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.

Artificial Insemination – How’s a Girl to Choose?

Image Copyright Carin Bondar 'The Nature of Human Nature'

Upon first consideration it might seem somewhat un-natural for a human female to attend a sperm bank for the purposes of propagating her genetic lineage.  Yes, the natural sex part is removed from the equation; however, when it comes to the selection of a donor she can be choosy with respect to several physical and behavioral characteristics like race, physical health, and even the IQ of the male with the winning seed.  In a perfect world we would all define the most important characteristics for our mates, find mates with said characteristics and procreate in order to obtain offspring with said characteristics.  However, reality in the natural world is harsh, whether you are human or otherwise, and sometimes things just don’t work out optimally.  In organisms where multiple males compete and copulate with a single female (polyandrous sexual system), females are often coerced into sexual activity with males that they wouldn’t otherwise choose (see ‘Not tonight honey, I have a headache’).  What’s a female to do if some un-desirable sperm happens to find its way into her reproductive tract?

Cryptic female choice (CFC) refers to the power of the female to bias sperm use towards that of preferred males, despite the availability of sperm from other (sub-optimal) males.  Females in several species have evolved ways to allow for the sperm of certain males to be the successful fertilizer of the precious eggs, not entirely unlike selecting such seed from a catalogue in a fertility clinic.  For example, female freshwater guppies (Poecilia reticulata) overwhelmingly prefer to mate with males that have bright body coloration, specifically with large orange spots¹.  Do they posess the ability to swing the insemination odds in the favor of a good looking suitor?  It appears that they do.  In laboratory experiments, female guppies were given a choice to mate with an intermediately colored male in two situations: 1) when he was the more attractive candidate (i.e. when he was paired with a dull colored indivudial), and 2) when he was the less attractive candidate (i.e. when he was paired with a very brightly colored individual).  In both cases the only male that had access to the female was the intermediately colored one, the comparative indivudials were visible by the female but not accessible.  The results were clear:  the intermediately colored males inseminated 68% more sperm into females when they were perceived as the more attractive candidate¹.  The mechanism by which this happens is as yet unclear, but there is no question that females exercised some control over the number of sperm that were successfully transferred to her reproductive tract subsequent to a copulation event.  If she mated with an attractive male she kept more of his sperm, simple as that.

Another example of females manipulating the insemination success of various types of sperm comes from the feral fowl Gallus gallus domesticus (aka wild chickens).  These organisms have a complex social system, with males being in an intricate hierarchy of social dominance.  Females prefer to copulate with dominant males (not with subordinate ones); however, the underdogs still undertake copulations, often violently coercing the female in order to do so.  Fortunately, the females have been found to get the last laugh: analysis of the fertilization success of dominant vs subordinate males showed that females eject the ejaculates of the latter subsequent to copulation².  So although the subordinate males utilize their strength to force copulations upon unwilling females, their chances at paternity are limited by the fact that she can subsequently discard his donation in favor of one that she actively seeks out.

In the natural world there is an abundance of examples of females biasing paternity in favor of specific male phenotypes or social ranks, kind of like a human female in a sperm bank selecting the seed of a successful entrepreneur over an unemployed couch surfer.  However, the major difference lies in the fact that in the natural world females are capable of undertaking such selection without the intervention of human-invented fertility procedures.  Even the lowly female chicken (who has proven to be more than just the ‘dumb’ animal we eat for dinner) displays a level of sophistication that seems unattainable for the Homo sapien.  In species where coersion is commonplace (and I would argue that our species is no exception), it is extremely advantageous for females to employ mechanisms to avoid having offspring that are fathered by undesirable sperm.  If that means making a well-informed decision after perusing a brochure from a sperm bank over a hasty choice after a few drinks at a night club, I’ll vote for the former.

¹Pilastro, A., Simonato, M., Bisazza, A. and Evans, J.P. 2004. Cryptic female preference for colorful males in guppies.  Evolution 58: 665-669.

²Pizzari, T. and Birkhead, T.R. 2000. Female feral fowl eject sperm of subdominant males.  Nature 405: 787-789.

The Crowded Buffet: Wait or Settle?

Image copyright Carin Bondar 'The Nature of Human Nature'

I’m not a huge fan of the ‘all you can eat buffet’.  I find it akin to a bunch of humans pulled up to the feeding trough plowing through as much as they can as though their lives depend on it.  It’s the crowding that I don’t like, the lineup of people at the prime-rib station, drooling as their cut of meat is hefted onto their overstuffed plates.  I think that my behavior at the buffet is directly correlated to the number of people are lingering around a specific area.  If I had the place all to myself, I would be more inclined to hit the hot ticket items; however, when it is busy and the best parts have been completely picked over it is probably best to explore the other available options.  Optimal foraging theory (OPT) predicts that when there is intense competition for preferred resources, organisms should increase their diet breadth to include other (less optimal) items¹.  In this way biological fitness is maximized by striking a balance between obtaining food and the amount of time and energy required to do so.  Prime rib becomes less valuable if there is a 20 minute wait attached to it.  I am in complete agreement with OPT on this one.  Instead of waiting for a meat slab or fighting over crab legs, I’d rather eat something that may be less ‘valuable’ but is all mine.

A field full of flowering plants can be thought of as an ‘all you can eat buffet’ for pollinating organisms.  Invertebrates from butterflies to bees can indulge on a plethora of plant items that are only too happy to share their wares (ingestion by pollinators = pollination = reproductive success of the plant involved).  Many plants have evolved specialized coloration, morphologies and scents in order to make themselves more attractive to potential pollinators, not entirely unlike the garnishes, scents and presentations of the various foodstuffs available at our buffets.  But what happens when the natural buffet becomes crowded?  Do pollinators wait in line for their chance at the hot ticket flowers or do they follow the tenets laid out by OPT and forage on something a little less exciting?  In an attempt to answer this question, laboratory experiments were conducted to assess the food choices made by the common bumblebee (Bombus terrestris) in crowded and non-crowded environments².  Artificial plant communities were created and comprised of a variety of species that included both high rewarding (i.e. prime rib) and poorly rewarding (i.e. peas and corn) types.  Individually marked bees were followed in two situations: with only one other conspecific present (low density), or with 6 conspecifics present (high density).  The number of visits made by the marked bees to each type of plant was recorded in each situation.  True to the predictions of OPT, the diet breadth of individual bumblebees was increased when the buffet was crowded.  Low-rewarding plant species that were visited only 6% of the time in the low-density treatment were visited 32% of the time in the high density situation, indicating that not all of the bees were willing to compete for the prime rib.  Interestingly, the diet of the bees was most specialized when the buffet was not crowded (i.e. exclusively high quality foods were selected).  Although this may be an optimal situation for the individual bee involved, it doesn’t help to maintain the diversity of the items available at the buffet.  In this context an increased level of competition may actually work to preserve the biodiversity of the plant community by forcing other (non-popular) plant species to become pollinated as well.

The Pollinator Buffet

Many plant-pollinator interactions are opportunistic³, meaning that the interactions can vary through space and time and have the effect of maintaining the integrity of the system.  It may be advantageous for a certain pollinating insect to feed on a particular food type at a specific time, but that insect maintains the ability to feed on other food sources if necessary.  The key is to have the diversity to be able to withstand temporary alterations in conditions.  When something like mad cow disease rears its ugly head and the popularity of the prime rib takes a nose dive, the buffet must be able to compensate by continuing to offer a variety of other things.  Although like the individual bees, individual humans might be inclined to indulge in a single hot-ticket item if such an opportunity exists; this strategy isn’t optimal for the overall maintenance of the buffet, natural or otherwise.  In addition, it isn’t optimal for maximizing biological fitness because organisms should be able to compensate for uncontrollable changes to their food supply.  If the ability to do this is lost, the quest to obtain an adequate amount of nutrition becomes a lot harder.  The overall message: a little crowding is beneficial to everyone.  Despite the fact that there is only a tiny spoonful of peas and corn on your overloaded plate, you’d probably miss them if they were gone.

¹MacArthur, R.H. and Pianka, E.R. 1966. On optimal use of patchy environment. The American Naturalist 100: 603–609.

²Fontaine, C., Collin, C.L. and Dajoz, I. 2008. Generalist foraging of pollinators: diet expansion at high density.  Journal of Ecology 96: 1002-1010.

³Alarcon, R., Waser, N.M. and Ollerton, J. 2008. Year to year variation in the topology of a plant-pollinator interaction network. Oikos 117: 1796-1807.