Another look at mortality crossovers

Abstract

Considerable debate still ensues on whether or not the so‐called “mortality crossover,” the intersection of age curves of mortality at the older ages, is artifactual or real. One school of thought argues that it is a function of misreporting of ages, greater for one population than another. The other school of thought counters that it persists apart from age misreporting and is due to selective frailty processes as age increases. An historical review of the debate is reported, after which an attempt at a balanced conclusion is offered.     

Causes of death which contribute to the mortality crossover effect

Abstract

Death rates vary over the life cycle in a standard fashion, with mortality probabilities being highest at infant and older ages. Nevertheless, when age curves of mortality are compared for different populations, they sometimes can be seen to intersect so that one population has higher death rates at younger and middle ages and lower rates at older ages. Past research has shown that this phenomenon is not due to erroneous data and is probably a result of some type of selection in survival patterns. A sample of pairs of mortality curves, 31 of which cross over and 31 of which do not, for combinations of countries and dates are analyzed to discover which causes of death are associated with the crossover phenomenon at the older ages. Cardiovascular and “other and unknown” diseases appear to contribute strongly to the crossover effect. Further research should deal with other comparisons and explore the underlying social and environmental factors.     

Causes and consequences of winter mortality in fishes

Winter mortality has been documented in a large number of freshwater fish populations, and a smaller, but increasing, number of marine and estuarine fishes. The impacted populations include a number of important North American and European resource species, yet the sources of winter mortality remain unidentified in most populations where it has been documented. Among the potential sources, thermal stress and starvation have received the most research attention. Other sources including predation and pathogens have significant impacts but have received insufficient attention to date. Designs of more recent laboratory experiments have reflected recognition of the potential for interactions among these co-occurring stressors. Geographic patterns in winter mortality are, in some cases, linked to latitudinal clines in winter severity and variability. However, for many freshwater species in particular, the effects of local community structure (predators and prey) may overwhelm latitudinal patterns. Marine (and estuarine) systems differ from freshwater systems in several aspects important to overwintering fishes, the most important being the lack of isolating barriers in the ocean. While open population boundaries allow fish to adopt migration strategies minimizing exposure to thermal stresses, they may retard rates of evolution to local environments. Geographic patterns in the occurrence and causes of winter mortality are ultimately determined by the interaction of regional and local factors. Winter mortality impacts population dynamics through episodic depressions in stock size and regulation of annual cohort strength. While the former tends to act in a density-independent manner, the latter can be density dependent, as most sources of mortality tend to select against the smallest members of the cohort and population. Most stock assessment and management regimes have yet to explicitly incorporate the variability in winter mortality. Potential management responses include postponement of cohort evaluation (to after first winter of life), harvest restrictions following mortality events and habitat enhancement. Future research should place more emphasis on the ecological aspects of winter mortality including the influences of food-web structure on starvation and predation. Beyond illuminating an understudied life-history phase, studies of overwintering ecology are integral to contemporary issues in fisheries ecology including ecosystem management, habitat evaluation, and impacts of climate change.     

Causes of mortality of the Wyoming toad

Wyoming toads (Bufo baxteri) that died from January 1989 to June 1996 were submitted to the Wyoming State Veterinary Laboratory (Laramie, Wyoming, USA) for postmortem evaluation. These consisted of 108 free-ranging toads and 170 animals from six captive populations. Ninety-seven (90%) of 108 free-ranging toad carcasses were submitted during September and October. From 1989 to 1992, 27 (77%) of 35 mortalities in the captive populations occurred in October, November, and December. From 1993 to 1996, mortality in captive toads occurred without a seasonal pattern and coincided with changes in hibernation protocols that no longer mimicked natural cycles. Cause of mortality was determined in 147 (53%) of the 278 cases. Mycotic dermatitis with secondary bacterial septicemia was the most frequent diagnosis in 104 (71%) of 147 toads. Basidiobolus ranarum was found by microscopic examination of skin sections in 100 (96%) of 104 of these mortalities. This fungus was isolated from 30 (56%) of 54 free-ranging and 24 (48%) of 50 captive toads. This research documents the causes of mortality for both free-ranging and captive endangered Wyoming toads over a 7 yr period.     

Causes of mortality of free-ranging Florida panthers

The Florida panther (Puma concolor coryi) is one of the most endangered mammals, with the entire population estimated to consist of only 30-50 adult animals. Between 1978 and 1999, 73 free-ranging Florida panther carcasses were submitted for postmortem evaluation, of which 47 (64%) were radiocollared and 26 (36%) were uncollared cats. Overall, mortality of panthers > 6-mo-old was due to vehicular trauma in 25 (35%), intraspecific aggression in 19 (26%), illegal kill in seven (10%), research activities in two (3%), infectious diseases in two (3%), esophageal tear in one (1%), pleuritis in one (1%), pyothorax in one (1%), aortic aneurysm in one (1%), atrial septal defect in one (1%), and causes of death were undetermined in 13 (18%) due to autolysis. Of the 25 panthers that were killed by vehicular trauma, 20 (80%) died between October and April. This coincides with increased number of winter visitors to south Florida. Among radiocollared panthers, intraspecific aggression was the primary cause of mortality for 19 (41%) dead cats. Of these cats, 16 (84%) were males and 14 (88%) were either less than 3 or more than 8-yr-old. These animals were probably fighting to establish or retain territory. Among the 26 uncollared panthers, vehicular trauma was the primary cause of mortality and was responsible for 16 (62%) deaths. This study documents the causes of mortality and the age, sex, and seasonal mortality trends for both radiocollared and uncollared free-ranging endangered Florida panthers over a 21-yr-period.     

Causes of neonatal mortality in Spain (1975-98): influence of sex, rural-urban residence and age at death

Neonatal mortality during the first week of life, corresponding to the years 1975-1998, was studied in Spain. The first week of life is the time in which the highest number of deaths occur. The temporal decrease of the neonatal mortality rate (NMR) was modelled according to log10(NMR+1)= 2.784 - 0.023 per year. This decline cannot be explained by an increase in the mean birth weight (MBW=23440.835 - 10.107 g per year). From the most frequent of the causes of death to the least were: congenital anomalies, preterm born or low birth weight, respiratory problems, pregnancy difficulties, hypoxaemia/asphyxia, delivery difficulties and infectious diseases. This sequence changed when the specific age at death was considered. The NMR descended evenly for both sexes for the causes indicated above, except for preterm born or low birth weight, in which the male mortality decrease was greater since its rate was more elevated at the beginning of the period studied. For all the causes listed, NMR was more elevated both in urban areas and for males. Early neonatal mortality (first 24 hours) was higher for pregnancy difficulties, preterm born or low birth weight, congenital anomalies and hypoxaemia/asphyxia.     

Virus-host arms race at the joint origin of multicellularity and programmed cell death

Unicellular eukaryotes and most prokaryotes possess distinct mechanisms of programmed cell death (PCD). How an “altruistic” trait, such as PCD, could evolve in unicellular organisms? To address this question, we developed a mathematical model of the virus-host co-evolution that involves interaction between immunity, PCD and cellular aggregation. Analysis of the parameter space of this model shows that under high virus load and imperfect immunity, joint evolution of cell aggregation and PCD is the optimal evolutionary strategy. Given the abundance of viruses in diverse habitats and the wide spread of PCD in most organisms, these findings imply that multiple instances of the emergence of multicellularity and its essential attribute, PCD, could have been driven, at least in part, by the virus-host arms race.     

Differential stability of DNA crossovers in solution mediated by divalent cations

The assembly of DNA duplexes into higher-order structures plays a major role in many vital cellular functions such as recombination, chromatin packaging and gene regulation. However, little is currently known about the molecular structure and stability of direct DNA–DNA interactions that are required for such functions. In nature, DNA helices minimize electrostatic repulsion between double helices in several ways. Within crystals, B-DNA forms either right-handed crossovers by groove–backbone interaction or left-handed crossovers by groove–groove juxtaposition. We evaluated the stability of such crossovers at various ionic concentrations using large-scale atomistic molecular dynamics simulations. Our results show that right-handed DNA crossovers are thermodynamically stable in solution in the presence of divalent cations. Attractive forces at short-range stabilize such crossover structures with inter-axial separation of helices less than 20 Å. Right-handed crossovers, however, dissociate swiftly in the presence of monovalent ions only. Surprisingly, left-handed crossovers, assembled by sequence-independent juxtaposition of the helices, appear unstable even at the highest concentration of Mg²⁺studied here. Our study provides new molecular insights into chiral association of DNA duplexes and highlights the unique role divalent cations play in differential stabilization of crossover structures. These results may serve as a rational basis to understand the role DNA crossovers play in biological processes.     

Causes of badger mortality in eastern counties of England

The circumstances of the deaths of 17 badgers in south-eastern England are described together with post-mortem examinations. The livers of eight animals were analysed to detect the levels of organochlorine insecticides. All contained dieldrin (0.01 to 46.3 ppm), five contained DDT and/or its derivatives (0.30 to 19.2 ppm) and three contained heptachlor epoxide (1.9 to 4.4 ppm). It is concluded that five of the 17 died after road accidents whilst of the remainder, six certainly and six very probably died of dieldrin poisoning. The behaviour of one poisoned badger maintained in the laboratory is described. The amounts of dieldrin contained and consumed by the badgers are discussed together with the possible sources of the poison.     

Causes of treatment failure and death in carcinoma of the lung

Studies of patterns of failure and causes of death have been undertaken based upon the WHO histopathologic classification. In a randomized trial of thoracic irradiation +/- chemotherapy (hydroxyurea and CCNU), patterns of failure did not seem to differ by cell type; the largest group was "death without progression." A subsequent clinical trial of thoracic irradiation +/- cranial irradiation permitted a more detailed evaluation. Patients with squamous cell carcinoma had a higher rate of local failure than distant metastasis. Those with small cell carcinoma had a lower local failure rate and a high rate of distant spread. Patients with adenocarcinoma and large cell carcinoma had the lowest local failure rate, but had a high rate of distant metastasis. In 300 consecutive patients with autopsies, 75 percent with squamous carcinoma died of complications of the thoracic tumor and only one-quarter had extrathoracic dissemination; 30 percent with small cell carcinoma died of local tumor complications and 70 percent had carcinomatosis; 40 percent of patients with adenocarcinoma and large cell carcinoma died of intrathoracic complications, and 55 percent had distant metastases. Half the patients with small cell carcinoma, large cell carcinoma, and adenocarcinoma had brain metastases at autopsy. Future clinical trials should emphasize better control of the most common sites of failure.