Willingness to initiate a fight but not contest behaviour depends on intruder size in Lethrus apterus (Geotrupidae).

In resident-intruder contests, residents are expected to win more often than intruders unless the intruder has significantly higher competitive ability that is often determined by its size. Therefore, small intruders are expected to be less motivated to engage in contests than large ones and intruder size is predicted to have a positive relationship with the duration and escalation of the encounters. In a field experiment we tested these hypotheses in Lethrus apterus, a biparental beetle breeding in underground tunnels, by placing either a small or a large male in front of a tunnel occupied by a resident male. In agreement with our predictions residents won most of the encounters. Small intruders were less willing to engage in a contest and were less successful in their takeover attempts than large intruders. Contrary to many studies however, the duration and escalation (measured by the occurrence and latency of the different contest stages) of the fight in front of the burrow did not differ between the two intruder size categories. These findings suggest that in this species, small and large intruders adjust their prior decisions to their competitive abilities but once a fight started, they behave similarly during the contest.

Male-biased sex ratio does not promote increased sperm competitiveness in the seed beetle, Callosobruchus maculatus.

Sperm competition risk and intensity can select for adaptations that increase male fertilisation success. Evolutionary responses are examined typically by generating increased strength of sexual selection via direct manipulation of female mating rates (by enforcing monandry or polyandry) or by alteration of adult sex ratios. Despite being a model species for sexual selection research, the effect of sexual selection intensity via adult sex-ratio manipulation on male investment strategies has not been investigated in the seed beetle, Callosobruchus maculatus. We imposed 32 generations of experimental evolution on 10 populations of beetles by manipulating adult sex ratio. Contrary to predictions, males evolving in male-biased populations did not increase their testes and accessory gland size. This absence of divergence in ejaculate investment was also reflected in the fact that males from male-biased populations were not more successful in either preventing females from remating, or in competing directly for fertilisations. These populations already demonstrate divergence in mating behaviour and immunity, suggesting sufficient generations have passed to allow divergence in physiological and behavioural traits. We propose several explanations for the absence of divergence in sperm competitiveness among our populations and the pitfalls of using sex ratio manipulation to assess evolutionary responses to sexual selection intensity.