Humans can be forgiven for assuming that successful reproduction requires both partners to be alive. Amazingly, this isn't the case for the Trinidadian guppy. Under laboratory conditions, female guppies are able to store sperm for up to a year, potentially allowing them to produce offspring even if the males are no longer around. However, several other studies have suggested that the last male to mate with a female ends up fertilizing the bulk of her offspring. This last-male-in advantage may mean that stored sperm is rarely used by guppies in nature. Studying this phenomenon in natural surroundings poses many logistical problems, but in a recent report, Andrés López-Sepulcre and his international team of collaborators not only proved that they were up to the challenge of tackling this problem in the wild but also discovered just how often female guppies use the sperm of deceased mates.
Undertaking a year-long mark–recapture study in Trinidad, the team introduced individually marked guppies with known genetic ‘fingerprints’ to a short stretch of the Guanapo River, which is isolated from existing downstream guppy populations by a swiftly flowing waterfall. Then, each month, the researchers painstakingly caught and identified almost every guppy longer than 13 mm present in the study site. These monthly assessments were used to determine when each guppy died, and also allowed the researchers to collect a few scales from each guppy born at the field site so that their mother and father could be determined using genetic fingerprinting. López-Sepulcre and colleagues then developed a complex mathematical model to estimate the reproductive contributions of dead males to the introduced guppy population.
The research team found that almost 50% of reproductively active males sired young after they had died and, amazingly, over 30% of reproductive males were successful only after they were dead. Some offspring were even fathered by males that had been dead for 8 months. Perhaps this allows relatively short-lived males (with an average lifespan of 3 months) to continue to mate with females over their much longer lives (15 months). The researchers also suggested that sperm storage would allow the genetic material of male guppies to survive the rainy season, when males die in much higher numbers than females. From the female perspective, the researchers proposed that using sperm from many males, both dead and alive, would produce offspring with higher genetic diversity. In a fluctuating habitat like a Trinidadian stream, this should increase the odds of producing some offspring that are genetically well suited to whatever environmental conditions the future has in store.
By the end of their year-long study, the researchers found that posthumously conceived guppies comprised almost 25% of the total reproductive population. This both highlights the importance of stored sperm to female guppies in nature and also raises red flags for measurements of genetic diversity in other species where females can store sperm: if the potential genetic contributions of dead males are not assessed, population sizes may be significantly underestimated. Apparently, you won't hear, ‘Till death do us part’, at a guppy wedding.
