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Friday, 12 February 2021

Cockroaches are CANNIBALS: Insect couples take turns to gnaw away each other's wings after they mate for life

  • Japanese researchers observed pairs of wood‐feeding cockroaches in the lab
  • The team found most of the couples engaged in mutual grooming and nibbling  
  • This unusual and mutual display differs from other forms of sexual cannibalismA species of cockroach eats a little bit of its partner's wings after mating, a new study reveals. 

    Japanese scientists who studied the wood‐feeding cockroach (Salganea taiwanensis) found both male and female consumed the other's wings.

    The species enjoys a post-coital process that consists of multiple stages, starting with a bit of flirtatious grooming, before progressing to eating.

    The cockroach that's being eaten can then violently shake its body to throw the other off once it's had enough of being nibbled at, researchers observed.

    Over time, S. taiwanensis cockroaches can gnaw their partner's wings down to a stub, becoming smaller and smaller as they get older.   

    Their 'mutual' nibbling is highly unusual and differs from the more common case of sexual cannibalism in insects, where the dominant female kills and eats the male after mating.  

    This species may be 'truly monogamous' – meaning it has sexual relationship with only one partner throughout its whole lifetime, researchers say. 

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    Adults of Salganea taiwanensis before (right) and after (left) mutual wing‐eating behaviour. The right individual has long intact wings. The left individual has short wings. Most of its wings have been eaten by its mate

    Adults of Salganea taiwanensis before (right) and after (left) mutual wing‐eating behaviour. The right individual has long intact wings. The left individual has short wings. Most of its wings have been eaten by its mateTrue monogamy is 'complete and lifelong' and is the only condition in which mating is freed of sexual conflict, but it 'may be compromised by betrayal behaviour'. 

    Nibbling on each other's wings may help keep them trim and 'reduce maintenance costs' of flight, the researchers suggest. 

    This may in some way let them invest further in reproduction or care for offspring. 

    The behaviour could also act as hygiene management against mites or mould, which were observed on their wings in the laboratory.'Within mating pairs of a wood‐feeding cockroach, males and females eat the mate's wings each other, which is the first “mutual” case in these behaviours,' say Haruka Osaki and Eiiti Kasuya from Kyushu University's Department of Biology.

    'Mutual wing‐eating may be an example of true cooperation and may help explain some interactions between females and males from a new viewpoint.

    'If the wing eating increases the ability of the mate to raise the offspring, this behaviour is adaptive for the eater as well as its mate.' 

    Researchers pointed out that since wings include no flesh, the value of wings as a food resource 'appears to be negligible'. 

    In the case of the species S. taiwanensis, new adults fly away from their natal colony, a rotten log in a forest, to search for a mate.  

    Osaki told the New York Times that she had been collecting insects from the woods as a hobby since becoming a biology student at Kyushu University.

    'When I caught the wood-feeding cockroaches in the wild, I noticed that their wings were chewed by something,' she said.  

    A drawing of a mating pair during mutual wing‐eating behaviour by study author Haruka Osaki. The individual in the front, which already has wings chewed, is eating the wings of its mate

    A drawing of a mating pair during mutual wing‐eating behaviour by study author Haruka Osaki. The individual in the front, which already has wings chewed, is eating the wings of its mate 

    For their experiments, Osaki and Kasuya collected 24 young adult pairs of the species from Yona Field, Okinawa, Japan, which were taken back to the lab, placed together in containers and video recorded for three days.

    In all, 12 of the pairs engaged in wing-eating until the 'wings of both sexes became as short as those of wild parents'.

    Overall, the area of the wings eaten was approximately 66.7 per cent of the entire wing area in males and 68.3 per cent in females in these 12 pairs. 

    Of the remaining pairs, nine ate wings partially and three pairs did not eat wings at all.  

    The team have also broken down the 'typical sequence' of mutual cannibalism involving the 'actor', the individual that is actively eating the wings, and the 'recipient', the mate of the actor that's being nibbled. 

    Both male and female can be an actor, meaning the roles are reversed after a single bout (which consists of grooming, nibbling and wagging).

    In 12 of the 12 pairs, the actor initially approached its mate and touched the antennae to the other insect and licked its body parts, including the head, wings and legs, as a form of grooming.  

    The actor then climbed onto the recipient's back and ate the recipient's wings, while the recipient was stationary, although in some cases the recipient actually leaned towards the actor. 

    Whenever the actor stopped eating, in 61.7 per cent of times the recipient remained motionless, while in the other 38.3 per cent of times, the recipient violently shook its body left and right (called 'wagging'), researchers observed. 

    After the recipients wagged, the actor stopped eating more often. 

    A typical sequence of mutual wing‐eating behaviour. Both male and female can be an actor in each bout

    A typical sequence of mutual wing‐eating behaviour. Both male and female can be an actor in each bout

    Once a bout was stopped, the pair usually did not groom and eat wings until the next bout. 

    During the intervals between bouts, both sexes were either motionless, walking, or digging – scratching the ground using its forelegs. 

    This new study, published in Ethology, suggests the wood‐feeding cockroach benefits from a much more peaceful and harmonious form of insect cannibalism than others found in the wild. 

    'Mutual wing‐eating is like sexual cannibalism, but it differs in that one mate is not killed,' they say. 

    Sexual cannibalism in insects is well documented and tends to be the dominant female killing and eating the male after mating. 

    Generally, the benefits of sexual cannibalism may include obtaining nourishment for reproduction or encouraging males to avoid mating with inferior females, previous studies show.  

    From the male's point of view, it is evolutionarily beneficial for him to sacrifice himself for the good of his mate and his offspring, which benefit from his vital nutrientsIn some cases, such as the praying mantis, females eat males before they get a chance to mate, especially if they're deemed unfit or unsuitable, and thus as seen to have more of a nutritional benefit than a reproductive benefit. 

    The male has evolved to put up a fight, often leading to a violent struggle between the pair, to increase his chances of mating successfully. 

    'Sexual cannibalism usually involves the female consuming the male – the reverse type is rare,' say Osaki and Kasuya.

    Another form of sexual cannibalism is called nuptial feeding, where the male presents a small 'gift' from some part of his body to the female for her to eat.   

    In nuptial feeding, males usually give materials other than sperm to improve their number of offspring, as detailed in a 2014 study in Biology Letters.  

    Female ‘sex cannibal’ spiders try to eat their mates as a TEST to find the most suitable male 

    Sexual cannibals, like the female raft spider, have long been thought to chow down on their mates as a result of uncontrollable aggression.

    A 2015 study from the University of Melbourne suggests that these femme fatales are more calculated than their reputation suggests, and are in fact putting potential suitors to the test by attacking them.

    The researchers, including Mark Elgar of the University of Melbourne, observed multiple mating trials between female and male raft spiders.

    In the first trial, 16 females copulated with the male spiders. 

    Of this group, 11 of the females attacked the male during or after mating; only four of the attacks were fatal.

    The next trial had similar results, with a high number of attacks, but few fatalities. Six of the eight females that mated with the males in this group then attacked them, but only two males were killed.

    'Females are sexually cannibalistic because they are testing the males, rather than just being inherently aggressive,' Dr Elgar said.

    'We conducted extensive experiments and found no correlation between female foraging aggression and that displayed toward males.'

    The female raft spiders showed no consistency in their aggression. While more than half were aggressive in sequential mating trials, some female spiders were aggressive in one trial, but not in the next.

    'It actually appears there are many other potential factors involved, including the size of the males in comparison to the female, the female's age and whether or not she is a virgin.'

    'Females were marginally more likely to attack smaller males, and that's perhaps because they are easier to capture,' Dr. Elgar said.

    Raft spiders are not the only arachnid species known to cannibalise their mates, and other studies reveal that females are not the only ones who are guilty.

    The 'Black Widow myth,' which assumes that only female spiders eat their partners, was refuted in a 2013 study.

    The study from Masaryk University tracked the behaviours of Micaria sociabilis spiders over two years, and found that the males also partake in sexual cannibalism.

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