Academics

Sperm Competition

Evans JP, Magurran AE. 1999. Male mating behaviour in sperm competition characteristics under varying sperm competition risk in guppies. Animal Behaviour 58: 1001-1006.

Kelly CD, Godin JGJ, Wright JM. 1999. Geographical variation in multiple paternity within natural populations of the guppy. Proceedings of the Royal Society of London 266(1436): 2403-2408

Pilastro A, Bisazza A. 1999. Insemination efficiency of two alternative male mating tactics in the guppy. Proceedings of the Royal Society of London 266: 1887-1891.

Competition among males in finding mates is prevalent in many animal species. However, this competition does not end after mating, but continues afterwards when the sperm from two or more males compete for the fertilization of eggs. The ultimate goal in studying this topic is to determine the mechanism of sperm competition and how it affects the evolution of male and female traits. A distinction must be made for the definition of sperm competition. One definition follows from the assumption that sperm compete numerically. The other, truer sense of the definition, is the actual competition of sperm from two or more males stored within the females' reproductive tract. Thus, either the physically more "healthy" and viable sperm fertilize the ovum, or the females have evolutionarily adopted a mechanism by which they can choose the sperm from males with "good genes".

Sperm competition in guppies, Poecilia reticulata, is a phenomenon describing the multiple insemination of a female by several males within a population. This concept is supported by the sexual behavior of guppies, the evidence for multiple paternity, females mating with several males, and a high incidence of sneak mating attempts by males (Matthews). In most studies on sperm competition, the method used to measure competition has been stripped sperm count. This involves the measuring and calculating of sperm count per stripped ejaculate. Male guppies are dissected and their sperm bundles were isolated to calculate this number (Evans and Magurran). (The assumption that more sperm produced is the direct advantage in sperm competition is being made here).

In the following articles, the idea of sperm production and male mating behaviors in guppies is presented. Some of the goals behind these studies were to investigate the effects of variation in population structure, insemination success, geographical variation in multiple paternity, and the relationship between male displays and the characteristics of guppy ejaculate.

Since guppies are promiscuous and female choice for colorful males plays a central role in the mating system, competition for the attention of their mate does not just depend on a male's ability to acquire mates. It also depends on other factors such as the number and timing of copulations with the mate and the number of sperm a guppy is able to distribute to each mating (Pilastro). Matthews et al. (1997) indicates that for two populations of guppies, the amount of sperm is positively correlated with display rate. The implications are that it is in the interest of males to advertise the fact that he has a high sperm count and that females may be able to assess ejaculate features via the male's behavior. This suggests females that select males on the basis of courtship rate will be choosing males with higher sperm counts, which is the foundation of the former definition of sperm competition. Studies have recognized that sperm competition effects the evolution of male and female traits in shaping mating systems.

With that in mind, male guppy behaviors were tested to see what the relationship between male displays and the characteristics of their ejaculate reveals (Matthews, Evans, and Magurran-1997). The data presents a strong correlation between sneak attacks per display rate and the number of sperm in female guppies. This result may have the capacity to challenge female choice in guppies. In a different study, Evans and Magurran (April 1999) investigated the effect of variation in population structure. They noted that the number of males is directly proportional to the frequency of sneak attacks. They also observed that with more females in a group, males display more frequently. With this information, the researchers were led to believe that males change their sexual activity as their environment changes with respect to sex ratio.

Evans et al. (1999) does not directly fall into either category of sperm competition. Instead, the researchers attempt to correlate sperm competition with male mating behavior in varying social environments. They have found that it is the sex ratio within a population, rather than density, that influences male sexual behavior. For example, when reared in male-biased groups, males performed more sneaky mating attempts and less courtship displays. Conversely, the opposite pattern of behavior was observed when reared in female-biased groups. Thus a male guppy's sexual behavior is phenotypically plastic in that males respond to differences in sex ratio by adjusting the degree to which they use courtship displays and sneaky mating attempts.

Pilastro and Bisazza (1999) compared the insemination success (i.e. the number of sperm delivered during a single copulation) of male guppies adopting sneaky copulation with that of males obtaining copulation with female consent after courtship. The outcome of their experiment was that on average, the number of sperm delivered after courtship was three times larger than that delivered via sneaky copulation. In addition, they have shown that there is a positive correlation between body size and available sperm in male guppies.

Since females are receptive to males for only two to three days during their breeding cycle and the rest of the time these females are faced with sneaky mating attempts by males, this may result in the transfer of high sperm numbers, thus potentially undermining female choice. Therefore, although less efficient on average in terms of both sperm delivered and frequency of successful copulations over the total number of attempts, sneaky copulations occasionally allow males to deposit large sperm numbers in the female reproductive tract. This follows the assumption that sperm compete numerically.

The study done by Pilastro and Bisazza compares the insemination success in guppies, i.e. the number of sperm delivered during single copulation. From the data collected, they concluded that the larger guppies produced more sperm while the smaller guppies delivered a larger proportion of their sperm. While these studies provide the reader with the knowledge on sperm competition between male guppies, geographic and environmental factors in sperm production provide us with a topic that is controversial.

One of these highly studied environmental factors influencing sperm competition is predation level. Due to increased risk factors in areas of high predation, there are more sneak mating attempts (sexual contact without the consent, or awareness, of the female). In areas of low predation, or low risk, more courtship displays are performed.

In an experiment conducted by Evans and Magurran, the relationship between risk and sperm competition was studied. Five natural populations from Trinidad (two from low-risk sites, two from high-risk sites, and one population consisting of descendants from a high-risk area to that of a low-risk area) were observed. The results showed that male guppies found in low predation environments had a significantly higher sperm count than those in high predation environments. This supports the fact that male guppies displaying in courtship behaviors have more sperm and therefore have a greater advantage in sperm competition.

While it is true that males from the low-risk sites had larger sperm reserves than their high-risk counterparts, this portion of the experiment seems to be redundant. This is because males from the high-risk sites were significantly smaller than those from the low-risk sites, and it is known from the preceding paper by Pilastro et al. (1999) that larger males have larger sperm reserves.

Another study by Kelly, Godin, and Wright, studied the same environmental factor. The results compiled were completely the opposite than those of Evans' and Magurran's. Ten natural populations from varying predation levels in Trinidad were observed in their study. Genetic analysis was done to estimate multiple paternity and genetic diversity in these different populations. The results showed that there was more multiple mating in the high predation populations than in the low predation population. This supports that male guppies performing more sneak attempts have a greater advantage in sperm competition.

Kelly et al. (1999) tested the hypothesis that the relative frequency of females that are inseminated by different males may be higher in populations experiencing strong predation pressure than in populations experiencing weaker predation pressure. They came to the same conclusion as Evans et al. (1999) that male guppies in populations exposed to high predation court females less often and attempt sneaky matings more often than males in populations with a lower risk of predation. According to their findings, the broods of female guppies originating from high-risk sites should be more frequently sired by two or more males than broods of females originating from low-risk sites. Thus they theorize that male sperm competition is more intense in high-risk populations implying that more sneaky mating attempts will contribute to multiple paternity, this is contradictory to Evans et al.'s (1999) paper.

While these studies provide different results, they attempt to explain part of the phenomenon of sperm competition. The hard truth about these topics is that there are more than one given variable controlling the targeted hypothesis. There are also many difficulties in conducting these types of experiments. Two of these difficulties are that sperm stripped artificially from a male guppy may bear no relationship to the amount naturally inseminated (Evans et al. 1999) and males may or may not have control over the quantity of sperm transferred during a single copulation. Also, the success of the sneaky tactic must be tested in natural conditions, where food is not available and predators can impose a cost on receptive females or courting males (Pilastro et al. 1999). Understanding sperm competition is thus an intricate and perplexing task; evolution luckily had time on its side.