Breeding Season 6.6.1
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Anestrus refers to the phase when the sexual cycle rests. This is typically a seasonal event and controlled by light exposure through the pineal gland that releases melatonin. Melatonin may repress stimulation of reproduction in long-day breeders and stimulate reproduction in short-day breeders. Melatonin is thought to act by regulating the hypothalamic pulse activity of the gonadotropin-releasing hormone. Anestrus is induced by time of year, pregnancy, lactation, significant illness, chronic energy deficit, and possibly age. Chronic exposure to anabolic steroids may also induce a persistent anestrus due to negative feedback on the hypothalamus/pituitary/gonadal axis. Other spellings include anoestrus, anestrum, and anoestrum.
Some species, such as cats, cows and domestic pigs, are polyestrous, meaning that they can go into heat several times per year. Seasonally polyestrous animals or seasonal breeders have more than one estrous cycle during a specific time of the year and can be divided into short-day and long-day breeders:
Monestrous species, such as canids and bears, have only one breeding season per year, typically in spring to allow growth of the offspring during the warm season to aid survival during the next winter.
Generally speaking, the timing of estrus is coordinated with seasonal availability of food and other circumstances such as migration, predation etc., the goal being to maximize the offspring's chances of survival. Some species are able to modify their estral timing in response to external conditions.
The female cat in heat has an estrus of 14 to 21 days and is generally characterized as an induced ovulator, since coitus induces ovulation. However, various incidents of spontaneous ovulation have been documented in the domestic cat and various non-domestic species. Without ovulation, she may enter interestrus, which is the combined stages of diestrus and anestrus, before reentering estrus. With the induction of ovulation, the female becomes pregnant or undergoes a non-pregnant luteal phase, also known as pseudopregnancy. Cats are polyestrous but experience a seasonal anestrus in autumn and late winter.
A feature of the fertility cycle of horses and other large herd animals is that it is usually affected by the seasons. The number of hours daily that light enters the eye of the animal affects the brain, which governs the release of certain precursors and hormones. When daylight hours are few, these animals "shut down", become anestrous, and do not become fertile. As the days grow longer, the longer periods of daylight cause the hormones that activate the breeding cycle to be released. As it happens, this benefits these animals in that, given a gestation period of about eleven months, it prevents them from having young when the cold of winter would make their survival risky.
In many floodplain systems, anthropogenic modifications of hydrology (dams, flood controlmeasures and road and rail embankments etc.) and of natural wetland habitats (changed toagriculture) have blocked migration pathways and disturbed or destroyed fish breeding areas anddry season habitats. This, combined with over-fishing, has reduced recruitment to indigenous fishstocks and, so, the potential production that could be expected from the remaining floodplain10.
The declining fish yields in an environment that provides an important livelihood for millions ofpeople world-wide has prompted much attention from governments, international donors andnon-governmental organisations (NGO's). Potentially, there are a variety of solutions that could beapplied. For example, release of introduced or indigenous species, habitat rehabilitation or changesto exploitation patterns (banning of gears harvesting juvenile fish or introducing reserves to protectdry season habitats). Of these, the release of seed fish (primarily species of carp) onto the floodplainis the solution that has been applied most extensively.
The objective that will most profoundly influence the structure of rules required, however, is that offinancial sustainability. Collection of revenue to fund enhancement in the subsequent seasonrequires that harvesting is controlled. In principle, this can be done directly, by managingoperations and deducting costs from sales revenues, or indirectly, through a system of gear/catch levies or by sub-leasing portions of the fishery. All imply that access to the fishery must berestricted for some gears and/or in some seasons, raising the possibility that some groups may loseout from enhancement.
Flexibility is essential as floodplains are complex environments, in both space and time, withhydrology the dominant force. In summary, a heterogeneous community of fishers harvest avariety of species, using many gears from a range of seasonal habitats. These characteristics arethe key technical constraint to introduction of successful stocking activities. Variability withinand between floodplains demands that projects tailor the overall enhancement strategy to theparticular characteristics of individual floodplains. Adaptive management, the process offormal learning through management experience, is the best approach to refining the strategyto local conditions.
The total population size (census population size) in natural populations is not the same as the effective population size (Ne), which is the size of the breeding population. Effective population size takes into consideration that many individuals that reach adulthood never breed in natural populations. Consequently, effective population size is almost always smaller than the census population size. Effective population size is particularly impacted by deviations from 1:1 sex ratios. In such cases, effective population size can be estimated as
where Nm is the number of males and Nf the number of females. If you assume census population of 100 with equal sex ratio, Ne is 100. If you assume a sex ratio of 1:9, Ne drops to just 36. Distinguishing between N and Ne is important for conservation biology and many population genetic analyses related to genetic drift and inbreeding. For example, when Ne is significantly smaller than N, the probability of fixation of an allele in response to drift can be much higher than estimated by census population sizes.
Similar to mutation, migration can introduce new genetic variants into a population upon which selection can act. Hence, human-facilitated migration is sometimes used as a tool in conservation biology, where new individuals are introduced into populations of endangered species suffering from low genetic diversity and inbreeding. This practice is also known as genetic rescue. In many instances, however, migration actually counteracts the effects of selection. Imagine two adjacent populations that are exposed to different environmental conditions. In every generation, selection favors alleles that mediate adaptation to the local conditions. But if there is migration between the two populations, new maladaptive alleles are continuously introduced from the other population. Hence, migration can prevent local adaptation of populations. Adaptive divergence between populations is only possible if the effect of divergent selection is stronger than the homogenizing force of migration (Figure 6.8).
One of the most common forms of non-random mating is inbreeding, where offspring are produced by individuals that are closely related. The epitome of inbreeding is selfing (self-fertilization), which essentially represents strict genotype-specific assortative mating and is particularly common in plants. If we assume a single, biallelic locus A, possible matings during selfing include AA x AA, Aa x Aa, and aa x aa. The consequences of selfing on the genotype frequencies across generations are depicted in Figure 6.10. As you can see, the frequency of heterozygotes declines rapidly until they are virtually gone after just 10 generations. This is because neither the self-crosses of AA and aa yield any heterozygotes, and self-crosses of Aa yield 50 % homozygotes. Accordingly, the frequency of heterozygotes is halved in every generation.
The degree of inbreeding can be described by the coefficient of inbreeding (F), which calculates the probability that two copies of an allele have been inherited from an ancestor common to both the mother and the father. You can find some examples for inbreeding coefficients in Table 6.1. Once we know F for a population, we can account for the effects of inbreeding on genotype frequencies by modifying the original Hardy-Weinberg formulas:
Equation (6.11) allows us to simulate the effects of different levels of inbreeding on the observed heterozygosity across successive generations. As you can see in Figure 6.11, the rate of decline in heterozygosity across generations is dependent on F, and declines can be rapid when inbreeding is common. Declines in heterozygosity are particularly common in small populations where the pool of potential partners is limited, inadvertently leading to mating between related individuals. This is also the case for many managed populations, including those associated with captive breeding programs for endangered species. Hence, many species maintenance programs strategically share individuals for breeding across institutions to avoid inbreeding.
If inbreeding is not really an evolutionary force, why is it important? Why is problematic for conservation and animal breeding? The excess of homozygotes generated by inbreeding increases the probability that individuals are homozygous for recessive deleterious alleles. As you know from simulations of selection, recessive deleterious alleles are usually rare; hence, matings that lead to individuals with two copies of recessive deleterious alleles are very unlikely (q2). That changes when inbreeding becomes common in a population. Along with negative fitness consequences for the individual, the increased probability of combining deleterious recessive alleles also reduces the average fitness in a population, which can be problematic for endangered species. Due to the costs associated with inbreeding, many species have evolved mechanisms for inbreeding avoidance, including disassortative mate choice or matrilineal group-living where male offspring are ostracized before they reach sexual maturity. 2b1af7f3a8