Why does reproductive performance tend to decline gradually in iteroparous animals?

In iteroparous animals, the gradual decline in reproductive performance is a manifestation of senescence, which occurs without the specific systemic or environmental triggers found in semelparous species.1

This decline is influenced by several factors:

• Age and Growth: In species that reach a fixed body size, reproductive capacity often begins to decline early and accelerates as the organism ages. In contrast, species that continue to grow throughout their lives, such as large egg-laying reptiles, may see an initial increase in reproductive output correlated with body size before it eventually levels off and declines.1
• Population Dynamics: The proportional contribution of an individual to population growth diminishes as the number of its existing progeny increases. This suggests an evolutionary "optimum" number of litters per lifetime, though whether this leads to adaptive senescence remains a subject of scientific debate.1
• Physiological Regulation: Reproductive senescence is linked to the function of the hypothalamus and pituitary gland. Research indicates that calorie restriction can delay this decline by enhancing the secretion of luteinizing hormone, which regulates the activity of the gonads.1