Infanticide Infanticide (or infant homicide) is the intentional killing of infants or offspring. Infanticide was a widespread practice throughout human history that was mainly used to dispose of unwanted children, its main purpose being the prevention of re ...

is the termination of a

neonate In common terminology, a baby is the very young offspring of adult human beings, while infant (from the Latin word ''infans'', meaning 'baby' or 'child') is a formal or specialised synonym. The terms may also be used to refer to Juvenile (orga ...

after it has been born, and in

zoology Zoology ( , ) is the scientific study of animals. Its studies include the anatomy, structure, embryology, Biological classification, classification, Ethology, habits, and distribution of all animals, both living and extinction, extinct, and ...

this is often the termination or consumption of newborn animals by either a parent or an unrelated adult. In

rodent Rodents (from Latin , 'to gnaw') are mammals of the Order (biology), order Rodentia ( ), which are characterized by a single pair of continuously growing incisors in each of the upper and Mandible, lower jaws. About 40% of all mammal specie ...

s, it is not uncommon for the mother to commit infanticide shortly after

parturition Birth is the act or process of bearing or bringing forth offspring, also referred to in technical contexts as parturition. In mammals, the process is initiated by hormones which cause the muscular walls of the uterus to contract, expelling the f ...

(giving birth) under conditions of extreme stress (parental infanticide), or for an unrelated male to kill neonates (nonparental infanticide).

Parental infanticide

Parental infanticide is perhaps the most confusing behaviour to understand, as in many cases it can seem

maladaptive In evolution, a maladaptation ( /ˌmælædæpˈteɪʃən/) is a trait that is (or has become) more harmful than helpful, in contrast with an adaptation, which is more helpful than harmful. All organisms, from bacteria to humans, display maladapt ...

for a parent to terminate offspring carrying its own genetic material. However, studies in mice have indicated infanticide may be a genetically heritable trait, and may even have a learned element, so there is clearly more to the behaviour than might be expected. The occurrence of infanticide seems to vary within rodent species between parents. For example, male

meadow vole The eastern meadow vole (''Microtus pennsylvanicus''), sometimes called the field mouse or meadow mouse, is a North American vole found in eastern Canada and the United States. Its range extends farther south along the Atlantic coast. The weste ...

s and house mice can be classed as either 'infanticidal' or 'non-infanticidal' depending on their history with other litters they have sired, although studies have shown that females do not discriminate between these classes when choosing a mate. Furthermore, recent studies in rodents have shown that infanticide is influenced by various hormones such as:

prolactin Prolactin (PRL), also known as lactotropin and mammotropin, is a protein best known for its role in enabling mammals to produce milk. It is influential in over 300 separate processes in various vertebrates, including humans. Prolactin is secr ...

corticosterone Corticosterone, also known as 17-deoxycortisol and 11β,21-dihydroxyprogesterone, is a 21-carbon steroid hormone of the corticosteroid type produced in the cortex of the adrenal glands. In the very rare case of congenital adrenal hyperplasia due ...

, and

progesterone Progesterone (; P4) is an endogenous steroid and progestogen sex hormone involved in the menstrual cycle, pregnancy, and embryogenesis of humans and other species. It belongs to a group of steroid hormones called the progestogens and is the ma ...

. Evidence from studies on female

cooperative breeding Cooperative breeding is a social system characterized by alloparental care: offspring receive care not only from their parents, but also from additional group members, often called helpers. Cooperative breeding encompasses a wide variety of group s ...

Mongolian gerbils indicated that type of living environment influences hormone concentrations of progesterone and prolactin. To investigate living environment effects, female gerbils in three housing groups (FH-family housed, PH-pair mate housed, and SH- singly housed) were observed and compared to each other. Unfamiliar pups were placed in a cage with each female to observe any

infanticide Infanticide (or infant homicide) is the intentional killing of infants or offspring. Infanticide was a widespread practice throughout human history that was mainly used to dispose of unwanted children, its main purpose being the prevention of re ...

response. Following the pup test samples of prolactin and progesterone were taken via blood. Analyses showed that females with lower progesterone (FH group) and prolactin levels rarely displayed infanticidal behavior toward the unfamiliar pup, compared to the females that possessed higher levels, in which they attacked the pup. These results show that

cohabitation Cohabitation is an arrangement where people who are not legally married live together as a couple. They are often involved in a Romance (love), romantic or Sexual intercourse, sexually intimate relationship on a long-term or permanent basis. ...

with family, contributes to lower progesterone and prolactin concentrations in female Mongolian gerbils, which is shown to decrease the likelihood that infanticide will occur. A further study investigated the role of living environment with younger siblings on infanticide in female and male cooperative breeding Mongolian gerbils. Female and male gerbils were put into one of four housing conditions (parents only, parents and siblings, siblings, and unrelated). Again, a pup test was conducted (same as previous study above), with each male and female to observe responses to an unfamiliar pup in the cage, while progesterone was measured via blood sample following the test. Females portrayed attacking behavior toward the unfamiliar pup when progesterone concentrations were elevated compared to the females with low concentrations (parents and siblings). Thus, these results have demonstrated further support that living with parents and siblings contributes strongly to lower levels of progesterone, causing inhibition of infanticide in female Mongolian gerbils. Researchers have found evidence on maternal care and infanticide in female mice that were

pregnant Pregnancy is the time during which one or more offspring gestation, gestates inside a woman's uterus. A multiple birth, multiple pregnancy involves more than one offspring, such as with twins. Conception (biology), Conception usually occurs ...

lactating Lactation describes the secretion of milk from the mammary glands and the period of time that a mother lactates to feed her young. The process naturally occurs with all sexually mature female mammals, although it may predate mammals. The proces ...

, when placed in a dangerous or safe environment. Female mice were placed in soiled bedding, with unfamiliar fecal cues of males which modeled a dangerous environment (UMB), or they were placed in a safe environment (NB) with neutral bedding. Corticosterone levels were measured via fecal metabolites after exposure to the assigned bedding. Female mice in the dangerous living environment group, showed higher levels of fecal corticosterone, and minimal maternal care compared to the females in the safe environment group. Furthermore, a dangerous environment inherently provides infanticide threats to female mice, increasing the likelihood that corticosterone concentrations will increase, while maternal care toward offspring decreases. In a study on mated male mice, there is evidence shown that when progesterone receptors are tampered with, using an

antagonist An antagonist is a character in a story who is presented as the main enemy or rival of the protagonist and is often depicted as a villain.Aggressive behavior was measured by response toward litters, and progesterone was measured by blood samples. Results indicated that PRKO mice (the knock-out mice) did not commit infanticide, but showed more paternal behavior, in comparison to the C57BL/6 strain mice that committed infanticide. Thus, this indicates that aggression and paternal behavior in mated male mice is linked to progesterone receptors; that is, when progesterone receptors are blocked by RU486 antagonist, males are less aggressive, and more paternal toward offspring.

Nonparental infanticide

Some rodent species (most typically males) will take the chance to kill neonates that are unrelated to them should opportunity permit. There is thought to be several benefits by doing so, which not only include nutrition benefits (particularly where food is in short supply) but also non-direct benefits, such as allowing access to more resources, improving reproductive opportunities and the retainment of energy and resources that might otherwise be spent on unrelated offspring. A common causal factor of infanticide, particularly among sciurid rodents, is direct resource competition, therefore killing the pups of an unrelated neighbour would prevent resource competition with those individuals in the future.

Species known to commit infanticide

Work by Blumstein (2000) highlighted the following rodent species which have previously been associated with infanticide in published scientific literature. Assessment of these traits lead Blumstein to conclude that infanticide evolved independently in sciuromorphs, myomorphs and caviomorphs.

Exceptions

Not all rodent species commit infanticide, and in fact some species are remarkable for their lack of this behaviour. One such species is the

common degu The common degu (''Octodon degus''; ), or, historically, the degu, is a small hystricomorpha rodent endemic to the Chilean matorral ecoregion of central Chile. The name ''degu'' on its own indicates either the entire genus '' Octodon'' or, mor ...

, a highly social caviomorph rodent from central Chile. Common degus have been found not to harm even unfamiliar pups, and male degus will even accept other males' litters into a group without problems. This is thought to be related to the communal nesting habits of the species and reflects genetic trends rather than a lack of the conditions that lead to infanticide.

References

{{DEFAULTSORT:Infanticide In Rodents Rodents Infanticide