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Some mouthbrooding fish eat their children to reduce stress
Some cichlid fish mothers snack on their own offspring to ensure their own survival.
New research suggests that the mothers can boost their chemical defences by using antioxidants found in their own children to reduce levels of DNA damage.
Fish raised in their mother's mouth can sometimes end up their parent's next meal.
Female Astatotilapia burtoni fish stop feeding for as long as two weeks after their eggs are fertilised, holding the eggs and the hatched young in their mouth until big enough to emerge. However, this mouthbrooding behaviour can cause the build-up of chemicals which damage cells and DNA in the parents body.
Researchers investigating how the mothers tolerate this damage inadvertently discovered that the offspring have a role to play.
Their findings, published in the journal Biology Letters, suggest that the female fish can reduce oxidative damage to their cells by selectively consuming their children, giving the mothers an energy boost they can use to bolster their antioxidant defences.
Dr Peter Dijkstra, who co-authored the study, says, 'We've worked with these fish for over a decade with hundreds of hours of observations, and we have never observed mothers spitting out their eggs or developing fish (known as fry).'
'While mouthbrooding, and cannibalism of children, is common in other cichlid fish, the discovery that filial cannibalism was taking place in this species was accidental.'
A wide range of animals, including the oriental pied hornbill, have been observed eating their young, though the exact reasons remain unclear. Image © sbw18/Shutterstock
Why do some animals eat their children?
Filial cannibalism is where a parent consumes one or more of their offspring. It is a surprisingly common behaviour in nature, with birds, insects, reptiles, amphibians and even some mammals observed to eat their own children.
As with other behaviours frowned upon by humans, such as nose picking, it is not well understood why filial cannibalism takes place. At face value, consuming offspring should be a behaviour selected against by evolution as it reduces the genetic material of the parent being passed on.
What research has taken place suggests a number of different theories to explain the behaviour, of which the most common is that the babies are simply a source of energy.
Raising offspring is costly to the parent, so consuming their offspring reduces some of these costs and makes the survival of the parent more likely, increasing their breeding chances in the future and thus an unlikely evolutionary advantage.
Eating some children can also, counterintuitively, help the survival of the rest. If faced with a lack of resources, animals may consume some of their offspring to help the rest survive. For instance, the burying beetle Nicrophorus orbicollis will consume its own children to match the size of a buried carcass they are feed on.
Filial cannibalism may also be an accidental side effect of rooting out imposters. In externally fertilised animals like fish, it's much easier for eggs to be fertilised by a male other than the partner of the mother. This can lead some males to accidentally eat their own children as they try to root out any imposters.
Even if it's not a very pleasant area of research, understanding why filial cannibalism takes place in the wild can help to explain how and why parental care evolved in the first place.
As one of the most extreme lengths a parent can go to for looking after offspring, the mouthbrooding of A. burtoni offers a new perspective on this behaviour.
The eye of a young Astatotilapia burtoni fry can be seen peeking through the transparent skin of its mother's lip. Image © Adrian Indermaur, Salzburger lab
How is child eating in fish related to stress?
Mouthbrooding is the most stressful time in a female A. burtoni's life. It reduces their ability to tolerate changes in their environment, with previous research showing that loud noises can cause mouthbrooding females to prematurely eject their offspring or eat them.
To investigate the impact of mouthbrooding on these fish, the researchers took females and separated them into three groups, brooders, non-brooders and a control group, before then testing them to see how much stress they were under at the start, in the middle, and at the end of the mouthbrooding period.
The researchers found that mouthbrooding increased the levels of reactive oxygen species (ROS) significantly across the brooding period as the fish were pushed to the limit. These chemicals can damage an individual's DNA, and are countered by the production of antioxidants.
In the early stages of mouthbrooding, DNA damage was almost 24% higher in brooding than control females. But the levels of damage fell during the full course of the experiment, which suggests that the beginning of mouthbrooding is the costliest point for the fish as it alters how its body functions.
Later in the process, consumption of offspring appears to help the mother's survival. Eating more offspring was linked to subsequent increases in the amount of antioxidants in the mothers, particularly towards the end of the two week brooding period.
This potentially protects the fish against the increase in ROS resulting from reproduction and subsequent mouthbrooding.
'Mothers can derive nutrients, and probably also antioxidants, from their babies,' Peter says. 'This in turn can also upregulate certain genes in the mother which produce other antioxidants.'
'However, it is also possible that the levels of ROS influence the probability of mothers ingesting their babies, which is something we are currently investigating by feeding mothers a certain number of fry and then testing the mother's liver antioxidant function.'
The researchers hope that these experiments will demonstrate whether child eating is a symptom of stress in these fish, or the mothers' unusual way of dealing with it.
