Genetic and phenotypic trends and parameters were estimated for reproduction, fleece weight and liveweight in a South African Merino population, divergently selected from 1986, either for (H line) or against (L line) maternal multiple rearing ability. Annual reproduction, ewe greasy fleece weight and pre-joining liveweight data were recorded on 809 Merino ewes, from 1986 to 2002. Phenotypic trends indicated divergence in reproduction traits between the H and L lines. The direct additive variance ratio (h2 ± s.e.) for day of lambing was 0.08 ± 0.02. Estimates of h2 for reproduction traits were: 0.10 ± 0.02 for number of lambs born per ewe; 0.04 ± 0.02 for number of lambs weaned per ewe; and 0.04 ± 0.02 for weight of lamb weaned per ewe, corrected for the gender of the lamb. Corresponding h2 estimates for annual production were 0.57 ± 0.06 for greasy fleece weight and 0.48 ± 0.06 for ewe liveweight at joining. Service sire only exerted a significant (P<0.05) effect on day of lambing, but it accounted for merely 2% of the overall phenotypic variation. Ewe permanent environment variance ratios (c2ewe) for the reproduction traits were: 0.07 ± 0.03 for number of lambs born per ewe; 0.11 ± 0.03 for number of lambs weaned per ewe; and 0.11 ± 0.03 for total weight of lamb weaned per ewe. Corresponding c2ewe estimates for annual production traits were 0.14 ± 0.05 for greasy fleece weight and 0.27 ± 0.06 for ewe joining weight. Genetic and ewe permanent environmental correlations between measures of reproduction exceeded 0.7. Genetic correlations of reproduction traits with greasy fleece weight were low and variable in sign. Genetic correlations of reproduction traits with ewe joining weight were positive and particularly high for weight of lamb weaned. Permanent environmental correlations of reproduction traits with greasy fleece weight and joining weight were generally low to moderate. Genetic trends for the H and L lines (derived from averaged direct breeding values within birth years) were divergent (P<0.01) for all reproduction traits. Expressed as percentage of the overall least squares means of the respective traits, breeding values in the H line increased annually, with 1.3% for lambs born per ewe, 1.5% for lambs weaned per ewe and by 1.8% for weight of lamb weaned per ewe. Corresponding trends in the L line were, respectively, –0.6%, –1.0% and –1.2% per year. Substantial genetic progress in annual lamb output was attainable, despite relatively small h2 estimates. This response was achieved without unfavourable genetic changes in wool and liveweight.

Data of 5,390 Merino lambs born from 1986 to 2007 (~6.9 generations) were used to derive genetic parameters and trends for age-specific and overall lamb survival on the underlying scale, as well as for lamb birth weight, using Gibbs sampling. The majority of lambs were descended from lines that were divergently selected for the ability of ewes to rear multiples. The line selected in the upward direction was denoted as the high line (H line), whereas the line selected in the downward direction was the low line (L line). Analyses included the covariance between direct and maternal genetic effects, except where it was not estimable owing to small direct additive variance components, a high incidence of lambs surviving, or both. Direct heritability estimates were 0.02 for lamb survival at birth, 0.12 for lamb survival from birth to tail docking, 0.39 for lamb survival from docking to weaning, 0.28 for overall lamb survival, and 0.17 for birth weight. Corresponding estimates for the maternal genetic effect were 0.26, 0.14, 0.16, 0.14, and 0.29. Dam permanent environmental variance ratios were, respectively, 0.14, 0.09, 0.05, 0.07, and 0.07. Estimates of the direct-maternal genetic correlation were −0.60 for lamb survival from docking to weaning, −0.61 for overall lamb survival, and −0.15 to −0.23 for birth weight. Genetic, maternal genetic, and dam permanent environmental correlations between lamb birth weight and overall or age-specific lamb survival were not different from zero. Expressed relative to overall means, annual direct genetic change in the H line amounted to −0.01% per annum for lamb survival at birth, 0.52% per annum for lamb survival from birth to docking, and 1.3% per annum for lamb survival from docking to weaning. Corresponding trends in the L line were, respectively, −0.01, −0.42, and 0.042% per annum. Maternal genetic trends amounted to, respectively, 0.23, −0.11, and −0.24% per annum in the H line and −0.78, −0.50, and −0.16% per annum in the L line. It was concluded that sustained genetic progress in lamb survival is feasible if directed selection is applied to a correlated trait such as the ability of ewes to rear multiples. Copyright © 2009. . Copyright 2009 Journal of Animal Science

Temperature has a crucial influence on the places where species can survive and reproduce. Past research has primarily focused on survival, making it unclear if temperature fluctuations constrain reproductive success, and if so whether populations harbour the potential to respond to climatic shifts. Here, using two decades of data from a large experimental breeding programme of the iconic ostrich (Struthio camelus) in South Africa, we show that the number of eggs females laid and the number of sperm males produced were highly sensitive to natural temperature extremes (ranging from −5 °C to 45 °C). This resulted in reductions in reproductive success of up to 44% with 5 °C deviations from their thermal optimum. In contrast, gamete quality was largely unaffected by temperature. Extreme temperatures also did not expose trade-offs between gametic traits. Instead, some females appeared to invest more in reproducing at high temperatures, which may facilitate responses to climate change. These results show that the robustness of fertility to temperature fluctuations, and not just temperature increases, is a critical aspect of species persistence in regions predicted to undergo the greatest change in climate volatility.

The evolutionary potential of species to cope with short-term temperature fluctuations during reproduction is critical to predicting responses to future climate change. Despite this, vertebrate research has focused on reproduction under high or low temperatures in relatively stable temperate climates. Here, we characterize the genetic basis of reproductive thermal tolerance to temperature fluctuations in the ostrich, which lives in variable environments in tropical and subtropical Africa. Both heat and cold tolerance were under selection and heritable, indicating the potential for evolutionary responses to mean temperature change. However, we found evidence for a negative, genetic correlation between heat and cold tolerance that should limit the potential for adaptation to fluctuating temperatures. Genetic constraints between heat and cold tolerance appear a crucial, yet underappreciated, factor influencing responses to climate change.