Testing avian polygyny hypotheses in insects: harem size distribution and female egg gallery spacing in three Ips bark beetles

A majority of females in Ipini bark beetles (Coleoptera: Scolytidae) mate polygynously, although larval competition within harems with >2 females is expected to reduce female reproductive success. Models generated for territorial polygyny in birds were examined in order to explain polygyny in bark beetles. At the same time, a test of the general validity of the avian models on an unrelated group of animals is provided. Three levels of models were tested in three Ips spp; 1) male coercion/female choice, 2) cost/no cost of polygyny, and 3) unequal sex-ratio (female biased). A fourth level; compensation/no compensation of female costs could be partly tested. In Ips typographus, I. cembrae, and I. duplicatus mean harem sizes were between 1.9 and 2.8, but between 51 and 79% of females were in galleries with harem size (H) >2. The hypothesis of male coercion (1) was not supported due to the non-Poisson distributions of H and the biology of bark living, where females can easily escape male dominance, in favour of female choice. A cost of polygyny (2) was demonstrated: Female egg tunnels were too closely spaced within harems to allow all eggs to develop into adults and the distance between egg tunnels were 50-100% greater between harems than within harems. The unbalanced sex-ratio hypothesis (3) could be rejected because a sizable fraction (3-35%) of males were unpaired or had H