Fecundity Patterns

The term 'parity' is used to describe a breeding event, and species can assume various modes of reproductive timing and frequency depending on their evolutionary past and the constraints of their current environment. There are too many variants of life-history strategies to describe succinctly given the large number of possible combinations of longevity, breeding frequency, and offspring number, but there are some major groups of strategies that have evolved. The two major fecundity patterns are semelparous and iteroparous reproduction.

Semelparity

Semelparous organisms reproduce only once during their lifetime. This may occur at the age of only 20 min in certain bacteria, a few hours in many protozoa, or up to a few weeks or months in some insects and mammals. Many semelparous species are annuals (live only one year), but some reproduce only after several years of maturation. Early work predicted that greater temporal variation in adult survival relative to juvenile survival favored the evolution of semelparity, although more recently it has been demonstrated that this also depends on the age structure of the population under selection. Semelparity may also provide other advantages over iteroparity in terms of offspring body size, leading possibly to increased juvenile growth rates and survival.

Semelparity in mammals is restricted to two marsupial families (Didelphidae and Dasyuridae), where all species demonstrate high post-reproductive senescence, but not all are semelparous. In some of the semelparous species, only the males die after the short, highly synchronous mating season, whereas in others, male die-offs are facultative. Some have argued that because the interval between conception and weaning is short and that these marsupials tend to live in highly predictable seasonal environments, there should be selection for a monoestrous reproductive pattern, high estrus synchrony, and a short mating season. These factors should therefore induce intense male-male competition and a low probability of multiseason male survival giving rise to the evolution of male semelparity. Additionally, the high rate of female mortality resulting from long lactation periods in some species also demonstrating female semelparity selects for a bet-hedging strategy by males and the evolution of male semelparity.

Iteroparity

Species that reproduce more than once during their lifetime are iteroparous. The time of maturation preceding first reproduction may vary from a few days in small crustaceans to greater than 100 years in some trees. The frequency of reproduction can also vary markedly - daily (e.g., some tapeworms), semiannually, annually, biennially, or irregularly (e.g., humans). Some have argued that iteroparity is favored over semelparity when high environmental variability induces large variation in offspring production and offspring survival. However, it has been demonstrated more recently that the evolution of either strategy does not depend on environmental variability alone, but also on the strength of intrinsic regulation (a tradeoff between fertility and survival - see below) operating on a population and the demographic rates most affected by extrinsic variation.

In many iteroparous species there are a certain proportion of individuals that reproduce only in alternate seasons (or less often), thereby foregoing a proportion of their potential lifetime reproductive success. Known formally as 'low frequency of reproduction', this phenomenon is most common among vertebrate ectotherms, but it has also been documented in endotherms (e.g., willow tits (Parus montanus); kittiwakes (Rissa tridactyla); fat dormice (Myoxus glis)). There is also considerable phenotypic variation within a species, with some individuals opting for lower-frequency reproduction than others. Ectothermic species employing this strategy appear to have accessory activity associated with reproduction (but independent of fecundity) such as breeding migrations, egg brooding through incubation, and live bearing. With its apparent ubiquity, low-frequency reproduction appears to offer some selective advantage in certain systems. It has been suggested that the strategy could evolve only under two scenarios: (1) when reproduction does occur, it confers a much higher fecundity benefit than would a regular-frequency breeding event; or (2) the probability of surviving the interval between reproduction is much higher than it would be during the same interval of regular-frequency breeding. Thus, lower-frequency reproduction may actually result in higher average fecundity over the individual's lifetime when at least one of the two previous conditions is met. There also appears to be a relationship between the occurrence of low-frequency breeding with habitat quality. Here, regular reproduction may become less favorable as the habitat becomes less suitable because resource availability declines and accessory activity cannot decrease to the same extent.

Age Specificity

In iteroparous organisms, fecundity often increases with age following reproductive maturity and then can decline at older ages. As such, age specificity of reproduction is an essential element for understanding the evolution of life-history strategies. Reproductive rate may increase with age when an individual devotes relatively more resources to reproduction with increasing age. For example, growth rate often declines after reproductive maturity is achieved, so more resources can thereafter be directed to reproduction, thereby increasing age-specific fecundity. This introduces the concept of 'primiparity', which is generally defined as the age of first reproduction. In some species, this age is rather constant suggesting genetic control. However, in other species (especially ectothermic organisms), it is environmentally mediated and may demonstrate high phe-notypic plasticity. The extent to which primiparity is delayed (i.e., the length of the pre-reproductive period) also depends on type of niche exploited by a species and the particular demographic configuration of a population.

Increasing fecundity is also observed in many organisms that do not grow following maturity, giving rise to three hypotheses to explain the pattern: (1) less-fit individuals are constantly eliminated from a population so that the average fecundity of surviving individuals increases; (2) increasing fecundity is a reflection of the gradual improvement in the competence of older, more experienced, and often higher-ranking individuals; and (3) as life expectancy decreases with age, individuals allocate more and more resources to reproduction at the expense of survival. The decline in fecundity with age is often referred to as 'reproductive senescence'. One hypothesis to explain this pattern is as an individual ages its resource acquisition rate deteriorates due to physiological ageing.

Recent work has attempted to combine the various hypotheses for age specificity in fecundity into a single conceptual model that incorporates the other key life-history parameter - age-specific survival. When extrinsic mortality is high, early maturity is favored allowing for high reproductive investment and fecundity early in life (Figure 1). Under this scenario, allocating resources to repair cumulative somatic damage is not beneficial when life expectancy is already low, and fecundity will still diminish with time due to physiological ageing. When extrinsic mortality is low, investing in somatic repair at the cost of reduced fecundity is beneficial because it increases life expectancy. Thus, reproductive rate is low early in life, but gradually increases toward a late-life maximum, and then inevitably declines again due to physiological deterioration late in life (Figure 1).

Figure 1 Patterns of age-specific fertility fecundity expressed as a percentage of maximum age under two different rates of extrinsic mortality. Reproduced from Cichon M (2001) Diversity of age-specific reproductive rates may result from ageing and optimal resource allocation. Journal of Evolutionary Biology 14: 180-185.

Figure 1 Patterns of age-specific fertility fecundity expressed as a percentage of maximum age under two different rates of extrinsic mortality. Reproduced from Cichon M (2001) Diversity of age-specific reproductive rates may result from ageing and optimal resource allocation. Journal of Evolutionary Biology 14: 180-185.

Allometric Scaling

Allometry is the relation between the size of an organism and aspects of its physiology, morphology, and life history. Typically, variation in body mass among individuals or species can be used to predict traits such as metabolic rate, dispersal capacity, survival probability, and fecundity. The ratio of combined offspring mass to maternal mass tends to remain approximately constant over a broad range of maternal sizes within a species. As such the ratio of offspring mass at independence to average adult body mass remains stable within major taxonomic groups. In other words, larger females tend to have more or larger offspring, especially among invertebrates and ectothermic vertebrates. An increase in fecundity with increasing female body size may constitute a selective advantage toward large female body size (the so-called 'fecundity advantage' model) in some taxa, especially in those for which energy is not limiting. However, this does not necessarily mean that larger females will always have higher reproductive success because of the increased risks due to predation, limited resources, and environmental uncertainty.

Density Dependence

It is in every individual's interest to maximize its reproductive output without compromising its own survival prospects; however, the production of too many individuals can lead to overcrowding and a reduction in

per capita resources such as food availability, breeding sites, and territories. Under such circumstances, it is clearly unprofitable for individuals to engage in potentially wasteful energy expenditure associated with fertility if the probability of their progeny surviving to breeding age is low. This observation is supported by the ample evidence that population density affects fertility negatively. In general, as population size increases, either the average number of offspring produced per female decreases (in litter or clutch sizes greater than one), or average offspring size decreases. However, the relationship between density and fertility is not necessarily linear. For example, for many long-lived, iteroparous species, the negative effects of population density are not expressed in average fertility until the population approaches its environmentally mediated carrying capacity. In many semelparous species with short life expectancies the opposite is true - fertility declines rapidly as soon as the population begins to grow. Figure 2 shows examples of fertility patterns relative to population density in different species.

Increasing density does not always result in depressed fertility. In many species, fertility is relatively invariant over the range of population densities experienced so that intrinsic regulation operates almost exclusively through the modification of age-specific survival rates. Some species do not appear to respond to density by adjusting fertility per se; rather, the age of first reproduction (pri-miparity) may change according to the per capita resources available. In other species, high-density living can actually stimulate production of offspring (known as inverse density dependence; e.g., Figure 2c). For example, many plant species require high population densities to attract grazers or pollinators that assist in cross-pollination and propagule dispersal. This Allee effect (a term used to describe a reduction in reproductive output with decreasing population density, thus affecting population growth rate) is another example of inverse density dependence that occurs when populations existing at low densities suffer from stochastic demographic events that, for example, skew the tertiary sex ratio (the ratio of the rarer sex to the total number of breeding adults). In these cases, the availability of potential mates for the more common sex is reduced so that not all reproductively capable individuals succeed in fertilization and the production of offspring. The type and strength of density dependence operating on a population also have implications for the patterns of age-specific fecundity. A reduction in extrinsic mortality favoring the evolution of reduced reproductive senescence is predicted only when density dependence acts principally on fertility and without differentially decreasing late-age fecundity. Alternatively, a reduction in extrinsic mortality can favor the evolution of faster senescence if density dependence acts mainly on the survival of older age classes.

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