Courtship and male ornaments as honest indicators of immune function

Abstract

Having an effective immune system can be very costly, sometimes at the expense of other important life history traits, including reproduction. This tradeoff can be exaggerated in males of species that have costly sexual signaling, where conditiondependent components of the signaling system reflect the health status of the bearer. It is therefore vital for a male to be able to adequately balance the costs of activating the immune system successfully while also expressing high quality sexual signals. We examined males of the brush-legged wolf spider Schizocosa ocreata to see whether static condition-dependent components of sexual signaling in adult males would be indicative of health status (immune stress response), and whether female preference for these traits could be influenced by infection. After experimental ingestion of a bacterial pathogen (Pseudomonas aeruginosa), symmetry of male foreleg tuft (a secondary sexual trait) was found to predict the intensity of the subsequent infection, such that males with more asymmetrical tufts had a higher level of bacteria found in the hemolymph when tested after ingestion. Females were equally likely to mate with both infected and uninfected males in live mating trials, but were found to have bacteria both in their hemolymph and on their body surface as a consequence of mating with infected males. Males that engaged in courtship had significantly lower encapsulation response than naïve males, but among those males that courted, larger tuft size indicated a higher encapsulation response even after energetically costly courtship. These results indicate that females may be able to use static sexual signaling traits to examine a male's overall health, but do not appear to discriminate against males who are actively

Male chemical cues as reliable indicators of infection in the wolf spider Schizocosa ocreata

“…However, these results suggest that females are able to detect a male's current health status via cuticle‐based chemical compounds and are less likely to show receptivity to a male that has consumed a bacterial pathogen recently. This is particularly interesting given that in live mating trials, females do not discriminate against males who are infected with the same pathogen at the same dose (Gilbert & Uetz, ). This could be because females may evaluate sexual signals differently, and can possibly gather important information about male quality from one signaling modality that outweighs information within another signaling modality.…”

“…One week after molting to sexual maturity, males were infected with the bacterial pathogen Pseudomonas aeruginosa strain PA‐14. This pathogen has been used successfully in previous studies to elicit an immune response (Gilbert et al, ; Gilbert & Uetz, ) and has been found in individuals that were sequenced using 16s rRNA primers (R. Gilbert, unpublished data). Stocks were kept at −80°C in Copan Cryovials and grown 18 hr before experiments on Luria broth media (1.0% Tryptone, 0.5% yeast extract, 1.0% NaCl, 1.5% agar).…”

“…Chemical signals have been shown to reflect information about health status and immune function in both vertebrates (López, Amo, & Martín, ; Penn & Potts, ; Reusch, Häberli, Aeschlimann, & Milinski, ; Zala, Potts, & Penn, ) and invertebrates (Ali & Tallamy, ; Rantala, Kortet, Kotiaho, Vainikka, & Suhonen, ; Richard, Holt, & Grozinger, ). This is potentially very important because females can become infected themselves by mating with a male that is infected with a parasite or pathogen (Gilbert & Uetz, ; Milinski & Bakker, ). By being able to avoid coming into contact with infected conspecifics, individuals can behaviorally reduce their risk of infection (Kiesecker, Skelly, Beard, & Preisser, ; Richard et al, ).…”

“…Previous research has shown that male foreleg tufts, a secondary sexual trait, are predictors of both male immune function and therefore the intensity with which a male is infected after being exposed to a bacterial pathogen, since males with a stronger immune system (and larger tufts) will fight off infection more quickly (Gilbert & Uetz, ). However, foreleg tufts are fixed at sexual maturity and will not fluctuate with male health status if infected after sexual maturity.…”

“…However, foreleg tufts are fixed at sexual maturity and will not fluctuate with male health status if infected after sexual maturity. While these static traits do correlate positively with male immune function as an adult and therefore may predict a male's ability to fight off an infection, they are not direct indicators of whether or not a male is actually infected (Gilbert, Karp, & Uetz, ; Gilbert & Uetz, ). A more dynamic trait, such as chemical cues, may be a more reliable cue for females to gather information about a male's current health status, because they can potentially change rapidly to more accurately convey current male condition.…”