Title:
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Semiochemical detection of infection status in house mice
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Female preference expressed through mate choice for attractive males represents a fundamental
mechanism for the selection and propagation of heritable advantageous traits in animal populations. The
presence of pathogenic infection in males can alter female mate choice, commonly rendering affected
males less attractive than uninfected conspecifics, although there are several exceptions. It has been
suggested that in wild populations, variation between males in their ability to cope with infection may be
reflected in differences in signalling. Signals may include secondary sexual characteristics, behaviour, or
other changes associated with clinical pathology. Recognition of these signals could provide a reliable
estimate of heritable resistance to locally relevant pathogens, as highly resistant males may invest more
heavily in signals of attractiveness or suppress indicators of pathology, and so experience greater
reproductive success. Heritable resistance in the context of local pathogens is likely to increase offspring
survival, so improving the reproductive success of both the male and female parent.
Interpretation of female preference behaviour from an ecological perspective demands that experimental
design takes into account the evolutionary history of the host-pathogen combination under study.
Natural host-pathogen combinations are likely to be mutually adapted through co-evolution, resulting in
optimisation of avoidance and transmission strategies respectively. Thus observations gained through
experimental use of naturally occurring pathogens in wild study populations are more likely to elicit
ecologically relevant responses than non-natural pathogens. A serological survey during this project
found variable prevalence of several viral pathogens in wild and wild-derived captive populations of
house mice (Mus domesticus). This survey also found that most viruses showed low or no transmission
in captivity, with the exception oflymphocytic choriomeningitis virus (LCMV). As this pathogen is an
important zoonosis, the captive population was screened to eliminate LCMV and thus remove any
associated risk to personnel. Transmission of LCMV occurred vertically (to the foetuses in utero) in
most cases, and several animals that were persistently infected with virus had no detectable antibody to
this pathogen. This has important implications for standard laboratory screening for LCMV, where the
use of serological methods alone could fail to detect persistent infections.
The effects of immune stimulation and viral infection on male competitive behaviour and female
preference for males and their scents were investigated. Previous studies had demonstrated infectionassociated
changes to urinary scent signalling. Thus urine of control, vaccinated and infected animals
was subjected to biochemical analysis. Immune stimulation was achieved by vaccinating animals with
the novel antigen keyhole limpet haemocyanin. During infection studies, animals were inoculated with
mouse adenovirus, a natural pathogen of wild mice that had been identified during the serology survey.
Immune stimulation caused small a reduction in male scent marking behaviour, but did not alter female
preference or urine biochemistry. Infection caused no change to male urine marking behaviour, but
reduced urinary protein concentration. This did not alter female preference.
Pregnancy block or the 'Bruce' effect may represent a mechanism females could use to avoid the
deletenous effects of infection on offspring. Laboratory studies have demonstrated that female mice will
termmate gestation if exposed to an unfamiliar male within a limited time after mating. As these studies
ofte.n use inbred laboratory strains, males are classed as 'unfamiliar' if they arw of a different inbred
stram to the stud male. However, scent changes associated with infection may provide a different source
of unfamlhanty. The 'Bruce' effect is traditionally assumed to have evolved as it offers potential
advantages.to both males and females. However, we show that female mice are able to control whether
pregnancy is blocked or maintained by altering their own exposure to male scent at times critical for the
activation of this physiological mechanism. Thus females may control'the 'Bruce' effect without
reference to male advantage, and this behaviour may have evolved solely to improve female
reproductIve success.
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