TESTING AVIAN POLYGYNY HYPOTHESES IN INSECTS - HAREM SIZE DISTRIBUTION AND FEMALE EGG GALLERY SPACING IN 3 IPS BARK BEETLES

A majority of females in Ipini bark beetles (Coleoptera: Scolytidae) m ate polygy nously, although larval competition within harems with > 2 females is expected to reduce female reproductive success. Models gene rated for territorial polygyny in birds were examined in order to expl ain polygyny in bark beetles. At the same rime, a test of the general validity of the avian models on an unrelated group of animals is provi ded. Three levels of models were tested in three Ips spp; 1) male coer cion/female choice, 2) cost/no cost of polygyny, and 3) unequal sex-ra tio (female biased). A fourth level; compensation/no compensation of f emale costs could be partly tested. In Ips typographus, I. cembrae, an d 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-Poiss on distributions of H and the biology of bark living, where females ca n easily escape male dominance, in favour of female choice. A cost of polygyny (2) was demonstrated: Female egg tunnels were too closely spa ced within harems to allow all eggs to develop into adults and the dis tance between egg tunnels were 50-100% greater between harems than wit hin harems. The unbalanced sex-ratio hypothesis (3) could be rejected because a sizable fraction (3-35%) of males were unpaired or had H < 2 in the field. At an extreme sex-ratio of 6:1: in the laboratory still as many as 31% of males had only one or two females. On the fourth le vel (4), compensation of female cost was best supported, but a limitat ion Of compensation by choice due to a 'search-cost' limitation could not be ruled out. 'Deception' hypotheses were ruled out: Female egg tu nnels were oriented more evenly than random between the sides of the g allery and eggs were predominantly laid (70%) on sides without within- harem competition. Thus, females acted adaptively based on knowledge o f their position relative to other harem females. The compensation for females could be of 'sexy-sons' type in I. typographus, as males with higher H may have higher pheromone content. It is concluded that on t he three first levels the 1) coercion, 2) no cost of polygyny, and 3) biased sex-ratio hypotheses could be rejected, while on the fourth lev el compensation was favoured.