The acanthocephalan parasite Pomphorhynchus laevis is transmitted by crustaceans such as Gammarus pulex to its paratenic or final hosts, fish. The conspicuous orangeyellow parasite is visible through the transparent cuticle of G. pulex. Infected gammarids are significantly less photophobic than uninfected ones. When hungry three-spined sticklebacks (Gasterosteus aculeatus), one of the hosts of this parasite, were offered equal numbers of uninfected and infected prey, G. pulex infected with P. laevis were eaten significantly more often. We tested experimentally whether parasite color and parasite-induced changes in host behavior affected the predation rate of G. pulex. Color effects were tested with uninfected G. pulex by painting an orange spot on their cuticle that simulated infection. Behavioral effects were tested with infected G. pulex by covering the place through which the orange parasite was visible with inconspicuous brown paint. We showed for the first time that both parasite color and changed intermediate host behavior promote the transmission of P. laevis to its next host. The evolution of orange parasite color, and why sticklebacks do not avoid infected prey, are discussed.
Males of many species produce scent marks and other olfactory signals that function to intimidate rivals and attract females. It has been suggested that scent marks provide an honest, cheat-proof display of an individual's health and condition. Here we report several findings that address this hypothesis in wild-derived house mice (Mus musculus domesticus). (1) We exposed males to female odor, which induces an increase in testosterone, and found that sexual stimulation significantly increased the males' scent-marking and the attractiveness of their scent marks to females. (2) We challenged sexually stimulated males with a nonreplicating strain of bacteria (Salmonella enterica C5TS) to activate immunity and found that this significantly decreased the males' scent-marking and the attractiveness of their scent marks to females. (3) We collected scent marks from infected and sham-infected males when they were sexually stimulated or not, and we found that females could significantly discriminate the scent marks of infected versus control males, but only when the males were sexually stimulated. Taken together, our results indicate that male mice modulate their scent-marking display depending on their health and perceived mating opportunities. Increased scent marking enhances males' attractiveness to females, scent marks provide an honest indicator of bacterial infection (and perhaps immune activation), and females are able to assess the health of males more easily when males mark at a high rate.
There is remarkable diversity in brain size among vertebrates, but surprisingly little is known about how ecological species interactions impact the evolution of brain size. Using guppies, artificially selected for large and small brains, we determined how brain size affects survival under predation threat in a naturalistic environment. We cohoused mixed groups of small- and large-brained individuals in six semi-natural streams with their natural predator, the pike cichlid, and monitored survival in weekly censuses over 5 months. We found that large-brained females had 13.5% higher survival compared to small-brained females, whereas the brain size had no discernible effect on male survival. We suggest that large-brained females have a cognitive advantage that allows them to better evade predation, whereas large-brained males are more colourful, which may counteract any potential benefits of brain size. Our study provides the first experimental evidence that trophic interactions can affect the evolution of brain size.
House mice (Mus musculus) emit ultrasonic vocalizations (USVs), which are surprisingly complex and have features of bird song, but their functions are not well understood. Previous studies have reported mixed evidence on whether there are sex differences in USV emission, though vocalization rate or other features may depend upon whether potential receivers are of the same or opposite sex. We recorded the USVs of wild-derived adult house mice (F1 of wild-caught Mus musculus musculus), and we compared the vocalizations of males and females in response to a stimulus mouse of the same- or opposite-sex. To detect and quantify vocalizations, we used an algorithm that automatically detects USVs (Automatic Mouse Ultrasound Detector or A-MUD). We found high individual variation in USV emission rates (4 to 2083 elements/10 min trial) and a skewed distribution, with most mice (60%) emitting few (≤50) elements. We found no differences in the rates of calling between the sexes overall, but mice of both sexes emitted vocalizations at a higher rate and higher frequencies during opposite- compared to same-sex interactions. We also observed a trend toward higher amplitudes by males when presented with a male compared to a female stimulus. Our results suggest that mice modulate the rate and frequency of vocalizations depending upon the sex of potential receivers.
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