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.

He’s Having a Baby!

Image copyright Carin Bondar, 'The Nature of Human Nature'

It’s a common topic of conversation among new (human) mothers:  how our population would cease to exist if males had to bear the children.  The physical costs (including a 9 month gestation, followed by giving birth to a 6-10 pound live young) are extremely daunting and may not be as readily undertaken even if the physiology of the human male permitted it.  It doesn’t end there.  The duties of lactation and child care are generally responsibilities of the female parent, and such tasks involve a great deal of time and energy that could otherwise be spent creating more offspring to represent us in future generations.  Human males seem to have it pretty good: biologically speaking they, like the majority of males in the animal kingdom, contribute little more than genetic material to their offspring.  Although it may at first seem as though males get off easily when it comes to their ability to contribute to future generations, it’s not all fun and games.  Males almost universally compete with each other for sexual partners (and in many cases this leads to the evolution of elaborate physical structures, coloration or behaviors¹).  In addition, the mere contribution of sperm to the reproductive tract of a female does NOT guarantee that a particular males’ seed will be the successful fertilizer (see ‘Artifical Insemination’).  So both males and females have their own difficulties when it comes to procreation, although without question the human female would argue that her male counterpart would not be willing to do the child rearing in her place.  Do any females in the animal kingdom have it figured out a little better than us?

Enter the family Syngnathidae, commonly referred to as pipefishes and seahorses.  These fish are ‘sex-role reversed’, which means that males take on the pregnancy and childcare role and females experience more intense competition for mates.  Females deposit their eggs into a males’ brooding pouch, and he is therefore guaranteed paternity once he fertilizes them².  The brood pouches found in different species are categorized from simple membranous egg compartments on the males’ ventral side to fully enclosed brooding pouches with placenta-like structures³ (aka male bellies).  The ‘pregnant’ males take on the duties of osmoregulating the environment within the brood pouch, aerating the eggs and providing nourishment.  However, females don’t get off without some investment of their own.  They are faced with an interesting conundrum when it comes to competing with each other for mates: egg production is still required (which has a high energetic cost, unlike the metabolically cheap sperm production), so females do not have the same energetic freedom as males do when it comes to producing expensive sexual ornaments.  If such ornaments are produced, they are done so at a potential cost to fecundity, which could make them less attractive to potential mates⁴.  So what can a female sygnathid do to increase her chances to fill a males’ brood pouch?  Although they are not as bizarre as structures seen on males in traditional sex roles, females do develop reproductive ornaments that are used both to attain mates and to deter other females⁴.  In addition to the reproductive ornaments, females busy themselves with attempts at ruining the reproductive efforts of other females.  ‘Mating disruption’ occurs when large females swim in between a male and female pair while they are mating (how rude!), effectively ending the transfer of eggs⁵.  Large females are effective ‘mating disrupters’, and they have also been shown to influence the behavior of smaller females through intimidation.  The mere presence of larger females has been shown to interfere with and substantially decrease reproduction in smaller ones⁵.

So the overall conclusion is this: when it comes to syngnathid fish males are choosy and females are competitive.  Perhaps it’s just the human in me but I feel like all that competing is rather undignified female behavior…it simply isn’t lady-like to disrupt a couple in the throws of passion (or egg transfer)!  Although child rearing and care are difficult jobs (and I maintain that the human population would cease to exist should the sex roles be reversed in our species), I think that it is more empowering to be the one who chooses.  No offence to all you males out there, but if we (females) are going to do all the work to rear and care for your offspring you’d better be prepared to compete for our affections.

¹Darwin, C. (1871). “The descent of man and selection in relation to sex.” Murray, London.

²Jones, A. G., G. Rosenqvist, A. Berglund, and J. C. Avise. 1999. The genetic mating system of a sex-role-reversed pipefish (Syngnathus typhle): a molecular inquiry. Behav. Ecol. Sociobiol. 46:357–365.

³Wilson, A.B., Ahnesjo, I., Vincent, A.C.J. and Meyer, A. 2003. The dynamics of male brooding, mating patterns, and sex roles in pipefishes and seahorses (family Syngnathidae). Evolution 57: 1374-1386.

⁴Berglund, A. and Rosenqvist, G. 2003. Sex role reversal in pipefish. Advances in the study of behavior 32: 131-167.

⁵Berglund, A. 1991. Egg competition in a sex role reversed pipefish: Subdominant females tradereproduction for growth. Evolution 45, 770-774.