Publisher Summary This chapter focuses on the seasonal control of fertility. It discusses two aspects in detail—the way in which changes in the secretion of luteinizing hormone-releasing hormone by the hypothalamus controls the activity of the pituitary and testis, and the way changes in day length influence the activity of the hypothalamus. The downstream events evoked by the hypothalamus are relatively easy to understand, whereas the mechanisms involved in the photoperiodic control are very complex and largely unresolved. While seasonal changes in temperature, rainfall, and food availability are the factors of the environment that dictate survival of adults and young—and are thus ultimately responsible for dictating the timing of the birth season—these are not necessarily the factors used as cues by the animals to regulate their reproductive endocrinology. This is because it is necessary to anticipate the timing of birth by dictating the timing of conception, as the duration of gestation in mammals is usually fixed. As accurate timing of conception is important, animals tend to become reliant on cues from the environment, which are the best predictors of the time of year. The seasonal cycle highlights the seasonal changes in daylight length, rutting behavior, testicular diameter, sexual skin flush, and concentrations of plasma follicle-stimulating hormone, prolactin, and testosterone in a group of rams throughout the year.

Seasonal breeding:nature's contraceptive.

Why Do Females Mate with Multiple Males? The Sexually Selected Sperm Hypothesis

This paper highlights the most critical aspects of the problem of predicting fertility. To determine if a laboratory test(s) is highly correlated with fertility it is essential to have: a) specific, precise and accurate laboratory tests, and b) precise and accurate fertility data. Acquisition of precise and accurate data for laboratory tests and fertility of spermatozoa in the same sample is not easy. Data derived from in vitro fertilization are not tests of fertility, because only a subset of the attributes important for fertilization in vivo are tested. Because of deficiencies in fertility data, there probably is no valid report for human spermatozoa correlating results of laboratory tests and fertility, and very few valid studies for laboratory or domesticated animals. There is little doubt that objective measures of sperm motion, acrosomal status, or other characteristics are significantly correlated with fertility. However, establishment of the correlations between a group of attributes and fertility is not the question of interest. The goal is prediction of fertility. There has been no recent effort to develop a prediction of fertility or fecundity based on sperm characteristics, and achievement of this goal may be elusive.

Can the fertility potential of a seminal sample be predicted accurately

Molecular techniques have enabled behavioural ecologists to reassess mating systems from a genetic perspective. Studies of paternity frequently reveal that mating behaviour does not always reflect parentage, and may bring to light alternative mating tactics. Here we present a comparison of behavioural and genetic measures of male reproductive success in a mammalian mating system in which both sexes are highly promiscuous. Rather than having a stable harem social structure, Soay rams (Ovis aries) on the island of Hirta, St Kilda, UK usually consort with individual oestrous ewes sequentially. Not all matings occur between consort pairs, however, and ewes have been seen to mate with up to 10 different rams on the same day. Using locus-specific polymorphism at five protein and 10 microsatellite DNA loci, we determined paternity for 236 lambs born into three cohorts, and compared paternity with estimates of mating success derived from more than one census of rutting behaviour. The correlation between the number of ewes with which each ram was witnessed in consort and the number of paternities assigned was positive and statistically significant, and rams that were observed in consort with a ewe were 18 times more likely to have sired her offspring than other candidate rams. However, most lambs (73%) were not sired by a ram seen in consort with the oestrous mother. Many juveniles, yearlings and some adult rams were rarely seen in consort with ewes, yet were assigned a significant number of paternities. These results suggest that mating tactics differ between age groups, and that alternative mating strategies among adults that do not involve forming consorts with many females also confer mating success. For these reasons, census-based observations of consort associations between individuals cannot be used to accurately estimate individual male reproductive success in this population.

Male reproductive success in a promiscuous mammal: behavioural estimates compared with genetic paternity.

When mixtures of semen containing an equal number of spermatozoa from two males are inseminated, the males usually sire disproportionate numbers of the offspring. In this study, when a mixture of equal numbers of spermatozoa from a Columbian (C) and a Leghorn (L1) cock was inseminated, the C male sired 34% of the offspring. The proportion of offspring was constant regardless of age of males, season, total number of spermatozoa, breed of hen or the interval from insemination to the time of egg-lay over a 15-day period after insemination. The proportion of C offspring observed from inseminating nine different ratios of spermatozoa arranged progressively from 1C :9L, 2C:8L and so on, was 10%, 20%, 23%, 34%, 38%, 44%, 58%, 74% and 82%. These results were in close agreement with mathematically derived estimates. It appears that the relationship between sperm ratios and the proportions of offspring sired by two males competing heterospermically is dependent on the ratio of the number of competing spermatozoa but not on total number, season, breed of hen or the interval from insemination to fertilization.

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The effect of ratios and numbers of spermatozoa mixed from two males on proportions of offspring

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Hall Jg

Papers

An experiment with heterospermicnsemination in cattle.