Researchers found female nematodes that mated with males from New York died more quickly than those mated to males from Ohio or Germany.
The research team looked at the nematode Caenorhabditis remanei and found females that mated with New York males for even one day lived half as long as they did spending their lives as virgins, the University of Oregon reported. Females that mated with males from New York also produced half as many offspring as those that mated with males from other regions. To determine what was causing these effects, the researchers put the males together in a competition. The team found sperm from the New York males strongly out-competed sperm from the Ohio and Germany males to fertilize female eggs.
"Despite their small size, nematode sperm is actually much larger than human sperm, and it is thought that the sperm from different males literally battle it out inside the female for access to her eggs," said study leader Patrick C. Phillips. "So a reasonable evolutionary explanation would be that these males make bad mates but highly successful fathers."
The team also looked at a species of nematode called Caenorhabditis elegans, which is a roundworm that usually exists as a hermaphrodite. These nematodes reproduce using sperm and eggs that are produced within the same individual. The scientists used a genetic mutation to block sperm in the hermaphrodites, transforming them into females. The females were mated with males for 60 generations. They observed the males developed larger sperm as a result of the competitive environment, and the females that mated with them died more frequently than those that mated with males not subjected to high-male competition.
"Overall, we were able to rapidly recapitulate the evolutionary pattern that we see in the male-female species by converting a hermaphroditic species to be male-female and then allowing them to evolve under these new circumstances," said Michael Palopoli, an evolutionary biologist at Bowdoin College in Brunswick.
The researchers are still not sure why the high-competition males have a lethal effect on females.
"It could be a change in the behavior of the males, or it could be something in the seminal fluid that they transfer during mating," Phillips said. "We are following up on this work to figure that out."
The findings were published in a recent edition of the journal BMC Evolutionary Biology.