilization.

Functional differences in the hypothalamic-pituitary-gonadal axis are associated with alternative reproductive tactics based on an inversion polymorphism.

The evolution of social behavior depends on genetic changes, yet, how genomic variation manifests itself in behavioral diversity is still largely unresolved. Chromosomal inversions can play a pivotal role in producing distinct behavioral phenotypes, in particular, when inversion genes are functionally associated with hormone synthesis and signaling. Male ruffs exhibit alternative reproductive tactics (ARTs) with an autosomal inversion determining two alternative morphs with clear behavioral and hormonal differences from the ancestral morph. We investigated hormonal and transcriptomic differences in the pituitary and gonads. Using a GnRH challenge, we found that the ability to synthesize testosterone in inversion carriers is severely constrained, whereas the synthesis of androstenedione, a testosterone precursor, is not. Inversion morphs were able to produce a transient increase in androstenedione following the GnRH injection, supporting the view that pituitary sensitivity to GnRH is comparable to that of the ancestral morph. We then performed gene expression analyses in a second set of untreated birds and found no evidence of alterations to pituitary sensitivity, gonadotropin production or gonad sensitivity to luteinizing hormone or follicle-stimulating hormone across morphs. Inversion morphs also showed reduced progesterone receptor expression in the pituitary. Strikingly, in the gonads, inversion morphs over-expressed STAR, a gene that is located outside of the inversion and responsible for providing the cholesterol substrate required for the synthesis of sex hormones. In conclusion, our results suggest that the gonads determine morph-specific differences in hormonal regulation.

Do alternative reproductive tactics predict problem-solving performance in African striped mice?

In changing environments, animals face unexpected problems to solve. Not all individuals in a population are equally able to solve new problems. It still remains unclear what factors (e.g. age and body condition) influence the propensity of problem solving. We investigated variation in problem-solving performance among males following alternative reproductive tactics (ARTs). We studied a free-ranging population of the African striped mouse (Rhabdomys pumilio). Adult male striped mice can employ 3 ARTs: (1) dominant group-living breeders, (2) philopatric living in their natal group, and (3) solitary-living roamers. ARTs in male striped mice reflect differences in competitiveness, sociality and physiology which could influence their problem-solving performance. We tested a total of 48 males in 2 years with two tasks: a string-pulling task to reach food and a door-opening task to reach the nest. Since male striped mice differ in personality traits independent of ARTs, we also measured activity, boldness and exploration. In addition, we assessed the association of body condition and age with problem solving. Problem solving was related the interaction of age and ARTs. The younger philopatrics had better performance in a food-extraction task whereas the older breeders were faster at solving the door-opening task. Individual differences in traits related to personality were significant correlates of problem-solving performance: pro-active mice (i.e. more active and explorative and bolder) performed better in both tasks. Finally, problem-solving performance was not consistent between the two tasks. Our study provides evidence of correlates of ARTs, age and personality on problem-solving abilities.

Alternative reproductive strategies provide a flexible mechanism for assuring mating success in the European badgers (Meles meles): An investigation from hormonal measures.

Selection-pressures differ with population density, but few studies investigate how this can affect reproductive physiology. European badger (Meles meles) density varies from solitary to group-living across their range, with reported mating periods throughout the entire year to specific seasonal periods. Badger reproduction is evolutionarily distinct, interrupting the direct progression from conception to gestation with delayed implantation (DI), allowing for superfecundation (SF). To establish the tactical mating flexibility afforded by DI*SF, we used cross-sectional population-level seasonal variation of circulating sex-steroids for 97 females from a high-density population. Oestradiol was highest in spring among non-parous females, then lower in summer, and remained low during following seasons, suggesting that the mating period was restricted to just spring. Oestrone was consistently higher than oestradiol; it was elevated in spring, lowest during summer, peaked in autumn, and remained elevated for pregnant females in winter. This suggests that oestrone sustains pre-implanted blastocysts throughout DI. Progesterone was low throughout, except during winter pregnancy, associated with implantation and luteal development. In contrast to multiple mating periods reported by lower-density studies, our oestradiol data suggest that, at high-density, females exhibit only one mating period (congruent with testosterone patterns in males studied previously in this same population). While additional mating periods during DI enhance fertility assurance at low-density, at high-density, we propose that when coitus is frequent, fertilisation is assured, precluding the need for further cycles and associated mating risks. This endocrinologically flexible DI*SF mating strategy likely represents a form of balancing selection, allowing badgers to succeed at a range of regional densities.

Fitness benefits of male dominance behaviours depend on the degree of individual inbreeding in a polyandrous lizard.

In polyandrous species, sexual selection extends beyond mating competition to selection for egg fertilization. As a result, the degree to which factors influencing mating success impact overall reproductive success becomes variable. Here, we used a longitudinal behavioural and genetic dataset for a population of eastern water dragons (Intellagama lesueurii) to investigate the degree to which male dominance, a pre-mating selection trait, influences overall reproductive success, measured as the number of surviving offspring. Moreover, we examine the interactive effects with a genetic trait, individual inbreeding, known to influence the reproductive success of males in this species. We found fitness benefits of male dominance, measured as body size and frequency of dominance behaviours displayed. However, individuals' propensity to display dominance behaviours had mixed effects, depending on the degree of inbreeding. While inbred males benefited from frequent displays, highly outbred males exhibited better reproductive outputs when displaying to a lesser extent. Given that outbred males have enhanced reproductive success in this species, the costs of displaying dominance behaviours may outweigh the benefits. Overall, our results demonstrate the fitness benefits of dominance in a polyandrous lizard, and suggest that these are modulated by an independent genetic trait. Our results may contribute to explaining the presence of alternative mating tactics in this species, owing to the variability in net fitness benefits of dominance. Our findings also reveal the challenges associated with investigating fitness traits in isolation, which may undermine the validity of results when important interactions are ignored.

Fitness Consequences of Female Alternative Reproductive Tactics in House Mice (Mus musculus domesticus).

Alternative reproductive tactics (ARTs) are defined as discrete differences in morphological, physiological, and/or behavioral traits associated with reproduction that occur within the same sex and population. House mice provide a rare example of ARTs in females, which can rear their young either solitarily or together with one or several other females in a communal nest. We assessed the fitness consequences of communal and solitary breeding in a wild population to understand how the two tactics can be evolutionarily stable. Females switched between the two tactics (with more than 50% of all females having two or more litters using both tactics), pointing toward communal and solitary breeding being two tactics within a single strategy and not two genetically determined strategies. Communal breeding resulted in reduced pup survival and negatively impacted female reproductive success. Older and likely heavier females more often reared their litters solitarily, indicating that females use a condition-dependent strategy. Solitary breeding seems the more successful tactic, and only younger and likely less competitive females might opt for communal nursing, even at the cost of increased pup mortality. This study emphasizes the importance of analyzing phenotypic plasticity and its role in cooperation in the context of female ARTs.

Morphological Polymorphism Associated with Alternative Reproductive Tactics in a Plethodontid Salamander.

Understanding polymorphism is a central problem in evolution and ecology, and alternative reproductive tactics (ARTs) provide compelling examples for studying the origin and maintenance of behavioral and morphological variation. Much attention has been given to examples where "parasitic" individuals exploit the reproductive investment of "bourgeois" individuals, but some ARTs are instead maintained by environmental heterogeneity, with alternative tactics exhibiting differential fitness in discontinuous reproductive niches. We use genomic, behavioral, karyological, and field observational data to demonstrate one such example in plethodontid salamanders. These ARTs ("searching" and "guarding" males) are associated with different reproductive niches and, unlike most other examples in amphibians, demonstrate substantial morphological differences and inflexibility within a reproductive season. Evidence suggests the existence of these ARTs within three putative species in the two-lined salamander (Eurycea bislineata) species complex, with other members of this clade fixed for one of the two tactics. We highlight directions for future research in this system, including the relationship between these ARTs and parental care.

Experimentally induced variation in neuroendocrine processes affects male reproductive behaviour, sperm characteristics and social interactions.

While extensive research has focused on how social interactions evolve, the fitness consequences of the neuroendocrine mechanisms underlying these interactions have rarely been documented, especially in the wild. Here, we measure how the neuroendocrine mechanisms underlying male behaviour affect mating success and sperm competition in the ocellated wrasse (Symphodus ocellatus). In this species, males exhibit three alternative reproductive types. "Nesting males" provide parental care, defend territories and form cooperative associations with unrelated "satellites," who cheat by sneaking fertilizations but help by reducing sperm competition from "sneakers" who do not cooperate or provide care. To measure the fitness consequences of the mechanisms underlying these social interactions, we used "phenotypic engineering" that involved administering an androgen receptor antagonist (flutamide) to wild, free-living fish. Nesting males treated with flutamide shifted their aggression from sneakers to satellite males and experienced decreased submissiveness by sneaker males (which correlated with decreased nesting male mating success). The preoptic area (POA), a region controlling male reproductive behaviours, exhibited dramatic down-regulation of androgen receptor (AR) and vasotocin 1a receptor (V1aR) mRNA following experimental manipulation of androgen signalling. We did not find a direct effect of the manipulation on male mating success, paternity or larval production. However, variation in neuroendocrine mechanisms generated by the experimental manipulation was significantly correlated with changes in behaviour and mating success: V1aR expression was negatively correlated with satellite-directed aggression, and expression of its ligand arginine vasotocin (AVT) was positively correlated with courtship and mating success, thus revealing the potential for sexual selection on these mechanisms.

Sperm maturation and male tactic-specific differences in ejaculates in the plainfin midshipman fish Porichthys notatus.

Using the plainfin midshipman fish Porichthys notatus, a species with alternative reproductive tactics (ARTs), we investigated how sperm maturation shapes sperm competitive abilities. We compared sperm performance and morphology before and after final sperm maturation by sampling sperm from the testes and stripped ejaculates of guarders and sneakers. In accordance with sperm competition risk theory, ejaculates from sneaker males had three times as much sperm as ejaculates from guarder males and sneaker males produced faster swimming sperm than guarder males, but this was only the case after final sperm maturation had occurred. Additionally, fully mature sperm found in ejaculates had larger heads and midpieces than sperm found in the testes. These results emphasize the important role played by non-sperm components of an ejaculate in mediating sperm performance and potentially also morphology.

Fitness consequences of threshold trait expression subject to environmental cues.

Most cases of alternative reproductive tactics (ARTs) are thought to represent conditional strategies, whereby high-status males express highly competitive phenotypes, whereas males below a certain status threshold resort to sneaky tactics. The underlying evolutionarily stable strategy (ESS) model assumes that males of high competitive ability achieve higher fitness when expressing the territorial phenotype, whereas the less competitive males are more fit as sneakers, caused by fitness functions for the ARTs having different slopes and intersecting at a threshold value of competitive ability. The model, however, is notoriously difficult to test as it requires access to low-status territorials and high-status sneakers, that rarely occur in nature. Here, we test the conditional ESS in the androdimorphic acarid mite Sancassania berlesei, where large males tend to develop into an armoured, aggressive 'fighter' morph, while small males become unarmoured, non-aggressive 'scramblers'. In addition to body size, male morph is affected by pheromones produced by big populations, with fighters being suppressed in dense colonies. By manipulating pheromone concentration, we obtained high-status scramblers and low-status fighters. We also estimated status- and size-dependent fitness functions for male morphs across a range of population sizes. Fighters had the highest fitness in small populations and their fitness declined with increasing density, whereas the reverse was true for scramblers, providing support for condition-dependent ESS with respect to demography. However, whereas male fitness increased with body size, the fitness functions did not differ significantly between morphs. Thus, although we found evidence for the intersection of morph fitness functions with respect to demography, we did not find such an intersection in relation to male body size. Our results highlight how demography can exert selection pressures shaping the evolution of the conditional strategy in species with ARTs.

Behavioural tactic predicts preoptic-hypothalamic gene expression more strongly than developmental morph in fish with alternative reproductive tactics.

Reproductive success relies on the coordination of social behaviours, such as territory defence, courtship and mating. Species with extreme variation in reproductive tactics are useful models for identifying the neural mechanisms underlying social behaviour plasticity. The plainfin midshipman (Porichthys notatus) is a teleost fish with two male reproductive morphs that follow widely divergent developmental trajectories and display alternative reproductive tactics (ARTs). Type I males defend territories, court females and provide paternal care, but will resort to cuckoldry if they cannot maintain a territory. Type II males reproduce only through cuckoldry. We sought to disentangle gene expression patterns underlying behavioural tactic, in this case ARTs, from those solely reflective of developmental morph. Using RNA-sequencing, we investigated differential transcript expression in the preoptic area-anterior hypothalamus (POA-AH) of courting type I males, cuckolding type I males and cuckolding type II males. Unexpectedly, POA-AH differential expression was more strongly coupled to behavioural tactic than morph. This included a suite of transcripts implicated in hormonal regulation of vertebrate social behaviour. Our results reveal that divergent expression patterns in a conserved neuroendocrine centre known to regulate social-reproductive behaviours across vertebrate lineages may be uncoupled from developmental history to enable plasticity in the performance of reproductive tactics.