Abstract
Males of many butterfly species secrete long-lasting mating plugs to prevent their mates from copulating with other males, thus ensuring their sperm will fertilize all future eggs laid. Certain species have further developed a greatly enlarged, often spectacular, externalized plug, termed a sphragis. This distinctive structure results from complex adaptations in both male and female genitalia and is qualitatively distinct from the amorphous, internal mating plugs of other species. Intermediate conditions between internal plug and external sphragis are rare. The term sphragis has often been misunderstood in recent years, hence we provide a formal definition based on accepted usage throughout most of the last century. Despite it being a highly apparent trait, neither the incidence nor diversity of the sphragis has been systematically documented. We record a sphragis or related structure in 273 butterfly species, representing 72 species of Papilionidae in 13 genera, and 201 species of Nymphalidae in 9 genera. These figures represent respectively, 13% of Papilionidae, 3% of Nymphalidae, and 1% of known butterfly species. A well-formed sphragis evolved independently in at least five butterfly subfamilies, with a rudimentary structure also occurring in an additional subfamily. The sphragis is probably the plesiomorphic condition in groups such as Parnassius (Papilionidae: Parnassiinae) and many Acraeini (Nymphalidae: Heliconiinae). Some butterflies, such as those belonging to the Parnassius simo group, have apparently lost the structure secondarily. The material cost of producing the sphragis is considerable. It is typically offset by production of a smaller spermatophore, thus reducing the amount of male-derived nutrients donated to the female during mating for use in oogenesis and/or somatic maintenance. The sphragis potentially represents one of the clearest examples of mate conflict known. Investigating its biology should yield testable hypotheses to further our understanding of the selective processes at play in an ‘arms race’ between the sexes. This paper provides an overview, which will inform future study.
Highlights
Male butterflies, like most animals, typically maximize their reproductive success by mating with as many females as possible
For our museum specimen searches, we especially targeted Nymphalidae and Papilionidae because only in these families has the sphragis been reliably reported by previous authors
The main outcomes are as follows: 1. A true sphragis was found in 232 species of butterflies from the families Papilionidae and Nymphalidae
Summary
Like most animals, typically maximize their reproductive success by mating with as many females as possible. Female butterflies possess sperm storage organs which can maintain vital sperm from a single mating for their entire life (Chapman 1969, Parker 1970, 1984, Ferro and Akre 1975, Ehrlich and Ehrlich 1978, Walker 1980, Thornhill and Alcock 1983, Drummond 1984, Gwynne 1984, Orr 1988, Solensky and Oberhauser 2009, Klowden 2013). This may result from the need for: more male-derived nutrients received during copulation, more sperm (especially if their first mate was depleted), increasing the genetic quality of the fertilized eggs by mating with a fitter male, increased genetic diversity of offspring in an unpredictable environment, or reducing the energy loss and risk of harm involved in resisting the attempts of copulation with new males. With many factors potentially influencing female mating, there is great interspecific variation in female mating frequencies, ranging from obligate monandry to regular polyandry (Scott 1972, Walker 1980, Thornhill and Alcock 1983, Knowlton and Greenwell 1984, Arnqvist and Nilsson 2000, Blanckenhorn et al 2002, Wedell 2005)
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