Introduction
What could be more fundamental to success of a species than reproduction? Understanding homeostasis and physiological controls are of limited use unless we can maintain our animals.
Natural selection has produced an astounding variety of successful reproductive strategies. From an evolutionary viewpoint, the most favored pattern is one that results in the largest number of offspring reaching sexual maturity. Most population biologists have recognized two general patterns, namely, r-selection or K-selection. These terms were coined from logistic equations that were used to model growth of populations of animals in the wild.For r-selection animals, the investment in each individual is small both in a physical and energetic sense. Examples of this pattern of reproduction include fishes or amphibians that can produce extremely large numbers of eggs. The trade-off in producing so many small offspring is that there is little if any care of the offspring. They are produced and released into the environment to fend for themselves. It is estimated that only six out of a million mackerel fry survive to also reproduce. On the other hand, K-selection species produce fewer but larger offspring. Because these offspring are cared for—at least to some degree—during development, the chance that they will successfully reach reproductive age is far greater. Compared with the mackerel, individual K-selection animals may have a 50% chance of success. Clearly, for most agricultural operations the K-Selection pattern predominates.
Anatomy and Physiology of Domestic Animals, Second Edition. R. Michael Akers and D. Michael Denbow. © 2013 John Wiley & Sons, Inc. Published 2013 by John Wiley & Sons, Inc.
Energetic costs of parental care occur in two categories. First, there is the cost that relates to transfer of nutrients to the developing neonate. For mammals, the demands of lactation can be very large. The lactating mother can expend 40% of her energy producing milk. This can be even greater in high-producing dairy cows that have been selected for copious milk production. Other animals' milk, for example, crop milk produced by pigeons and doves, also represents a direct energetic expenditure cost. In addition, there are costs of behavioral responses to protect and nurture neonates.
As with most of the other chapters, entire courses are devoted to these physiological specialties. Many animal science majors will take courses devoted to nutrition, lactation, and reproduction. Our goal for this chapter is not to substitute for a specialized course in reproduction but rather to provide some basics and stimulate your interest in more detailed study (Randall et al., 2002).