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

Abstract

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.

Biology and associated fungi of an emerging bark beetle pest, the sweetgum inscriber Acanthotomicus suncei (Coleoptera: Curculionidae)

“…For Ips acuminatus , the number of females per nuptial chamber was also found to range from 1 to 5 (Bakke, 1968). The sex ratio of Ips and the degree of monogamy versus polygamy may differ depending on environmental conditions (Bakke, 1968; Griffin et al., 2020). We speculate that the sex ratio of SI adults may be affected by both the interior and exterior environmental conditions of host trees.…”

Environment and Local Substrate Availability Effects on Harem Formation in a Polygynous Bark Beetle

“…Hence, females may trade off costs of dispersal with staying at the first log they land on and the increased costs of breeding at high densities [ 19 ]. Previous studies of I. grandicollis have indicated that females choose males at random and do not select on the basis of quality of the male (e.g., male size) or the number of females already associated with the male [ 18 , 48 , 49 ]. Random selection of bore holes may be a consequence of predator avoidance by females by selecting the first available hole that they encounter [ 17 ].…”