Effects of male size and mate quality on male-male contest in the hermit crab <i>Pagurus filholi</i>

Abstract

Abstract:Males of the hermit crab Pagurus filholi guard females until spawning, and male-male contest often occur when guarding males encounter other males. A contest comprises two phases, initial contact and physical combat, and larger males typically win. We conducted male-male contest experiments to examine whether body sizes of both sexes of P. filholi and female receptivity affect whether and when a contest escalates to physical combat, as well as contest duration and outcome. For each experimental trial, a pair, comprising a pairing male and a pairing female , and a second male intruder were placed into a small container, and their behaviors were observed for 15 minutes. Each intruder had been guarding a female in the field but we removed this female removed female just before the trial. All pairs in the field were reconstructed each after trial to quantify female receptivity. This was done by checking each female for spawning every day. As an index of size, we measured shield length length of calcified anterior portion of cephalothorax of all crabs. In the trials, large size of the intruder and high receptivity of the removed female significantly shortened the time until the onset of physical combat. Intruders needed more time to take over the pairing females and were less likely to win the contests when the intruders were small, the pairing males were large, and the removed females were large. Body size and receptivity of the pairing female had no effect on the process and the outcome of male-male contest. These results suggest that a male P. filholi can assess the quality of female it is guarding and remember its assessment for at least a short period. Asymmetry between contestants in this memory store for female quality could affect the process and the outcome of male-male contests in P. filholi.

Reason for being single: some males do not guard receptive females in the hermit crab Pagurus filholi

“…However, in P. filholi various exceptions occur. Some males do not guard receptive females that have been guarded by other males in the field (Tanikawa, Yasuda, Suzuki, & Wada, 2012;see Results). Intruders of this species stopped competing for females in some trials of a maleemale contest experiment even when they were intact and larger than the guarding males (Tanikawa et al, 2012; see https:// www.youtube.com/watch?v¼ygA1Cj4Y4Kw for details of behaviour).…”

“…Intruders of this species stopped competing for females in some trials of a maleemale contest experiment even when they were intact and larger than the guarding males (Tanikawa et al, 2012; see https:// www.youtube.com/watch?v¼ygA1Cj4Y4Kw for details of behaviour). Furthermore, large males sometimes stop guarding females just after they have succeeded in taking over the females of their opponents (Tanikawa et al, 2012; see https://www.youtube.com/ watch?v¼rKnvtFF9QBk for details of behaviour).…”

“…In the laboratory experiment of Tanikawa et al (2012), males were removed from guarding females, and just after the removal the males were used as intruders in the experiment. This manipulation may cause an artificial 'predatory threat' or 'losing experience' in the males.…”

Rapid regeneration of the major cheliped in relation to its function in male-male contests in the hermit crab Pagurus middendorffii

“…This period of observation is the same duration as in other studies dealing with malemale contests in Pagurus hermit crabs (P. nigrofascia; Yasuda et al 2011P. filholi;Tanikawa et al 2012). It was then recorded whether or not the interaction escalated into a contest (initiating physical combat, N=47) and which contestant (intruder or owner) won the contest after escalation, based on which male guarded the female (N=33).…”

“…They possess a large right (major) cheliped, and it is considered to be a sexually selected weapon for males (Yasuda et al 2011. In the reproductive season, male pagurid hermit crabs directly compete for mates during precopulatory guarding (Hazlett 1968a, Wada et al 1999, Tanikawa et al 2012, when the male grasps the aperture of the gastropod shell occupied by a sexually mature female with his left (minor) cheliped over several days. Males use their major cheliped as defensive and offensive weapons during physical combat in male-male contests, and the major cheliped size strongly affects the probability of winning in P. nigrofascia (Komai) (Yasuda et al 2011) and P. middendorffii (Brandt) .…”