No Eggs? No Problem!
According to our biological mantra, any energy spent either finding, courting and or fornicating with a member of the opposite sex is only justified if such an act is a bona fide attempt at spreading one’s genetic blueprints. Where’s the fun in that? Homo sapiens males engage in copious amounts of sex without the thought of reproduction, and actively seek out partners that are sexually sterilized (see ‘Yes I’m on the pill’) in order to engage in it without the ‘worry’ of the potential side-effects (i.e. offspring). Biologists generally assume that most other boys in the animal kingdom are much more asute than this when it comes to leaving your share of genes in the pool for subsequent generations. Indiscriminate sex should be a rare occurrence due to the fact that energy is wasted on dead-end sex as opposed to being used for other forms of survival (e.g. food gathering or avoiding predators) or reproduction (e.g. courting a viable mate or creating a favorable environment in order to attract one). However, there are always exceptions…and like the human male, there are others who do not necessarily wait for Ms. Right to come along before attempting sexual relations. Animal species that live in both sexual and asexual forms present an interesting conundrum when it comes to mate selection. Females are generally the gender with both sexual and asexual morphs, leaving the males to determine where his sperm will be most usefully spent. However, many are incapable of discriminating between sexually competent or sterile females, meaning that human males aren’t the only ones to discard sperm without regard for its future…
The New Zealand mud snail (Potamopyrgus antipodarum) is a lake-dwelling mollusc whose females can be either sexually reproducing (requiring male ‘input’ for successful embryo production), or asexually reproducing (clonally reproducing without sexual activity). Further, many native populations of this organism are infected with a parasitic trematode that causes castration (sterilization) in females. Hence, males in these populations have several factors acting against their sexual success, leaving them in quite a conundrum when it comes to allocating energy to reproduction. One might imagine that the powers of evolution would have dealt these poor fellas a little help in the mate-discrimination department; however, that doesn’t seem to be the case. Mate choice experiments in which males were given a choice of either a) sexual vs asexual females OR b) healthy vs castrated females revealed that they don’t do a whole lot of discriminating1. Males showed no preferance for viable over non-viable females, appearing instead to simply attempt copulation with whichever females they could find. In this species the average copulation event lasts approximately 2 hours, during which both the male and the female involved in the act are relatively immobilized…leaving them more susceptible to predation. Conclusion: a copulation event represents a fairly large cost to a male if he is mating with an asexual or a sterilized female. So would he do it? Although the possibility exists that there may be an even larger cost to a male (in terms of time and energy lost) if he were to attempt to discriminate between fertile and sterile females, the scientists conducing this study surmise that at some level the male mud snails are engaging in behavior that is simply not contributing positively to their biological fitness in any way1.
Rotifers are tiny freshwater-dwelling organisms that also have two distinct female forms: sexual and asexual. Akin to the mud snail and the human, there are no clear physical differences between sexual and asexual females; although those females that are sexual must be fertilized when they are at a very early age (they are no longer fertile after 9-20 hours of life2). Male rotifers show a distinct preference for fertilizing very young females (2-3 hours old) which slightly improves the liklihood of fertilizing a sexual female, although they do not specifically discriminate between sexual and asexual individuals3. Why don’t the males preferentially select females with the capability to propagate their genetic lineages? They have a short lifespan (approximately 48 hours) and a large-enough supply of sperm so as not to become completely tapped out during this short time (it takes a total of about 13 copulations for him to be spent), drastically decreasing the need to discern between sexual and asexual females. If he had a lower amount of sperm to work with, it may lead to increased selection pressure to find the right girl rather than any girl.
As these examples show, if males cannot distinguish between fertile and sterile females, several of his sexual conquests may be in vain. This could mean big trouble if you are a rotifer or a mud snail: reproduction is as important as survival to any particular individual, and if the chances to do so are impaired then biological fitness is automatically lowered. So where does that leave the Homo sapien? Far from the priorities of our cousins in the animal kingdom, many of ours (with respect to reproduction anyway) have been altered in order to minimize biological fitness. Human males, unlike their snail and rotifer counterparts, actually seek out sterility in a potential partner….and for good reason: could you imagine if each of your own sexual conquests had resulted in offspring? You might have the highest biological fitness of all of your friends, but to the Homo sapien this situation would be far from optimal.
1Neiman, M. and Lively, C.M. 2005. Male New Zealand mud snails (Potamopyrgus antipodarum) persist in copulating with asexual and parasitically castrated females. American Midland Naturalist 154: 88-96.
2Snell, T.W. and Childress, M. 1987. Aging and loss of fertility in male and female Brachionus plicatilis (Rotifera). International Journal of Invertebrate Reproduction and Development 12: 103-110.
3Gomez, A. and Serra, M. 1996. Mate choice in male Brachionus pllicatilis Rotifers. Functional Ecology 10: 681-687.