Reduced sexual size dimorphism in a pipefish population where males do not prefer larger females

Within a species' distribution, populations are often exposed to diverse environments and may thus experience different sources of both natural and sexual selection. These differences are likely to impact the balance between costs and benefits to individuals seeking reproduction, thus entailing evolutionary repercussions. Here, we look into an unusual population (Baltic Sea) of the broadnosed pipefish, Syngnathus typhle, where males do not seem to select females based on size and hypothesize that this pattern may derive from a reduction in direct benefits to the male. We further hypothesize that if larger females do not persistently secure a higher reproductive success, either through pre- or postcopulatory sexual selection, a decrease in sexual size dimorphism in the Baltic population should be apparent, especially when contrasted with a well-studied population, inhabiting similar latitudes (Swedish west coast), where males prefer larger females. We found that, in the Baltic population, variation in female quality is low. We were unable to find differences in abortion rates or protein concentration in oocytes produced by females of contrasting sizes. Direct benefits from mating with large partners seem, thus, reduced in the Baltic population. We also found no evidence of any postcopulatory mechanism that could favor larger mothers as embryo development was unrelated to female size. While female size can still be selected through intrasexual competition or fecundity selection, the pressure for large female body size seems to be lower in the Baltic. Accordingly, we found a noticeable decrease in sexual size dimorphism in the Baltic population. We conclude that, although far from negating the significance of other selective processes, sexual selection seems to have a decisive role in supporting pipefish sexual size asymmetries.

Pipefish embryo oxygenation, survival, and development: egg size, male size, and temperature effects

In animals with uniparental care, the quality of care provided by one sex can deeply impact the reproductive success of both sexes. Studying variation in parental care quality within a species and which factors may affect it can, therefore, shed important light on patterns of mate choice and other reproductive decisions observed in nature. Using Syngnathus typhle, a pipefish species with extensive uniparental male care, with embryos developing inside a brood pouch during a lengthy pregnancy, we assessed how egg size (which correlates positively with female size), male size, and water temperature affect brooding traits that relate to male care quality, all measured on day 18, approximately 1/3, of the brooding period. We found that larger males brooded eggs at lower densities, and their embryos were heavier than those of small males independent of initial egg size. However, large males had lower embryo survival relative to small males. We found no effect of egg size or of paternal size on within-pouch oxygen levels, but oxygen levels were significantly higher in the bottom than the middle section of the pouch. Males that brooded at higher temperatures had lower pouch oxygen levels presumably because of higher embryo developmental rates, as more developed embryos consume more oxygen. Together, our results suggest that small and large males follow distinct paternal strategies: large males positively affect embryo size whereas small males favor embryo survival. As females prefer large mates, offspring size at independence may be more important to female fitness than offspring survival during development.

The 'Woman in Red' effect: pipefish males curb pregnancies at the sight of an attractive female

In an old Gene Wilder movie, an attractive woman dressed in red devastated a man's current relationship. We have found a similar 'Woman in Red' effect in pipefish, a group of fish where pregnancy occurs in males. We tested for the existence of pregnancy blocks in pregnant male black-striped pipefish (Syngnathus abaster). We allowed pregnant males to see females that were larger and even more attractive than their original high-quality mates and monitored the survival and growth of developing offspring. After exposure to these extremely attractive females, males produced smaller offspring in more heterogeneous broods and showed a higher rate of spontaneous offspring abortion. Although we did not observe a full pregnancy block, our results show that males are able to reduce investment in current broods when faced with prospects of a more successful future reproduction with a potentially better mate. This 'Woman in Red' life-history trade-off between present and future reproduction has similarities to the Bruce effect, and our study represents, to our knowledge, the first documentation of such a phenomenon outside mammals.

Why do some animals mate with one partner rather than many? A review of causes and consequences of monogamy

Why do some animals mate with one partner rather than many? Here, I investigate factors related to (i) spatial constraints (habitat limitation, mate availability), (ii) time constraints (breeding synchrony, length of breeding season), (iii) need for parental care, and (iv) genetic compatibility, to see what support can be found in different taxa regarding the importance of these factors in explaining the occurrence of monogamy, whether shown by one sex (monogyny or monandry) or by both sexes (mutual monogamy). Focusing on reproductive rather than social monogamy whenever possible, I review the empirical literature for birds, mammals and fishes, with occasional examples from other taxa. Each of these factors can explain mating patterns in some taxa, but not in all. In general, there is mixed support for how well the factors listed above predict monogamy. The factor that shows greatest support across taxa is habitat limitation. By contrast, while a need for parental care might explain monogamy in freshwater fishes and birds, there is clear evidence that this is not the case in marine fishes and mammals. Hence, reproductive monogamy does not appear to have a single overriding explanation, but is more taxon specific. Genetic compatibility is a promising avenue for future work likely to improve our understanding of monogamy and other mating patterns. I also discuss eight important consequences of reproductive monogamy: (i) parentage, (ii) parental care, (iii) eusociality and altruism, (iv) infanticide, (v) effective population size, (vi) mate choice before mating, (vii) sexual selection, and (viii) sexual conflict. Of these, eusociality and infanticide have been subject to debate, briefly summarised herein. A common expectation is that monogamy leads to little sexual conflict and no or little sexual selection. However, as reviewed here, sexual selection can be substantial under mutual monogamy, and both sexes can be subject to such selection. Under long-term mutual monogamy, mate quality is obviously more important than mate numbers, which in turn affects the need for pre-mating mate choice. Overall, I conclude that, despite much research on genetic mating patterns, reproductive monogamy is still surprisingly poorly understood and further experimental and comparative work is needed. This review identifies several areas in need of more data and also proposes new hypotheses to test.

Costs and Benefits to Pregnant Male Pipefish Caring for Broods of Different Sizes

Trade-offs between brood size and offspring size, offspring survival, parental condition or parental survival are classic assumptions in life history biology. A reduction in brood size may lessen these costs of care, but offspring mortality can also result in an energetic gain, if parents are able to utilize the nutrients from the demised young. Males of the broad-nosed pipefish (Syngnathus typhle) care for the offspring by brooding embryos in a brood pouch. Brooding males can absorb nutrients that emanate from embryos, and there is often a reduction in offspring number over the brooding period. In this study, using two experimentally determined brood sizes (partially and fully filled brood pouches), we found that full broods resulted in larger number of developing offspring, despite significantly higher absolute and relative embryo mortality, compared to partial broods. Male survival was also affected by brood size, with males caring for full broods having poorer survival, an effect that together with the reduced embryo survival was found to negate the benefit of large broods. We found that embryo mortality was lower when the brooding males were in good initial condition, that embryos in broods with low embryo mortality weighed more, and surprisingly, that males in higher initial condition had embryos of lower weight. Brood size, however, did not affect embryo weight. Male final condition, but not initial condition, correlated with higher male survival. Taken together, our results show costs and benefits of caring for large brood sizes, where the numerical benefits come with costs in terms of both embryo survival and survival of the brooding father, effects that are often mediated via male condition.

Kinship and the evolution of social behaviours in the sea

Until recently, little attention has been paid to the existence of kin structure in the sea, despite the fact that many marine organisms are sessile or sedentary. This lack of attention to kin structure, and its impacts on social evolution, historically stems from the pervasive assumption that the dispersal of gametes and larvae is almost always sufficient to prevent any persistent associations of closely related offspring or adults. However, growing evidence, both theoretical and empirical, casts doubt on the generality of this assumption, not only in species with limited dispersal, but also in species with long dispersive phases. Moreover, many marine organisms either internally brood their progeny or package them in nurseries, both of which provide ample opportunities for kinship to influence the nature and outcomes of social interactions among family members. As the evidence for kin structure within marine populations mounts, it follows that kin selection may play a far greater role in the evolution of both behaviours and life histories of marine organisms than is presently appreciated.

Polyandry as a mediator of sexual selection before and after mating

The Darwin–Bateman paradigm recognizes competition among males for access to multiple mates as the main driver of sexual selection. Increasingly, however, females are also being found to benefit from multiple mating so that polyandry can generate competition among females for access to multiple males, and impose sexual selection on female traits that influence their mating success. Polyandry can reduce a male's ability to monopolize females, and thus weaken male focused sexual selection. Perhaps the most important effect of polyandry on males arises because of sperm competition and cryptic female choice. Polyandry favours increased male ejaculate expenditure that can affect sexual selection on males by reducing their potential reproductive rate. Moreover, sexual selection after mating can ameliorate or exaggerate sexual selection before mating. Currently, estimates of sexual selection intensity rely heavily on measures of male mating success, but polyandry now raises serious questions over the validity of such approaches. Future work must take into account both pre- and post-copulatory episodes of selection. A change in focus from the products of sexual selection expected in males, to less obvious traits in females, such as sensory perception, is likely to reveal a greater role of sexual selection in female evolution.

Exclusive male care despite extreme female promiscuity and low paternity in a marine snail

Males exhibit striking variation in the degree to which they invest in offspring, from merely provisioning females with sperm, to providing exclusive post-zygotic care. Paternity assurance is often invoked to explain this variation: the greater a male's confidence of paternity, the more he should be willing to provide care. Here, we report a striking exception to expectations based on paternity assurance: despite high levels of female promiscuity, males of a marine snail provide exclusive, and costly, care of offspring. Remarkably, genetic paternity analyses reveal cuckoldry in all broods, with fewer than 25% of offspring being sired by the caring male, although caring males sired proportionally more offspring in a given clutch than any other fathers did individually. This system presents the most extreme example of the coexistence of high levels of female promiscuity, low paternity, and costly male care, and emphasises the still unresolved roles of natural and sexual selection in the evolution of male parental care.

The ovarian structure and mode of egg production in two polygamous pipefishes: a link to mating pattern

In this study, the ovarian structure and mode of egg production were examined in two pipefishes, the broad-nosed pipefish Syngnathus typhle and the straight-nosed pipefish Nerophis ophidion, which show different types of polygamous mating patterns. Syngnathus typhle showed an ovary with one germinal ridge and asynchronous egg production, corresponding to previous findings in other polygamous Syngnathus pipefishes. In contrast, the ovary of N. ophidion had two germinal ridges and eggs were produced synchronously in groups, similar to what has been observed in monogamous syngnathids. The egg production of N. ophidion, however, is clearly distinguished from that of monogamous syngnathids by the additional egg production after an ovulation. It is suggested that the differences in female mating strategies result from the difference in egg production process and that this is related to the difference in mating pattern between these two polygamous species.