How and why do sex roles and secondary sexual characters evolve in animals, why do sex roles reverse sometimes, and how are the costs of sexual selection divided upon males and females? I target these questions in my research on pipefish.

Fishes in the family Syngnathidae, pipefishes and seahorses, rely heavily on crypsis and relative immobility to evade predation (e.g. by crabs, birds and larger pelagic fishes) in their typical seagrass habitats. However, courtship in many species is prolonged and relatively conspicuous.

Female syngnathids continuously mature eggs which are deposited in the male's brood pouch, or attached to his ventral surface. Females play no further role in parental care. The eggs are fertilised and the developing embryos are protected, aerated, osmoregulated and also nourished by the male brooding structure. Independent young are born after several weeks of pregnancy, after which remating may occur within hours.

There are several hundred species of syngnathids in about 35 genera, found throughout the world. Different genera of syngnathids show varying degrees of male brood pouch development, culminating in the sealed seahorse pouch.

We have studied two pipefish species, Syngnathus typhle and Nerophis ophidion, with the ambition to empirically verify the different elements of the following long chain of events: it begins with the two sexes investing differentially in offspring, which affects the relative potential reproductive rates of males and females, which in turn influences the operational sex ratio (males and females willing to mate), which in its turn decides which sex should be choosy and which becomes competitive. The outcome of these complex interactions and relationships governs the process of sexual selection, and the costs for sexual characters will constrain their evolution. This is what we have shown:

• Parental investment: male pregnancy length and male brooding capacity is not sufficient for all the eggs a female can produce ⇒

• Potential reproductive rates: males repoduce slower than females, even if partner availability is unlimited ⇒

• Operational sex ratio: males willing to mate are outnumbered by females willing to mate ⇒

• Females compete, males choose (but this is also influenced by predation, mate encounter rate, mate quality, parasites and more) ⇒

• Fecundity depends on number of mates, but more so in females than in males ⇒

• Sexual selection acts primarily on females ⇒

• Females evolve ornaments and female status signals

The beginning of the chain of events is already well understood: in neither of the two pipefish species does the male spend more energy on offspring than does the female, but the long male pregnancy lowers the potential reproductive rate of males below that of females. Temperature may modify but not reverse this sex difference. As females are faster reproducers the operational sex ratio becomes skewed to an excess of females.

Female pipefish compete for males through dominance hierarchies, and males are choosy. Choosiness is a plastic male trait that can be modified by predation threat (choosiness disappears in the presence of a predator), by the operational sex ratio (choosiness disappears under male excess), and by mate encounter rate (choosiness disappears under low encounter rates). In nature male S. typhle actively choose among and reject some females, whereas females vigorously display, often in groups in a lek-like fashion.

In the pipefish Syngnathus typhle females advertise their quality to males using two routes: first, body size is an important determinant of male mate choice because larger females provide more and larger eggs, which give rise to heavier and higher quality offspring. Body size is difficult to fake convincingly, and may therefore constitute a low-cost honest signal of quality.

Second, a temporary sexual coloration that females display during female-female interactions and during the nuptial dance preceding mating functions as an ornament and as a status badge, preferentially used by dominant females. This temporary female ornament:

• facilitates female size assessment by males

• predicts female mating success

• attracts males

• is costly in terms of predation risk

• is socially provocative

• is energetically inexpensive.