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Parental investment is generally defined as as any expenditure by parents on an individual offspring that reduces their potential to invest in other offspring, either at present or in the future.

Parental care, in general, is presumed to increase growth rates, quality, and survival of young, and hence ultimately increase the inclusive fitness of parents. In a variety of vertebrate species (e.g., ~80 of birds and ~6% of mammals, both males and females invest heavily in their offspring. Many of these biparental species are socially monogamous, so individuals remain with their mate for at least one breeding season.

Mammals in General
Female mammals invest heavily in reproduction, in large part due to lactation. Because these costs are obvious, the physiological costs of female reproductive effort, especially care of offspring, have been studied extensively in mammals. These costs have been found to include reductions in longevity and in such processes as thermogenesis, physical activity, and immune function, apparently reflecting the high energetic and nutritional demands of parenting.

Male mammals may also invest heavily in reproduction through efforts to enhance mating success (e.g., courtship displays, intrasexual combat) or to provide paternal care. However, the costs of paternal care have rarely been studied in mammals, in large part because only 5-10% of mammals exhibit such care. Nonetheless, in those species in which males do provide extensive care for their offspring (i.e., biparental species, including humans), indirect evidence suggests that its costs can be substantial. For example, mammalian fathers that care for their young may undergo systematic changes in body mass and in circulating or excreted concentrations of a number of hormones (e.g., androgens, glucocorticoids, leptin) as a function of reproductive status , and several of these hormones have important effects on body composition, metabolism, and organismal performance. Nonetheless, the energetic and performance consequences of male parental investment have not been investigated directly in mammals.

I mammals, paternal care is found most commonly in primates, rodents, and canids.

Humans
Human cultures and societies vary widely in the expression of paternal care. Some cultures recognize paternal care via celebration of Father's Day. According to CARTA|CARTA, human paternal care is a derived characteristic (evolved in humans or our recent ancestors) and one of the defining characteristics of Homo sapiens. Different aspects of human paternal care (direct, indirect, fostering social or moral development) may have evolved at different points in our history, and together they form a unique suite of behaviors as compared with the Great Apes.

Non-human Primates
Paternal care is rare in the Great Apes.

Rodents


Several species of rodents have been studied as models of paternal care, including Prairie voles (Microtus ochrogaster), Campbell's dwarf hamster, the Mongolian gerbil, and the African striped mouse. The California mouse (Peromyscus californicus) is a monogamous rodent that exhibits extensive and essential paternal care, and hence has been studied as a model organism for this phenomenon.

Birds
Fathers contribute to the care of offspring in many species of birds, sometimes including incubating the eggs.

Squamate "Reptiles" (lizards & snakes)
Although some maternal care and considerable sociality (especially in lizards ) occurs in squamates, paternal care per se apparently has not been reported.

Fishes
Paternal care occurs in various species of fish. One well-known example of paternal care is in seahorses, where males brood the eggs in a pouch until they are ready to hatch.

Arthropods
Paternal care is rare in arthropods, but occurs in some species, including the giant water bug.

Theoretical Models of the Evolution of Paternal Care
Mathematical models related to the Prisoner's Dilemma suggest that when female reproductive costs are higher than male reproductive costs, males cooperate with females even when they do not reciprocate. In this view, paternal care is an evolutionary achievement that compensates for the higher energy demands that reproduction typically involves for mothers.

Other models suggest that basic life-history differences between males and females are adequate to explain the evolutionary origins of maternal, paternal, and bi-parental care. Specifically, paternal care is more likely if male adult mortality is high, and maternal care is more likely to evolve if female adult mortality is high. Basic life-history differences between the sexes can also cause evolutionary transitions among different sex-specific patterns of parental care.

Consequences of Paternal Care for Offspring Survival and Development
Care by fathers can have important consequences for survival and development of offspring in both humans and other species.

Proximate Mechanisms of Paternal Care
The proximate |proximate mechanisms of paternal care are not well understood for any organism. In vertebrates, at the level of hormonal control, vasopressin apparently underlies the neurochemical basis of paternal care; prolactin and testosterone may also be involved. As with other behaviors that affect Darwinian fitness, reward pathways in the brain may reinforce the expression of paternal care and may be involved in the formation of attachment bonds.

The mechanisms that underlie the onset of parental behaviors in female mammals have been characterized in a variety of species. In mammals, females undergo endocrine changes during gestation and lactation that "prime" mothers to respond maternally towards their offspring.

In contrast, paternal males do not undergo these same hormonal changes and so the proximate causes of the onset of parental behaviors must differ from those in females. There is little consensus regarding the processes by which mammalian males begin to express parental behaviors. In humans, evidence ties oxytocin to sensitive care-giving in both women and men, and with affectionate infant contact in women and stimulatory infant contact in men. In contrast, testosterone decreases in men whobecome involved fathers and testosterone may interfere with aspects of paternal care.

Placentophagia (the behavior of ingesting the afterbirth after parturition) has been proposed to have physiological consequences that could facilitate a male’s responsiveness to offspring