Barbara Hellriegel

Mathematical modelling: a tool for hospital infection control

Health-care-associated infections caused by antibiotic-resistant pathogens have become a menace in hospitals worldwide and infection control measures have lead to vastly different outcomes in different countries. During the past 6 years, a theoretical framework based on mathematical models has emerged that provides solid and testable hypotheses and opens the road to a quantitative assessment of the main obstructions that undermine current efforts to control the spread of health-care-associated infections in hospitals and communities. We aim to explain to a broader audience of professionals in health care, infection control, and health systems administration some of these models that can improve the understanding of the hidden dynamics of health-care-associated infections. We also appraise their usefulness and limitations as an innovative research and decision tool for control purposes.

Female-mediated differential sperm storage in a fly with complex spermathecae, Scatophaga stercoraria

Multiple spermathecae potentially allow selective sperm use, provided that sperm from rival males are stored differentially, that is, in different proportions across storage compartments. In the yellow dung fly, Scatophaga stercoraria, females have three spermathecae arranged as a doublet and singlet. To test whether females store the sperm of rival males actively and differentially, we mated fixed male pairs to three females. After copulation, females were (1) dissected immediately before they could start laying a clutch of eggs, (2) left awake for 30 min but prevented from oviposition, or (3) anaesthetized with carbon dioxide for 30 min to interfere with the muscular control presumably required for sperm transport from the site of insemination to the spermathecae. For each female, we estimated the proportion of the second males sperm stored in her spermathecae (S(2)value), using sperm length as a male marker. After copulation, the S(2)values in the singlet and doublet spermathecae differed significantly, indicating differential sperm storage during copulation. Postcopulatory treatment affected differential sperm storage significantly. Females dissected immediately had lower S(2)values in the doublet than in the singlet spermatheca, while females left awake showed the reverse pattern for the same two males. This reversal did not occur when females were treated with carbon dioxide. The results indicate differential storage of sperm from different males during copulation and that female muscular activity can affect storage and separation of competing ejaculates beyond copulation. Copyright 2000 The Association for the Study of Animal Behaviour.

A new method to determine bird and bat fatality at wind energy turbines from carcass searches

Abstract Wind energy is of increasing importance for a sustainable energy supply worldwide. At the same time, concerns about the number of birds and bats being killed at wind turbines have been growing. In this situation, methods for a reliable estimation of bird and bat fatality numbers are needed. To obtain an unbiased estimate of the number of fatalities from fatality searches, the probability to detect the carcass of an animal being killed at a turbine has to be assessed by considering carcass persistence rate, searcher efficiency and the probability that a killed animal falls into a searched area. Here, we describe a new formula to determine the detection probability of birds or bats that are killed at wind turbines and which can estimate the number of fatalities from the number of carcasses found. The formula was developed to analyse a large data set of bats killed at wind turbines in Germany. In simulations, we compared it to three other formulas used in this context. Our new formula seems to have unbiased results when searcher efficiency and carcass removal rate are constant over time. When searcher efficiency or carcass removal rate varied with time, all four formulas showed a similar bias. These comparative results can be used to choose between methods depending on the quality of information available. Our estimator can, for instance, be adapted to different situations including temporal changes of searcher efficiency or carcass removal rate because it is based on an explicit process model.

The collective action problem in primate territory economics

Group-living animals often do not maintain territories, but instead have highly overlapping ranges, even though in principle these are economically defendable. We investigate whether this absence of range defence reflects a collective action problem, since a territory can be considered a public good. In a comparative analysis comprising 135 primate species, we find a positive association between range overlap and group size, controlling for economic defendability and phylogenetic non-independence. We subsequently demonstrate that groups with multiple adults of both sexes suffer levels of range overlap twice as high as groups with only a single adult representative of either sex, consistent with the presence of a collective action problem. Finally, we reveal that this collective action problem can be overcome through philopatry of the larger sex. These results suggest that a social complication of group living is a stronger determinant of between-group relations among social animals than ecological factors, but also that collective defence is still achieved where the dominant sex is philopatric and effective defence is critical to reproductive success and survival. In addition, our findings support the idea that human-like warfare, defined as escalated collective territorial conflict, has an evolutionary basis reflected by cases of convergent evolution among non-human primates.

SEXUAL CONFLICT OVER FLORAL RECEPTIVITY

Abstract In flowering plants, the onset and duration of female receptivity vary among species. In several species the receptive structures wilt upon pollination. Here we explore the hypothesis that postpollination wilting may be influenced by pollen and serve as a general means to secure paternity of the pollen donor at the expense of female fitness. Taking a game-theoretical approach, we examine the potential for the evolution of a pollen-borne wilting substance, and for the coevolution of a defense strategy by the recipient plant. The model without defense predicts an evolutionarily stable strategy (ESS) for the production of wilting substance. The ESS value is highest when pollinator visiting rates are intermediate and when the probability that pollen from several donors arrives at the same time is low. This finding has general implications in that it shows that male traits to secure paternity also can evolve in species, such as plants, where mating is not strictly sequential. We further model coevolution of the wilting substance with the timing of stigma receptivity. We assume that pollen-receiving plants can reduce the costs induced by toxic pollen by delaying the onset of stigmatic receptivity. The model predicts a joint ESS, but no female counter-adaptation when the wilting substance is highly toxic. This indicates that toxicity affects the probability that a male manipulative trait stays beneficial (i.e., not countered by female defense) over evolutionary time. We discuss parallels to male induced changes in female receptivity known to occur in animals and the role of harm for the evolution of male manipulative adaptations.

Theoretical and experimental evidence indicates that there is no detectable auxin gradient in the angiosperm female gametophyte

The plant life cycle alternates between a diploid sporophytic and a haploid gametophytic generation. The female gametophyte (FG) of flowering plants is typically formed through three syncytial mitoses, followed by cellularisation that forms seven cells belonging to four cell types. The specification of cell fates in the FG has been suggested to depend on positional information provided by an intrinsic auxin concentration gradient. The goal of this study was to develop mathematical models that explain the formation of this gradient in a syncytium. Two factors were proposed to contribute to the maintenance of the auxin gradient in Arabidopsis FGs: polar influx at early stages and localised auxin synthesis at later stages. However, no gradient could be generated using classical, one-dimensional theoretical models under these assumptions. Thus, we tested other hypotheses, including spatial confinement by the large central vacuole, background efflux and localised degradation, and investigated the robustness of cell specification under different parameters and assumptions. None of the models led to the generation of an auxin gradient that was steep enough to allow sufficiently robust patterning. This led us to re-examine the response to an auxin gradient in developing FGs using various auxin reporters, including a novel degron-based reporter system. In agreement with the predictions of our models, auxin responses were not detectable within the FG of Arabidopsis or maize, suggesting that the effects of manipulating auxin production and response on cell fate determination might be indirect.

Repeatability of sperm size in outbred and inbred Scathophaga stercoraria (Diptera: Scathophagidae)

Variability in male gametic traits can depend on several genetic and environmental factors such as developmental instability as a consequence of inbreeding, developmental noise during spermatogenesis, or age-or condition-dependent changes in allocation to sperm cells. Variation in sperm size is particularly evident in species that produce more than one sperm morph but also occurs among males in sperm-monomorphic species. Both discrete and continuous sperm size variation have been implicated in male fertilization success when the sperm of several males directly compete for fertilization of the same set of ova. In this study, we investigated among-male variation in sperm length in field-collected, outbred male Scathophaga stercoraria (L.) flies, as well as in flies from the same natural population that had been subjected to 15 and 16 generations of inbreeding under laboratory conditions. Among-male variation in sperm length was significant and repeatable over subsequent matings in both inbred and outbred flies. We conclude that sperm length can be used as an individual male marker in sperm competition studies and that significant repeatability of sperm length supports heritability for this trait.