Sex ratio and female allocation to harems in a polygynous bark beetle

Varying forms of polygyny are observed across many animal groups. In some species, groups of females may remain with a single male for an extended length of time. This is often referred to as harem polygyny. A female‐biased sex ratio has been associated with harem polygynous species. In such species, formation of harems may be an inevitable consequence of the relative lack of available males, rather than multiple females actively choosing to mate with a subset of specific males. The five‐spined bark beetle, Ips grandicollis, is a secondary pest of pine plantations (Pinus spp.). Harem‐like groups are formed when males bore into the bark creating a small nuptial chamber from where they release pheromones, attracting multiple females that build individual galleries associated with the males' chamber. We estimated the sex ratio of I. grandicollis in the flying population compared with those in the logs, and investigated whether females are actively choosing to join the harem of particular males, or if they simply randomly allocate themselves to harems. We also investigated how this female behaviour changes over time since the start of colonisation. The sex ratio in the flying population of I. grandicollis was slightly male biased while the population in pine billets was female biased. Early colonising females (within 2 to 8 days of log infestation) joined males non‐randomly – being more evenly distributed than expected by chance. However, the pattern of allocation differs between these days, with females allocating themselves more randomly later in the colonisation process. In a separate experiment, data collected from logs colonised after 14 days suggest completely random allocation of females to males. These results suggest that when there is a higher proportion of males to females (during early colonisation), females allocate themselves evenly among males, reducing potential between‐female competition. However, as males become rarer, females must join larger harems, but do not appear to exhibit any specific choice behaviour in which harems they join.