Copulatory plugs are handy little things for males to have: Once they’re in the female, plugs stop sperm leakage and prevent females from mating with other males. These non-sperm components of ejaculate are usually cheap and easy to make, but males of some promiscuous species actually set aside a surprisingly substantial amount of energy for plug production. For a male red-sided garter snake, it can cost up to 18 percent of his daily energy expenditure. The findings were published in the Journal of Experimental Biology last week.
Previous work has suggested that seminal fluid production is costly in red-sided garter snakes (Thamnophis sirtalis parietalis). These males produce and store their sperm late in the summer, but most of their seminal fluid components (like the plugs) are produced in the spring. So, a team led by Christopher Friesen from the University of Sydney wanted to tease apart the costs of seminal fluid production from that of sperm production.
The team collected dozens of males in Manitoba, Canada, as they emerged from their hibernation chambers, and then presented them with females. (To get an idea of what that’s like, check out this video of the world’s largest snake orgy.) Half of the males successfully mated with the females, while the other half were thwarted by tape strategically placed over the cloaca—the opening that females use for both mating and pooping.
Over the course of nine days, the team measured the energy consumption of the males, Inside JEB reports, and found that it was around 50 percent higher than that of males during non-mating seasons. And since males don’t eat during mating season, the team tracked their mass loss: Males who mated at least five times lost up to eight grams, while males who mated just once lost between four and six grams.
When the team estimated the cost of producing seminal plugs, they discovered that males invested between five and 18 percent of their daily energy expenditure on a single plug (pictured above in a female).
Additionally, the team calculated the energy consumption for each male as they courted and mated with females. "Small males have a harder go of it,” Friesen tells Live Science. The smallest males invested up to eight times more energy in seminal fluid production than the largest males. During courtship, the metabolic rates of the largest males barely rose at all, while that of the smallest males rose by 30 percent—and soared by nearly 50 percent when they mated successfully. The researchers think these smaller males throw everything they can into each and every mating opportunity because they may not live long enough to become the older, larger snake. Manitoba winters are harsh (and they’re coming).