The little-known phenomenon that's making it harder for some animals to mate in the wild
Mating calls and rituals can be intercepted, or drowned out, by invasive species. Scientists are just starting to understand the consequences.
Invasive species are everywhere. Burmese pythons are literally eating their way through the Florida Everglades, and bullfrogs are making an all-you-can-eat buffet out of the American West. Consuming resources and taking over habitats are well-studied tactics for ecological invaders. But some introduced species may be taking an invasion of privacy to entirely new levels by interfering with the most intimate moments in the lives of native wildlife.
A review study recently published in the journal Trends in Ecology & Evolution found that invaders are masking mating calls, wasting precious time in a small window of mating opportunity, and even cannibalizing tragically misled bachelors. “Sometimes the mere presence of an invader can inhibit the reproduction of native species,” says Morelia Camacho-Cervantes, a biologist at the National Autonomous University of Mexico who co-authored the review with her colleague Constantino Macías Garcia.
These are all forms of a phenomenon called reproductive interference. In total, the team found 11 independent examples from six major taxonomic classes. This number is a conservative estimate for how common reproductive interference from invasive species might be in the wild, Camacho-Cervantes says, because they only included examples that had been experimentally or observationally demonstrated. This may be part of the reason vulnerable native species are decreasing in numbers.
“I am somewhat disappointed from a conservation perspective, because what we found suggests that invasive species are affecting native organisms through mechanisms that have barely been studied, and in some cases not even considered,” Camacho Cervantes says.
(Inside Florida’s war on invasive species.)
All flash and no substance
Camacho-Cervantes first became interested in this unexplored aspect of biological invasions while working on fish in Mexico.
Invasive guppies (Poecilia reticulata) originally native to South America and some Caribbean islands have been introduced to river systems across the world, including those in Mexico, either as released pets or in an effort to control mosquitos.
In the Central Mexican Plateau, the guppies quickly began to displace native fish species, particularly similarly-sized Skiffia bilineata goodeids, and bad sex may give the invaders an edge. Male guppies will try to mate with female Skiffia bilineata, but they simply aren’t very good at it. Guppies try to mate either by displaying their flashy iridescent colors, or by skipping the preamble and just sneaking in to mate quickly with relatively elongated sexual organs while a female isn’t paying attention too closely.
At best, these attempts can interrupt mating opportunities for the goodeids, which only reproduce seasonally rather than the prolific guppies. At worst, the overzealous guppies may injure or even kill native female Skiffia bilineata, says Camacho-Cervantes. The Skiffia bilineata “get harassed, and they need to keep moving and avoiding guppies.”
That means the goodeids spend less time mating, and thereby reproducing, with their own species—a problem that can lead to a drop in population for a sensitive group of fish already facing threats from pollution and human changes to water flow and conditions. Some of Mexico’s endemic goodeids are endangered or vulnerable while others have already gone extinct in recent decades.
It’s unclear how much guppies were responsible for these extinctions, but they could have played a role. “Many goodeids went extinct before the mechanisms could be studied, so we cannot determine retrospectively whether invasive [guppies] played a decisive role, but we think they did,” Camacho-Cervantes says.
Love unto death
Invasive guppies may occasionally kill the unfortunate subjects of their affection, but it’s probably not the guppies’ intentions.
This isn’t so with female praying mantises, which are notorious for sexual cannibalism. Mating has always been a zero-sum game for many mantis males. Females will sometimes forego any pillow talk in favor of eating their former suitor after mating—a way to gain some of the energy needed to produce and lay eggs.
But the males of New Zealand’s only native mantis species, Orthodera novaezealandiae, are throwing their love and lives away for nothing. Research has revealed that the native males are attracted to invasive females from the South African springbok species (Miomantis caffra)—perhaps due to confusing the chemical signaling. The invasive females are sexually cannibalistic while the native species usually aren’t. So, the invasive females, perhaps also confused, allow the mating attempt, but the whole affair still leaves them with an appetite. Meanwhile their male New Zealand suitors never realized the mortal peril of their courtships. “That paper was very illustrative of the worst outcome,” Camacho-Cervantes says.
(Female invasive joro spiders may dabble in cannibalism.)
Losing the language of love
Invasive species can also block native species from mating in less direct ways. In 2012, Camila Both, now an ecologist at Federal University of Rio Grande do Sul in Brazil, and her colleague conducted experiments playing back the calls of American bullfrogs, which are invasive in Brazil, to white-banded tree frogs native to the imperiled Atlantic Rainforest.
They found that the bullfrogs seemed to shout down the native species. After a while, the native tree frogs had to change the way they advertise their suitability. “This tree frog, they completely changed the parameters of their calls,” says Both.
Since then, other cases of invasive frogs masking the mating calls of native frogs have emerged—invasive cane toads drown out the sounds of marbled frogs in Australia and invasive Cuban tree frogs interfere with the mating calls of amphibians native to the U.S. Southeast.
This sound masking likely makes native amphibians waste energy, causing stress that could affect their immune systems. “The mating call is an energetically costly activity,” Both says. In some cases, the native species will just leave areas where their serenades are being drowned out by the invasive racket.
It’s encouraging that in white-banded tree frogs and a few other species around the world males have learned to change their calls. Both says some evidence suggests that females might be learning to be more receptive to the new calls. But they could still feel the negative effects of breeding interference.
(New Zealand wants to kill millions of invasive predators to save these fragile birds.)
Dire outcomes for natives—and invasives
Even invasives themselves can fall victim to sexual interference; sometimes successive waves of different invasive species can sexually eliminate each other. Native to Africa, Aedes aegypti mosquitoes, responsible for spreading diseases like yellow fever, dengue and Chikungunya, likely arrived in the United States on slave ships centuries ago. The species quickly invaded southern states, until a new invader arrived in the 20th century—another disease spreader called the Asian tiger mosquito, Aedes albopictus.
Whenever the newer Asian tiger mosquitoes arrived in an area with established populations of Aedes aegypti, the latter’s populations would collapse within three years. Both species are fairly similar in appearance, and researchers found that even the mosquitoes themselves couldn’t always tell each other apart. When a male Asian tiger mosquito tried to mate with a female Aedes aegypti, she couldn’t produce viable offspring and became sterile—forever.
In this case, there was no real harm done to the ecosystem as both species were invasive. But these kinds of discoveries about failed mating trysts can inform human strategies to control invasive disease-carrying insects. “Sometimes the mistake can shed light on what’s going on normally,” says Irka Bargielowski, the lead author of a study on the behavior published in 2015. She wasn’t involved in Camacho-Cervantes’ new review, but says reproductive interference is “way more common than we realize.”
It’s hard to quantify the impact that this interference has on native species, but it’s clear that sexual misdirects and mismatches have an impact. “I’m certain that invasive species impair breeding,” says Both. “We just don’t focus on these subtle effects.”
For Camacho-Cerventes, these effects may be more than subtle. The fact that reproductive interference is happening in so many different types of animals on different continents means that it’s likely one of the factors that causes invasives to outcompete native species, reducing populations and even contributing to extinction. The collection of studies she and her colleagues identified in their recent review may just be the tip of the iceberg. “It could be happening,” she cautions, “but we are not looking for it.”
