Parental care is an evolutionary adaptation, widespread in a large number of species, in which parents try to increase the chances of success of their children. However, there are decisions that parents must make and they will directly affect the survival not only of their descendants, but of themselves and their own species. We will see what happens in the case of birds.
1. PARENTAL INVESTMENT
According to the Theory of parental investment (Trivers, 1972), the animals that reproduce sexually must assess the cost to them to invest in their children.
Reproduction is costly, and individuals are limited to what they can devote time and resources to raising and growing their offspring, and such an effort can be determinant in their survival and future reproductive activities. According to the Principle of Allocation, the energy that an individual obtains must be distributed among the requirements derived from its maintenance, growth and reproduction. Extra energy being channeled to any of these activities will result in less energy available to the remaining ones.
Caring for the offspring consists of a series of activities carried out by the parents and an increase in the probabilities of survival of offspring, effects that will be considered as benefits. At the same time, these activities will have negative consequences on the parents, affecting their survival and the probability of producing new offspring in the future, since they involve an expense of time and energy or costs. Each individual must consider both, costs and benefits, to make the most beneficial choice.
2. FORMS OF PARENTAL INVESTMENT
Parental investment must be considered from the beginning of reproduction, and not only from the birth of offspring.
We can distinguish different stages in the parental investment of birds:
Investment prior to fertilization: birds need to establish nesting and feeding grounds with conditions conducive to raising their offspring, such as the availability of food. In addition, once the territory is selected, they will have to choose a safe place for predators to set up their nest. In some cases they will also dedicate energy to the construction of the same, adding costs to the parental investment. The production of gametes is another process that supposes an energetic expense for the individual.
Placement and incubation: The laying of the eggs implies a great investment for the female, who is the one who does it. In relation to egg production, the energy investment of the female will vary depending on the development of the chicken at birth. In precocial birds or nidifugous (that present a state of advanced development at birth and can leave the nest, being able to move and Regulate its own temperature), the percentage of yolk will be greater and therefore, the greater the energy demand in its production. On the other hand, in altricial birds (born in premature developmental state, with eyes and ears not developed, body without feathers and without capacity to move), the percentage of yolk has been seen that is smaller and with this also the energy investment of the female. However, this initial differential investment may be later compensated for in the parental care necessary after hatching, which will be higher in altricial birds.
Once the female makes the egg laying, a very delicate stage begins in which the correct development of the embryo will be determined by the incubation conditions: temperature, humidity, ventilation and egg turnover.
Care after birth. After the hatching of the eggs, the offspring will need food, temperature regulation, and protection, by the parents. But this care will vary depending on their development at birth, being smaller in the precocial than in the altricial.
Precocial and superprecocial birds are characterized by patterns of simple parental care, with minimal assistance in the nest. As an example are galliformes and anseriformes, who seek their own food since they are born, but will depend on their parents to protect themselves. At the other extreme, altricial species are characterized by sophisticated parental care, with a high level of offspring assistance. These features associated with altricial development are also related to an increase in the variety of flight styles, flight speed, and ecological habits (Dial, 2003).
Finally, we can find different models of parental care according to the individuals involved in the care of the young. In breeding parasitism, individuals try to reduce the costs of parental care by involving other individuals in caring for their offspring. (Lying birds: Brood parasitism in birds, the continual struggle for survival). Another possibility is that only one member of the pair, male or female, cares for the offspring; Or that both engage in that task (male and female). Finally, cooperative breeding is a system in which adult individuals (assistants) provide parental care, such as feeding, thermoregulation, grooming and advocacy, to juveniles who are not their direct descendants. If only a pair is reproduced, it will be cooperative breeding, if they reproduce more, it is called communal breeding.
3. SEX CONFLICT
The conflict of interests between males and females begins in the production of gametes. The male gametes, smaller and simpler, need less investment on the part of the individual. In contrast, as we have seen, female gametes need more investment of female resources.
From the point of view of the male, the most advantageous would be to fertilize as many females as possible and let them be the ones who would care for the young, while he is engaged in seeking and fertilizing more females. On the contrary, the most advantageous for a female would be for the males she mates to take care of the pups so that she could devote her time, energy and resources to mating again and producing more pups.
However, the choice of one or another strategy will be determined mainly by several factors: physiological limitations, types of life cycles and ecological factors. According to the balance of costs and benefits for males and females in each ecological context, each sex will try to maximize its reproductive success, even at the expense of the reproductive interests of the other sex.
The conflict between the sexes in parental care can be explained through the classic Maynard-Smith model (1978), represented by the Matrix of Game Theory, which will determine the parents’ decisions about whether or not to care for their offspring as a function of Success or benefit they obtain. Success will depend on the number of offspring produced (W), their chances of survival when they receive more or less parental care (P), and the male’s chances of mating again if he deserts (p).
The selection will favor the desertion of one of the progenitors when the progenitor has a high probability of re-pairing, when the desertion has a small effect on the survival of the offspring and when the contribution of this progenitor is small (Lazarus, 1989). Even when both parents care for the offspring, there are conflicts of interest with respect to the level of investment that males and females provide, so that what each sex is willing to invest will depend in part on their partner’s level of investment.
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