Nagasaki University unveils water bug mating behavior: 35% of carried eggs are not their own

A research group led by Associate Professor Shin-ya Ohba from Nagasaki University, Assistant Professor Tomoya Suzuki from Hiroshima Shudo University (Shinshu University when the research began), and Professor Koji Tojo from Shinshu University has revealed that approximately 35% of the eggs carried and cared for by male water bugs "are not their own offspring." Water bugs are a unique bug whose males carry and care for eggs themselves. Males who carry more eggs, including those not fertilized by themselves, are more likely to be chosen as mates. Conversely, some males have their own fertilized eggs cared for by other males. The reproductive behavior of fathers raising offspring alone appears to conceal complex and diverse reproductive strategies. The group's outcomes were published in Ecology and Evolution.

Female water bugs lay eggs on the males' backs, and males provide sole protection until they hatch. Male water bugs can carry over 100 eggs, performing paternal care with such large egg clusters that they become unable to fly and face increased predation risks. Females mate with multiple males and have a sperm storage organ (spermathecae) that temporarily stores sperm from different partners. As a result, it's possible that not all the eggs carried by a male are fertilized by his own sperm.

The research group identified and freely bred 20 pairs of water bugs in a breeding case for about a month, conducting parentage analysis based on DNA information from hatched larvae and parent individuals. The results showed that many males and females mate with multiple partners, revealing an extremely promiscuous reproductive system. The average paternity of males was about 65%, with individual variations ranging from 10% to 100%.

Essentially, males who protected more eggs compared with other males had more offspring. This supports previous research hypotheses that "the very act of egg protection becomes a condition for being chosen as a mate," in other words, "paternal care evolved through female selection of males with egg-caring behaviors."

However, complex male interactions were also detected. Many males end up caring for others' offspring while having their own offspring cared for by other males. Some males were even confirmed to have offspring without performing any paternal care at all - essentially mating, transferring sperm to the female's spermathecae, and then leaving without providing paternal care.

Furthermore, the study revealed that multiple females cooperatively form a single egg clutch. Male water bugs discard eggs if they carry too few, but a single female cannot lay enough eggs to trigger male paternal care. This suggests their egg formation might have evolved into cooperative formation of a single egg clutch by multiple females. Additionally, females were observed distributing eggs across multiple males' backs, potentially an adaptive strategy to prevent total egg loss if a male dies during paternal care.

These results support the hypothesis that "paternal care can evolve even with low paternity certainty," challenging the traditional view that "high paternity is essential for the evolution of paternal care."

Journal Information
Publication: Ecology and Evolution
Title: Reproductive Strategies in Paternal Care and Remarkably Low Paternity Level in a Giant Water Bug
DOI: 10.1002/ece3.71316