Maternal behavior in vertebrates

Vertebrate maternal behavior is a form of parental care that is specifically given to young animals by their mother in order to ensure the survival of the young. Parental care is a form of altruism, which means that the behaviors involved often require a sacrifice that could put their own survival at risk. This encompasses behaviors that aid in the evolutionary success of the offspring and parental investment, which is a measure of expenditure (time, energy, etc.) exerted by the parent in an attempt to provide evolutionary benefits to the offspring. Therefore, it is a measure of the benefits versus costs of engaging in the parental behaviors. Behaviors commonly exhibited by the maternal parent include feeding, either by lactating or gathering food, grooming young, and keeping the young warm. Another important aspect of parental care is whether the care is provided to the offspring by each parent in a relatively equal manner, or whether it is provided predominantly or entirely by one parent. There are several species that exhibit biparental care, where behaviors and/or investment in the offspring is divided equally amongst the parents. This parenting strategy is common in birds. However, even in species who exhibit biparental care, the maternal role is essential since the females are responsible for the incubation and/or delivery of the young. Although maternal care is essential in many classes of vertebrates, it is the most prevalent in mammals, since the care from the mother is essential for feeding and nourishing their young. Because the care exhibited by the mother plays such a large role in mammals, the role of the male is often very limited. Maternal care begins during fertilization and pregnancy however, the most prominent array of maternal behaviors are exhibited by the mother after the birth of their offspring and until the offspring are capable of being independent. The duration that the offspring stays with the mother and receives care, and the types of care behavior displayed by the mother varies amongst different species, forms of reproduction, and the level of development in the offspring at birth.

Mammals Mammals are a class of vertebrates that contain mammary glands which secrete milk that nourishes their offspring. This maternal behavior is unique from all other vertebrate classes. The basic maternal care patterns in mammals involve the internal incubation during gestation, the delivery of young, and maternal care until the young are weaned. The way a female mammal cares for their young depends on the level of maturation of the offspring at birth. There are three main categories of newborn maturation that different species of mammals exhibit. Altricial young (marsupials, rodents) are completely helpless and dependent on their mothers, precocial young (elephants, cattle) have highly developed sensory and motor functions at birth, and primates who are semi-helpless young, have an intermediate level of maturation. This variation in maturation at birth requires different maternal care from the mother.

Altricial species

Altricial young are born deaf, blind, almost completely hairless and have very limited motor functioning. The maternal behavior in these species is primarily focussed on providing warmth for the young since they are unable to thermoregulate. This is achieved through the utilization of pouches in marsupial species, such as kangaroos, and the construction of nests in rodents and other altricial species. Because the young are not capable of locomotion, many other maternal behaviors take place at the nest as well, including feeding, nursing and grooming (licking). Members of this species are unable to recognize their own young and therefore will care for all the young in the nest equally, regardless of whether or not it is their own offspring. Once the young reach a point in their sensory, and motor development and no longer require the nutrients from their mothers milk, they will leave the nest and the mother will discontinue her maternal care behaviors.

Precocial species Precocial young are born with a highly developed sensory and motor system. At birth they are able to see, hear, and most can walk or will learn to walk within the first few days after birth. The females in these species have adapted to be able to recognize their own young, which allows them to be selective about who they choose to care for. They will reject other young who try to nurse from them. The primary focus of these species is to develop an exclusive relationship with their young which allows for the resources and energy expenditure of the mother to be optimized by the offspring and not wasted on other young. This is essential for their success and fitness.

Primates Primates are born with intermediate maturation of altricial and precocial species. Newborns have limited motor functioning, but have highly developed sensory systems and the ability to thermoregulate. A unique characteristic in the maternal behavior in primates is that the mothers carry their young until their locomotive abilities develop. The mother will either carry the young in their arms, or the young will attach itself to the mother and cling to her. This is known as matricolia.

Amphibians Most amphibians are oviparous and lay their eggs in water, though some have adapted to lay them on land. There are also some species that are viviparous, giving birth to live young. Eggs can be fertilized either internally or externally depending on the species. Many amphibians exhibit maternal behaviors such as egg and young brooding and transport, as well as varying methods of feeding their young.

Frogs

Frogs have developed many different maternal behaviors across varying species. The most common maternal behavior among frogs is egg attendance, wherein the mother will remain near her eggs as they develop to protect them from predators. A mother may also stay with her clutch in order to prevent desiccation, as she can rehydrate them by positioning themselves above the eggs, a behavior known as ventral brooding. Some species have also been observed oxygenating their eggs by moving them around occasionally. Frogs of the genus Hemiphractus carry their eggs around on their back until they hatch. Some species of this genus develop a dorsal pouch to carry their young inside while other leave them exposed. Some frogs, such as poison dart frogs, will lay unfertilized eggs for her clutch to eat.

Caecilians

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