Sex ratio in the spider Pityohyphantes phrygianus affected by winter severity



The sex ratio of subadult Pityohyphantes phrygianus was recorded before and after five winters in a natural population in coniferous forest in south-west Sweden. In autumn, the proportion of males was on average 33–8% (range 28–7–40–1) and the proportion of females was 66–2% (59–9–71–3). In each of the winters 1981 82, 1982–83, 1984–5, the proportion of males decreased significantly. The proportion of males decreased more the lower the February mean temperature. Field experiments showed that low ambient temperature in winter caused high mortality among spiders. Experimental data also suggest that males are more vulnerable to low winter temperatures than are females. Indirect evidence indicates that neither predation pressure nor starvation alone are likely to cause the observed changes in sex ratio in the three winters. However, the combined effect of sex differences in predation by birds and cold-induced mortality may explain why males disappear faster than females in certain winters.