Courtship pheromones

The chemoreceptive mechanisms in amphibia are undoubtedly worthy of further analysis, not only for their own sake, but to provide clues as to the origination of advanced chemosignal systems. As noted above, a pheromonal signal from the mental gland acts as a courtship/ receptivity inducer. The plethodontid receptivity factor (PRF) (Chap. 3) despite its size (22 kD), seems to have been converted from its internal role as an inter-cellular cytokine, to an inter-individual coordinator of reproductive activity (Rollmann et al., 1999). Endocrine or... 

Feldhoff R.C., Rollman S.M. and Houck L.D. (1999). Chemical analysis of courtship pheromones in a Plethodontid Salamander. In Advances in Chemical Signals in Vertebrates (Johnston R.E., Miiller-Schwarze D. and Sorenson P., eds.). Kluwer, New York, pp. 117-126. 

Some male arctiid moths produce their courtship pheromone from dietary pyrrolizidine alkaloids acquired during feeding by the larvae [ 126]. Conversion of monocrotaline to hydroxydanaidal by males is accomplished by aromatiza-tion, ester hydrolysis and oxidation of an alcohol to the aldehyde [7]. In the case of Utetheisa ornatirx the stereo-configuration at C7 of the dietary alkaloid is the same as the pheromone released (R). In contrast, another arctiid, Creatono-tos transiens, can convert a dietary precursor alkaloid with the (S) configuration at C7 (heliotrine) to (l )-hydroxydanaidal. The biosynthesis occurs by first oxidation-reduction at C7 to convert the stereochemistry and then proceeds through aromatization, hydrolysis, and oxidation [7]. 

Pheromones are used as sex attractants in courtship, warning substances, or aggregation compounds (to cause members of their species to congregate). 

The function of the male courtship pheromone in plethodontids was first documented in a study of Desmognathus ocoee Houck and Reagan (1990) showed that the diffusion delivery of a crude extract from the male pheromone gland increased... 

After 30 min, a deglanded male was added to each courtship box, and a combination of scan and focal sampling was used to record all occurrences of courtship behaviours. These behaviours included the time of the first occurrence of the male orienting to and physically contacting the female, and the time when the male completed spermatophore deposition. The same male-female pair was observed on a subsequent night when the female received a treatment (saline control or pheromone extract) that was different from the treatment she received on an earlier trial. In this way, each male-female pair was its own control. On each trial night, equal numbers of females were treated with each solution. 

Courtship duration was defined as the time from first tactile contact (typically the male contacting the female) until spermatophore deposition was completed. We selected spermatophore deposition as the end of courtship because (a) this is a distinct behaviour that any observer can score, (b) the time between spermatophore deposition and insemination typically is only 1-2 min, and (c) some females do not get inseminated (e.g., walk past the spermatophore) and so the end of courtship would be ambiguous for these females. In addition, treatment (pheromone or saline) was not correlated with insemination success for D. ocoee, given that a pair reached the spermatophore deposition stage (Houck, unpublished data). 

Twenty male-female pairs each mated on two different trial nights, once with a pheromone treatment (the 20-25 kDa fraction of the male courtship pheromone) and once with a saline treatment (n = 20 paired values). The duration of courtship was reduced significantly when pairs were treated with the pheromone fraction vs the saline control (t = 1.73, df= 19, P < 0.02). For these 20 pairs, the average courtship duration was 54 min when treated with the pheromone fraction, and 70 min when treated with the saline control. Thus, the control treatment resulted in courtships that were approximately 30% longer. 

In the plethodontid salamander D. ocoee, courtship duration was reduced for male-female pairs in which the female received a protein signal from the 20-25 kDa fraction of the male courtship pheromone. We interpret this reduction in courtship duration as an increase in receptivity for females receiving the pheromone. 

The delivery of male courtship pheromones is widespread among plethodontid salamanders (Houck and Arnold 2003), and other courtship pheromones are being discovered for this group (Houck, Palmer, Watts, Arnold, Feldhoff and Feldhoff 2007). The mode by which these pheromones are transferred to the female apparently has been modified from delivery via diffusion into the circulatory system to delivery that directly stimulates vomeronasal receptors (Fig. 20.1 Houck and Sever 1994 Watts et al. 2004 Palmer et al. 2005 Palmer et al. 2007). The behavior patterns and morphologies associated with these two delivery modes often remain static for millions of years. In contrast, evolution at the level of pheromone signals is apparently an incessant process that continuously alters the protein sequence and composition of pheromones both within and among species (Watts et al. 2004 Palmer et al. 2005 Palmer et al. 2007). 

Houck, L.D. 1986. The evolution of salamander courtship pheromones. In D. Duvall, D. MiHler-Schwarze and R.M. Silverstein. (Eds.) Chemical Signals in Vertebrates, Vol. IV Ecology, Evolution, and Comparative Biology. Plenum Press, New York. Pp. 173-190. 

Houck, L.D., Bell, A.M., Reagan-Wallin, N.L. and Feldhoff, R.C. (1998) Effects of experimental delivery of male courtship pheromones on the timing of courtship in a terrestrial salamander, Plethodon jordani (Caudata Plethodontidae). Copeia 1998, 214-219. 

Houck, L.D. and Reagan, N.L. (1990) male courtship pheromones increase female receptivity in a plethodontid salamander. Anim. Behav. 39, 729-734. 

Palmer, C., Watts, R.A., Gregg, R., McCall, M., Houck, L.D., Highton, R. and Arnold, S J. (2005) Lineage-specific differences in evolutionary mode in a salamander courtship pheromone. Mol. Biol. Evol. 22, 2243-2256. 

Rollmann, S.M., Houck, L.D. and Feldhoff, R.C. (2000) Population variation in salamander courtship pheromones. J. Chem. Ecol. 26, 2713-2724. 

Courtship pheromones are not necessarily species specific. Pairs of the woodland salamander, Plethodon shermani, courted for an equally long time (about 35 to 50 minutes) whether male pheromone from the mental gland of conspecifics or the allopatric species P. montanus or P. yonahlosscc was present, even though the composition of the proteinacous pheromones (plethodontid receptivity factor of these three species differ considerably (Rollmann et al, 2003). 

Chemical cues are important to advertise one s sex and to attract the opposite sex as the first step in sexual behavior. Other functions of sexual signals are to signal current sexual status and to alter the behavior of the potential partner(s) via courtship or scent marking to facilitate mating. Typically, the odor of the opposite sex is attractive, at least in the breeding season. (Priming pheromones are covered separately in Chapter 8.)... 

---