Growth and survival of Azolla filiculoides in Britain. II. Sexual reproduction

Sporulation in the floating fern Azolla filiculoides Lam. is both frequent and widespread in Britain and might therefore play a greater part in the population dynamics of the species than has been suggested by earlier reports. In laboratory experiments, increasing plant density and/or phosphorus supply resulted in increased sporulation. It was estimated that a thick mat of 8 kg m² fresh biomass can produce 380000 microsporocarps and 85000 megasporocarps per m².
Light and temperatures >10°C were necessary for sporocarp germination. Sporocarps could survive exposure to both low temperatures (5°C for at least 3 months) and sub-zero temperatures (−10°C for at least 18 d). Sporocarps were found to survive storage in water for 3 yr and to germinate from mud samples collected in the field. In laboratory culture, sporeling growth and survival were optimal at 15°C.
There is some evidence that A. filiculoides might have adapted to the British climate since its introduction.     

Time is money. Waiting costs explain why selection favors steeper time discounting in deprived environments

Individuals exposed to deprivation tend to show a characteristic behavioral syndrome suggestive of a short time horizon. This pattern has traditionally been attributed to the intrinsically higher unpredictability of deprived environments, which renders waiting for long term rewards more risky (i.e. collection risks are high). In the current paper, based on a simple dynamic life history model, we show that a significant portion of individuals' propensity to discount future rewards might have a completely distinct origin. Upon collecting a resource, individuals have the opportunity to accumulate “capital” (e.g. grow muscular tissue, build a protective shelter, buy a car, etc.), which eventually increases their productivity and/or their chances of survival. As a result, delaying the collection of a resource creates an opportunity cost in the sense that during the waiting time, the benefits otherwise generated by the increment in capital are lost. These forgone benefits are independent of collection risks and constitute waiting costs per se. Using optimal control theory we show that these costs can lead to the evolution of short time horizons even in the complete absence of collection risks. Moreover, assuming diminishing returns to capital, we show that the evolutionarily stable time horizon increases with the amount of capital already owned by individuals. When individuals possess little capital, they have a lot of room to improve their productivity and/or survival, hence they should be impatient to collect resources; that is, their time horizon should be short. On the contrary, when individuals already possess a lot of capital, the benefits of further accumulation are plateauing, hence patience becomes a more profitable strategy and individuals should lengthen their time horizon. This means that individuals get more patient as they age and that people in deprivation, who still have important productive and survival needs that can be satisfied, should have a shorter time horizon. This result leads us to interpret the behavioral constellation of deprivation in a new way.     

Growth and survival of Azolla filiculoides in Britain I. Vegetative production

Azolla filiculoides Lam. causes serious weed problems in Britain, but its long-term survival might be limited by winter death. The aim of this study was to establish the low temperature responses and limitations of A. filiculoides sporophytes.
In the laboratory, normal vegetative growth was shown to continue at 5°C. Reddening of plants was a response to low temperature and high light conditions which could be prevented by shading. Adult plants died after short (18 h) exposure to −4°C but survived sub-zero temperatures >−4°C. Evidence was found of seasonal changes in chill tolerance, but not in freeze tolerance.
In outdoor culture, plants survived encasement in ice and air temperatures to −5°C. Additional evidence suggested that natural populations can readily survive air temperatures much lower than this. Microclimatic effects are likely to be responsible for this discrepancy between laboratory and outdoor culture results.
Three phenotyes were identified; survival, colonizing and mat forms.     

Maternal survival costs in an asocial mammal

Maternal characteristics, social dynamics, and environmental factors can all influence reproduction and survival and shape trade‐offs that might arise between these components of fitness. Short‐lived mammals like the golden‐mantled ground squirrel (GMGS; Callospermophilus lateralis) tend to maximize effort toward current reproduction at the expense of survival but may be complicated by other aspects of the species’ life history and environment. Here, we use 25 years of data (1995–2020) collected from a population of GMGS at the Rocky Mountain Biological Research Laboratory in Gothic, Colorado, to test the effect of several maternal characteristics (e.g., age, experience, and timing of litter emergence), social context (e.g., litter sex ratio and kin density), and environmental context (e.g., date of bare ground and length of vegetative growing season) on survival of reproductive female GMGS using Cox proportional hazard models. Our results indicated that social dynamics (i.e., density) and environmental conditions (i.e., standardized first day of permanent snow cover and length of growing season) explained significant variation in annual maternal survival, while maternal characteristics did not. A higher density of related breeding females and the total number of females (both related and unrelated to the focal mother) were associated with an increase in the mortality hazard. A later standardized date of the first day of permanent snow cover and a shorter growing season both reduced the maternal mortality hazard. Together, our results suggest that factors extrinsic to the squirrels affect maternal survival and thus may also influence local population growth and dynamics in GMGS and other short‐lived, territorial mammal species.     

Cancer chemotherapy: identifying novel anticancer drugs.

There are now around 60 cytotoxic drugs licensed for use in cancer therapy in the United Kingdom. For certain malignancies such as childhood cancers, haematological malignancies, and germ cell tumours chemotherapy has been pivotal to the substantial improvement in therapeutic outcome achieved over the past 10 years. In contrast, improvements in the systemic management of adult solid tumours have been less dramatic. There is a clear and urgent need for new, more effective drugs for lung, breast, and colorectal malignancies. This paper examines the processes in identifying, developing, and evaluating new drugs with anticancer activity.     

Population based survival rates for childhood cancer in Britain, 1980-91.

OBJECTIVES--To investigate the survival of children with cancer diagnosed during 1980-91 in order to assess the impact of developments in medical care on a population basis. DESIGN--Retrospective cohort study. SETTING--Great Britain. SUBJECTS--14973 children with cancer diagnosed during 1980-91 and included in the population based National Registry of Childhood Tumours. MAIN OUTCOME MEASURES--Actuarial survival rates. RESULTS--For all cancers combined, two year survival increased from 66% to 76% between 1980-2 and 1989-91, and five year survival increased from 57% to 65% between 1980-2 and 1986-8. Significant increases in survival rates occurred among children with acute lymphoblastic leukaemia, acute nonlymphocytic leukaemia, retinoblastoma, osteosarcoma, Ewing''s sarcoma, rhabdomyosarcoma, and malignant gonadal germ cell tumours. No trend in survival was seen for children with Hodgkin''s disease, central nervous system tumours, neuroblastoma, or Wilms''s tumour. CONCLUSIONS--Nearly two thirds of children who have cancer diagnosed can now expect to survive at least 10 years.     

Cumulative reproduction and survival costs in female red deer

Successful reproduction in a single breeding event has consistently been shown to reduce condition, fecundity and survival to the following breeding season. Few studies have examined the cumulative costs of frequent reproduction on survival. Here we use a dataset of female red deer ( Cervus elaphus ) from the Isle of Rum, Scotland, to model survival probability within a mark–recapture framework. By including both recent reproduction and long-term cumulative reproductive effort in the models we tested whether knowledge of lifetime reproductive effort improves our estimates of survival probability. We found that the fit of the model was significantly improved with the inclusion of longer-term measures of reproductive history. Heterogeneity in the reproductive performance of individuals influenced the expected survival cost of reproduction, with high cumulative reproductive effort associated with high survival, except with individuals reproducing in their first year where reproduction was associated with a decrease in survival. This work emphasises the need to account for reproductive history when estimating the survival probabilities of animals.     

Sex-specific costs of reproduction on survival in a long-lived seabird

Costs of reproduction on survival have captured the attention of researchers since life history theory was formulated. Adults of long-lived species may increase survival by reducing their breeding effort or even skipping reproduction. In this study, we aimed to evaluate the costs of current reproduction on survival and whether skipping reproduction increases adult survival in a long-lived seabird. We used capture–mark–recapture data (1450 encounters) from two populations of Bulwer''s petrel (Bulweria bulwerii), breeding in the Azores and Canary Islands, North Atlantic Ocean. Using a multi-event model with two different breeding statuses (breeders versus non-breeders), we calculated probabilities of survival and of transitions between breeding statuses, evaluating potential differences between sexes. Females had lower survival probabilities than males, independent of their breeding status. When considering breeding status, breeding females had lower survival probabilities than non-breeding females, suggesting costs of reproduction on survival. Breeding males had higher survival probabilities than non-breeding males, suggesting that males do not incur costs of reproduction on survival and that only the highest quality males have access to breeding. The highest and the lowest probabilities of skipping reproduction were found in breeding males from the Azores and in breeding males from the Canary Islands, respectively. Intermediate values were observed in the females from both populations. This result is probably due to differences in the external factors affecting both populations, essentially predation pressure and competition. The existence of sex-specific costs of reproduction on survival in several populations of this long-lived species may have important implications for species population dynamics.     

The energetic and survival costs of growth in free-ranging chipmunks

The growth/survival trade-off is a fundamental aspect of life-history evolution that is often explained by the direct energetic requirement for growth that cannot be allocated into maintenance. However, there is currently no empirical consensus on whether fast-growing individuals have higher resting metabolic rates at thermoneutrality (RMRt) than slow growers. Moreover, the link between growth rate and daily energy expenditure (DEE) has never been tested in a wild endotherm. We assessed the energetic and survival costs of growth in juvenile eastern chipmunks (Tamias striatus) during a year of low food abundance by quantifying post-emergent growth rate (n = 88), RMRt (n = 66), DEE (n = 20), and overwinter survival. Both RMRt and DEE were significantly and positively related to growth rate. The effect size was stronger for DEE than RMRt, suggesting that the energy cost of growth in wild animals is more likely to be related to the maintenance of a higher foraging rate (included in DEE) than to tissue accretion (included in RMRt). Fast growers were significantly less likely to survive the following winter compared to slow growers. Juveniles with high or low RMRt were less likely to survive winter than juveniles with intermediate RMRt. In contrast, DEE was unrelated to survival. In addition, botfly parasitism simultaneously decreased growth rate and survival, suggesting that the energetic budget of juveniles was restricted by the simultaneous costs of growth and parasitism. Although the biology of the species (seed-storing hibernator) and the context of our study (constraining environmental conditions) were ideally combined to reveal a direct relationship between current use of energy and future availability, it remains unclear whether the energetic cost of growth was directly responsible for reduced survival.