Wildlife is both beautiful and cruel world. If is also full of phenomenon that is not exactly easy to understand. One of them is infanticide, which is rather common in many mammal species, including lions, mountain gorillas, monkeys, and mice. It is a phenomenon when male mammals attack and kill the offspring of other males.
However, males that usually are prone to infanticide change when they themselves become parents. Now scientists at the RIKEN Brain Science Institute in Japan have figured out the mechanism behind this change.
Scientists conducted experiments with mice and discovered two small brain regions that control which of these very opposite behaviours – aggressive towards infants or fatherly – a male mouse will exhibit. In short, there is a mechanism, which basically regulates whether mouse will attack mouse pups of will protect them. Infanticide is a rather simple part of male reproductive strategy – if a female mammal is not taking care of pups of other father, she is available to mating attempts for other males.
Therefore, mice kill other mice pups in order to free up the mother from her duties to take care of her offspring. However, only a virgin male mouse shows such aggression towards newborn pups fathered by other males. After mating and taking care of a pregnant female, a male mouse will not only keep offspring alive, but will also demonstrate fatherly behaviour towards newborns, regardless if they are his.
Researchers wanted to see how this switch happens and studied brain activation patterns that are induced by parenting and infanticide. During the experiment scientists allowed male mice to meet mouse pups and recorded whether their behaviour was paternal or infanticidal. Then researchers measured the level of c-Fos—an indicator of recent neuronal activity—in nine forebrain regions.
This allowed them to find out if activity in these brain regions was related to demonstrated behaviour. Such analysis showed that c-Fos expression in the cMPOA brain region was associated with paternal behaviour, while expression in the rhomboid nucleus of the BSTrh was associated with infanticidal behaviour. Furthermore, tendency to kill offspring can be noticed just by looking at the c-Fos expression patterns. In fact, scientists found that determining social behaviour by looking at the level of c-Fos was as accurate as 95−97%.
Scientists wanted to know whether the cMPOA and BSTrh are necessary in determining which kind of behaviour animal will express. Therefore, they conducted some additional experiments. Firstly, they discovered that lesioning the BSTrh inhibited infanticidal behaviour in virgin males. Secondly, they demonstrated that lesioning the cMPOA inhibited parental behaviour and even made fathers to attack newborn pups. Researchers also performed experiments using wire mesh to prevent acts of infanticide or parental behaviour.
Kumi Kuroda, leader of the research team, explained: “c-Fos expression patterns in these regions were just as accurate at predicting attempted behaviours. Along with the lesion results, this indicates that the cMPOA and BSTrh are responsible for the urge, or drive, behind these very different social acts.”
Scientists were not sure how these two brain regions cooperate with each other, so conducted some testing and examined the neural connections between them. Study showed that most connections travel from the cMPOA to the BSTrh and are inhibitory. Since inhibition of the BSTrh through lesions had reduced infanticide and c-Fos expression analysis also showed that cMPOA is strongly activated during mating, hypothesis was made that the activation of the cMPOA is involved in the mechanism of switching between infanticidal behaviour and parenting.
Researchers performed a test, using optogenetic light stimulation over several days to selectively activate the cMPOA neurons of virgin males. In favour of the hypothesis, this did reduce the amount of infanticide in the group. Switch, however, was not immediate and required a certain period of stimulation to work.
Now scientists are already thinking about further steps to enhance the research. Firstly, scientists will try to determine how social experience with a female leads to paternal behaviour. Secondly, researchers will also try to study the function of said brain regions in primates and have already started investigating them in marmosets. Now scientists do not know brain regions of primates that are related to parenting.
This research is providing knowledge about how interaction between different brain regions can radically change the behaviour of mammal males. It is also showing how social interaction and puberty changes mindset of rather aggressive young males. But we will have to wait what other results this research may demonstrate in the near future as scientists will be investigating primates.