PhD Research - University of Toronto at Scarborough
Supervisor: Dr. Maydianne Andrade
Title: Female reproductive strategies in the Australian redback spider
Abstract - Sexual selection arises due to variation in the reproductive success of individuals arising from male-male competition and female choice. Studies that isolate one of these two mechanisms mask a critical aspect of sexual selection by overlooking the interaction between the two and their effect on net reproductive success. My thesis focuses on the interaction between male-male competition and female choice and how this shapes traits important to sexual selection in the Australian redback spider
(Latrodectus hasselti ). First, I show that males discriminate female maturity and mating status based exclusively on web-borne chemicals. Extracts from webs of virgin females triggered elevated male activity but this was not the case for webs of juveniles, non-virgin females and solvent controls. I also show that pheromone production ceases rapidly after females mate. This suggests there is a limited window for inter-male competition, tightly focused around the arrival of the first male on a virgin female's web, so courtship and competition would often occur simultaneously. Given this context, I investigated male reproductive tactics and their outcomes. To do this, it was first critical to discern how females make mating decisions. Theory suggests two broad categories of female decision rules. In threshold-based decisions a given male is selected as long as he exceeds the female's internal standard, regardless of whether another male of higher quality is available. This differs from sample-based rules in which females mate with the best of a series of sampled males. I hypothesized that female choice in redback spiders is largely based on a courtship duration threshold, and this might explain why courtship typically lasts for 5 hours. I used previously published data to estimate that the female's threshold is approximately 100 minutes of courtship. I supported my threshold hypothesis by showing that, whereas males who court for shorter periods are penalized by females (killed before mating is complete, which decreases paternity), this does not occur when courtship is prolonged past the threshold. This may often lead to females mating with higher quality males as courtship can decrease male survival and I use respirometry to show that it entails substantial metabolic cost. I then asked whether females discriminate the source of courtship when rival males compete, as this would determine the effect of the threshold on mating outcomes. I staged competitions where ‘resident' males courted females and exceeded the courtship threshold before ‘intruding' males were introduced. Intruding males mated rapidly but were not penalized by females, in contrast to cases where competition starts before the threshold is surpassed. This suggests females do not distinguish which male reaches the threshold, allowing intruders to parasitize the courtship efforts of residents. Such exploitation of mating effort by rival males mediated by a female choice threshold has not been demonstrated elsewhere. Ironically, this female choice threshold and the attendant possibility of courtship parasitism may lead to selection for lower quality males to recognize and seek out (rather than avoid) webs in which competitors are already present. This raises interesting questions about the benefits to females of having such a threshold-based decision rule. This research highlights how the interplay of male-male competition and female choice can affect sexual selection on male traits, including selection for alternative mating tactics that can ultimately circumvent choice.
MSc Research - University of Western Ontario
Supervisor: Dr. Bryan Neff
Title: Sperm competition and mechanisms of fertilization success
Abstract - In the first data chapter of my thesis, I dissected the mechanisms used during sperm competition for fertilization success by males with alternative mating tactics in the bluegill sunfish. The difference in competitiveness during fertilization of parental males and cuckolder males has been suggested to be due to differences in position or sperm number released (Leach & Montgomerie 2000; Fu et al . 2001; Neff et al . 2003). I hypothesized that there would be differences in position or sperm number released between parental and cuckolder males. I predicted that cuckolder males should obtain a position closer to the female and/or release more sperm than parental males during spawning. The reason for my prediction was that cuckolders fertilize the majority of eggs released by females when in competition with parental males (Fu et al . 2001). Because the probability of fertilization increases with proximity to eggs in many fishes (Foote et al . 1997; Mjølnerød et al 1998; Blanchfield & Ridgway 1999; Blanchfield et al . 2003), cuckolders should be closer to the female at egg release than the parental.
In the 2nd data chapter of my thesis, I determined how ecological and phenotypic attributes (specifically body size and peripheral vegetation) affect the proximity that males using alternative mating tactics can obtain to females during spawning. In this chapter I also investigated how ecological and phenotypic attributes affect the level of sperm competition in the bluegill sunfish. I hypothesized that there would be differences in the proximity to females obtained by males with alternative mating tactics based upon body size and peripheral vegetation. A sneaker's body length should be positively related to their closest proximity to females because smaller sneakers should be more cryptic and less detectable by parentals than larger sneakers. Alternatively, I predicted that satellite body length should be negatively related to their closest proximity to females because satellites are at the lower end of the mature female size distribution (Gross 1982). Thus, larger satellites may better mimic females and enable them to get closer to the female. For parentals, I predicted that body length would have no relationship with proximity to females during egg release when spawning alone with females because the parental and female should attempt to maximize fertilization by positioning themselves as close as possible regardless of the parental's size. Additionally, I hypothesized that there should be differences in the level of sperm competition based upon the amount of peripheral vegetation between males with alternative mating tactics. I predicted that there would be a positive relationship between peripheral vegetation and sneaker streak rate and no relationship for satellites. Sneakers use the peripheral vegetation to access the parental's nest, whereas satellites do not. Sneakers should be able to use more peripheral cover to covertly position themselves on the periphery of the nest and therefore increased peripheral vegetation should have higher streak rates.
In the third data chapter of my thesis, I investigated if a commonly used measure in fishes for the investment into sperm production is appropriate. Gonad size is a male morphological trait that appears to show a consistent adaptation to the level of sperm competition, with a positive relationship between gonad size and the level of sperm competition having been documented in a variety of taxa (Harcourt et al . 1981; Møller & Briskie 1995; Jennions & Passmore 1993; Gage 1994; Stockley et al . 1997). An increase in the level of sperm competition should lead to an increase in the relative investment into sperm production (Parker 1990). In fishes, relative investment into sperm production is commonly measured during the breeding season using the gonadal somatic index (GSI) (reviewed by Taborsky 1998). GSI is calculated by dividing gonad mass by total body mass, expressed as a percent. However, Tomkins & Simmons (2001) have recently criticized GSI because it only controls for differences in body size if there is an isometric relationship between gonad mass and body mass. I tested whether or not an allometric relationship exists between gonad mass and body mass in fishes. I calculated the scaling exponent from 23 different fish species and compare species means, individuals within species, and individuals within mating tactics (bourgeois or parasitic).
Hon BSc Research - University of Toronto at Scarborough
Supervisor: Dr. Maydianne Andrade
Title: Male-male competition in the Australian redback spider
Abstract - Despite widespread recognition that inter-sexual interactions shape reproductive strategies, studies of male competition do not typically include effects imposed by females. In cannibalistic redback spiders, escalated fighting between rival suitors is predicted since males are unlikely to mate with more than one female, and strong first-male sperm precedence favours mating with virgins. In staged competitions for matings between size mis-matched rivals, smaller males adopted an alternative, sneaking strategy. However, despite initial agonistic interactions, larger males did not pursue or incapacitate smaller males. When inter-male competition occurred, it was suppressed by f emales, which struck at males frequently, although strikes were rarely seen when males courted in the absence of a rival. After minimal fighting, larger males engaged in significant courtship (3 hours) rather than killing inferior rivals. Prolonged courtship was also favoured by female behaviour as males that attempted rapid copulation (smaller, sneaking males) were cannibalized before mating was completed. This premature cannibalism significantly decreases paternity. Thus, significant features of male competitive behaviour are predicted only with consideration of female response to male reproductive strategies. Although the effect of females may be more subtle in systems without the extreme reversed size-dimorphism of redbacks, these results strongly suggest female interests should be explicitly considered when studying inter-male interactions.
©2006