In species with both male-male competition and male mate choice, inferior males may make different mate choice decisions from superior males. Males of the intertidal hermit crab, Pagurus middendorffii, are known to conduct precopulatory guarding and to adjust the threshold for guarding according to social parameters, such as encounter rate with females, competitor size and sex ratio. Larger males are stronger in male-male competition during guarding in this species. We here tested whether male of P. middendorffii initially guarding a smaller female exchanged partners when the male encountered a larger receptive female, and whether large and small males chose potential mates on the basis of body size and/or time needed for guarding when a male simultaneously encounters two females. When a male guarding a smaller receptive female encountered a larger receptive female, the male assessed the larger female and exchanged his partner only in cases of a large difference in body size between the two females, suggesting that males of this species could choose their mates based on female quality even during guarding. When a male simultaneously encountered two receptive females, small males showed the prudent mate choice by balancing female traits between larger body size and shorter time until breeding, while large males showed preference for larger females. The distinct preference exhibited by males of different size classes is concluded to be an adaptive response to the size-dependent risk of losing the female during guarding.
Male mate choice has recently been reported in some animals with male-male competition. In the laboratory, we examined whether males choose their mates based on female quality that was indicated by body size and/or days to prenuptial molt, and the effects of female quality on malemale competition in the hermit crab Pagurus nigrofascia. We collected samples from April to May 2009 at an intertidal shore in Hokkaido, Japan (41°N, 140°E). When a male simultaneously encountered two receptive females in the mate choice experiment, males chose females which require less time to molt. When a male guarding a female with less time to molt was challenged by an intruder, the guarding male defended the female for a longer period and was more likely to win the contest. These results indicate that male P. nigrofascia use time to molt to discriminate between females.3
Errors in decision-making in animals can be partially explained by adaptive evolution, and error management theory explains that cognitive biases result from the asymmetric costs of false-positive and false-negative errors. Error rates that result from the cognitive bias may differ between sexes. In addition, females are expected to have higher feeding rates than males because of the high energy requirements of gamete production. Thus, females may suffer relatively larger costs from false-negative errors (i.e. non-feeding) than males, and female decisions would be biased to reduce these costs if the costs of false-positive errors are not as high.Females would consequently overestimate their capacity in relation to the probability of predation success. We tested this hypothesis using the Japanese pygmy squid Idiosepius paradoxus. Our results show that size differences between the squid and prey shrimp affected predatory attacks, and that predatory attacks succeeded more often when the predator was relatively larger than the prey. Nevertheless, compared to male predatory attacks, female squid frequently attacked even if their size was relatively small compared to the prey, suggesting that the females overestimated their probability of success. However, if the females failed in the first attack, they subsequently adjusted their attack threshold: squid did not attack again if the prey size was relatively larger. These results suggest a sex-specific cognitive bias, that is females skewed judgment in decisionmaking for the first predation attack, but they also show that squid can modify their threshold to determine whether they should attack in subsequent encounters.
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