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.

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.

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.

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

First born offspring have it pretty good.  Mom is generally healthy and well-rested (having no other children to look after during her pregnancy), and the newly emerged micro-human has the exclusive attention of both parents and quite likely some grandparents as well.  The first born doesn’t have to share the resources of his/her parents like second and third children do.  Being a mother of three children, I think about this quite often.  I feel kind of bad for my youngest child, who gets dragged around to preschool, music lessons, play-dates and the other endeavors of his older siblings.  He inevitably gets his toys snatched away or his snacks devoured before he can get to them…and really there’s not a whole lot that can be done to change that.  A major difference between humans and many other animals with respect to sibling competition is this: first born offspring often attain independence before further offspring are born, leaving the parents to care for only one infant at a time.  This is certainly not the case in our species, where offspring remain dependent for too long for mothers to wait for independence between them (our physiology just won’t allow for 18 years between children!).  Instead, sacrifices are made and a lower amount of parental care is provided to all siblings.  So why not just have one offspring and avoid the need to sacrifice any kind of parental commitment?  This could be risky…if all reproductive effort is placed on a single offspring, and (for whatever reason) said offspring fails to reproduce, your biological fitness is doomed.  The ‘bet-hedging’ strategy¹ has been coined to account for uncertain conditions in the future: there are clear advantages to having more offspring but providing a lower amount of parental care to each.

Other (non Homo sapiens) mammals face a similar conundrum when it comes to repeat child rearing when the first offspring is not yet independent.  Fur seal and sea lion females rear a single offspring at a time, and nurse it exclusively for a period of 2-3 years.  However, many females give birth to another offspring during this nursing phase.  A long term study of Galapagos fur seals (Arctocephalus galapagoensis) and sea lions (Zalophus wollebaeki) addressed the potential conflict between siblings competing for their mothers’ resources, and also the conflict between mothers and offspring (i.e. which offspring should she nurse?  In what situations should she choose the juvenile over the newborn and vice versa?²).  Between-sibling conflict was found to be especially strong when resource levels were low, and the older sibling was unable to forage effectively on its own (away from the supplementation of the mother’s milk).  During El Nino years, (resulting in low ocean productivity) a high level of between sibling conflict was documented, which often resulted in the continued nursing of the older juvenile and the death (by starvation) of the newborn.  However, during periods of high resource availability mothers aggressively defended their newborns against the juveniles’ attempts to nurse.  You might find yourself wondering: why should a female continue to reproduce when there is a chance for her newborn to starve or when her juvenile offspring is not ‘ready’ to stop nursing?  The fluctuating conditions of the Galapagos make it impossible for the adult females to predict what is going to happen, making a bet hedging strategy important.  If food resources are plentiful when a new pup is born, it makes sense for the mother should defend the younger offspring from aggressive attacks from the older one (i.e. get off my boob and go forage for yourself, there’s plenty of food out there).    However, if food resources are scarce, the mother is expected to allow the intimidation (and starvation) of the younger sibling since the older one has already received more of her resources and is more likely to survive to adulthood.  Although the death of a pup is a high price to pay for a miscalculation in available resources, the mother still gets one healthy offspring when times are bad and has the potential for more when times are good.

The Galapagos study shows that bet hedging is an effective reproductive strategy when environmental conditions and resource availablity are unpredictable.  Over the course of her reproductive lifetime, a female seal or sea lion can maximize her overall fitness by bet hedging rather than by waiting for favorable conditions.  Humans on the other hand, have taken a lot of the unpredictability out of our ability to attain resources, so our need for bet hedging is reduced.  While an El Nino year might affect the availability of jack tuna in the fresh seafood section of the grocery store, it really has no bearing on whether there will be enough food to go around.  Although at times my youngest child has to fight a little harder for his share of resources, starvation is out of the question.  Taking the nature out of foraging has its advantages.

¹Mock, D.W. and Forbes, S. 1995. The evolution of parental optimism.  Trends in Ecology and Evolution 7:409-413.

²Trillmich, F. and Wolf, J.B.W. 2008. Parent-offspring and sibling conflict in Galapagos fur seals and sea lions.  Behavioral Ecology and Sociobiology 62:363-375.

Image Copyright Carin Bondar 'The Nature of Human Nature'

As parents, most of us vow to keep things equal between our offspring.  In theory there should be an equal amount of time investment into Suzie’s piano lessons as there is into Bobby’s tennis practice.  However, in reality these things are never quite equal are they?  Bobby’s tennis practice is across town, making the investment into his achievements larger than when Suzie walks over to the lady down the street for her piano lesson.  Next, the inevitable:  Suzie hates piano but Bobby looks as though he’s heading straight to the Olympic trials with his perfected backswing.  Resource allocation into the two offspring is suddenly a far cry from 50:50.  As humans we may say that we make equal investment into each of our offspring, this is an almost impossible task in the animal kingdom.  An additional complicating factor is this:  what about offspring with different fathers?  Perhaps Bobby was destined to be a champion based on the fact that his biological father is an admirable member of society and a physically fit athlete himself, but poor little Suzie was the outcome of a one night stand after a few too many cocktails with a character whose name you’d rather erase from memory.  Life history theory predicts that maternal investment into offspring should reflect the likelihood of said offspring to contribute to the fitness of said mother…which could be bad news for Suzie.

Many species in the animal kingdom mate with different partners in different mating seasons.  Based on a number of environmental, physical and random factors a female may not always end up with the mate that she most desires.  However, offspring are the inevitable result and so an interesting conundrum presents itself:  invest equally in all offspring despite the sub-optimal paternity of some?  Biologically speaking the answer should be a resounding no!  According to the ‘differential allocation hypothesis¹’, females of many species do not contribute equally to the health and well being of offspring sired by fathers of different quality.  In experimental mating research on mallard ducks (Anas platyrhynchos), researchers paired individual females with both high ranking and low ranking males in order to investigate whether they would invest differently in their offspring based on the identity of the father².  Egg size is entirely determined by the female, and is a critical trait influencing fitness in birds: larger eggs produce larger chicks that have an increased chance of surviving to adulthood.  Sure enough, it was demonstrated that eggs are significantly larger when females mate with a high ranking male.  Invest equally in all offspring?  I think not!

Blue footed booby (Sula nebouxii) males have a clear signal to show females that they are highly fit and ready to sire the next generation: their feet.  Female boobies prefer males with bright blue-green feet, and will actively discriminate against males without this characteristic.  Once copulation has occurred, the female generally lays two eggs, the second approximately 4 days after the first.  Both parents incubate the clutch and raise the young together until they are ready to leave the nest.   In an experiment designed to assess differential allocation hypothesis, researchers painted the feet of the male partner to a dull (unattractive) color just after the first egg was laid³.  As a result of this change in the males’ foot morphology, the second egg laid by the female partner had a lower volume as well as a lower hormonal content.  So perceptive is the female blue footed booby that within a few days of her mate quality being ‘lowered’, she reacts accordingly in her egg investment!

The effects of partner ‘attractiveness’ do not end at the physiological stage of egg-laying.  It has been shown for Zebra finches (Taeniopygia guttata) that the amount of post-natal parental care (by both mothers and fathers) varies with partner quality as well.  Parental care activities like nest maintenance, watching over young offspring, and time spent brooding, feeding and grooming them are generally performed by both Zebra finch parents; however, levels of all aforementioned activities are lower when a partner has an unattractive mate⁴.  It’s as if parents are reluctant to place all of their reproductive resources into a set of offspring that may have mediocre genes…tough luck for said offspring!

I suppose most human parents would argue against application of the differential allocation hypothesis in our species….and in many cases this would be with just cause.  Most Homo sapiens do an admirable job of investing equally in their offspring. There are many parents out there raising children that are not only born of ‘unattractive’ parents, they are being raised by parents that are not even biologically involved (adoption/foster parenting).  Despite the fact that at times Bobby’s tennis talents may overshadow the accomplishments of his sister, as a human offspring Suzie is quite likely to be well provided for.  Perhaps the animal kingdom could learn a thing or two about the importance of the nurture part of ‘nature vs nurture’ argument.

¹ Burley N. 1986. Sexual selection for aesthetic traits in species with biparental care. American Naturalist 127(4):415–445

²Cunningham, E.J.A. and Russell, A.F. 2000. Egg investment is influenced by male attractiveness in the mallard.  Nature 404: 74-77.

³Dentressangle, F., Boeck, L. and Torres, R. 2008. Maternal investment in eggs is affected by male feet color and breeding conditions in the blue-footed booby, Sula nebouxii.  Behavioral Ecology and Sociobiology 62: 1899-1908.

⁴Burley N. 1988. The differential allocation hypothesis: an experimental test. American Naturalist 132:611–628

Image copyright Carin Bondar 'The Nature of Human Nature'

It’s a tough world out there.  For the average Homo sapiens, having security in your relationship is a critical part to feeling successful.  If someone out there loves you enough to be faithful to you and to forsake the advances of all others, it re-enforces our self confidence and thereby makes us happier (and more productive) beings.  In the real world however, not all partnerships involve parties that adhere quite as strictly to the faithfulness that I mentioned above…what happens if you are feeling less than secure about the fidelity of your partner?  In the 15^th century humans invented a nifty contraption that provided a guarantee to insecure males that their partners would not accept genetic donations in his absence: the chastity belt.  Made out of tough material (usually a combination of steel and leather), the belt covered her sensitive areas and prevented any kind of sexual interaction.  In my opinion it seems a rather drastic measure to take, but as I mentioned above it’s a tough world out there.  It turns out that the ‘chastity belt’ method of preventing your mate from copulating with any others isn’t limited to our species.

A ‘mating plug’ is defined as a structure that is utilized to block the female genital reproductive tract.  Males of many species (including insects, crustaceans, reptiles and even mammals) utilize such plugs to prevent further mating by a female once they have deposited their DNA into her reproductive tract.  In species where there is intense sperm competition, biologists have described some sophisticated mating plugs.  For example, the spermatophore of ground beetles (Oedothorax retusus) contains sticky substances that allow it to act as a mating plug by physically blocking the females’ reproductive opening.  In addition to its physical blockage, the seminal fluid of ground beetles induces refractory behavior in mated females (i.e. it effects a change in her behavior that makes her avoid further copulation events with other males)¹.  Dwarf spider (Leptocarabus procerulus) males utilize glandular secretions that harden once they are deposited inside a female².  There is an extremely high level of sperm competition in dwarf spiders, as the sperm from a single mating event can remain fertile in a females’ sperm storage organ for several months.  The glandular-produced plugs utilized by the dwarf spiders are highly effective at preventing further mating of females: an experimental study showed that a large plug (produced by an uninterrupted copulatory event) prevented remating of the female 93% of the time².

If you think that the ‘chastity belt’ strategies utilized by male ground beetles and dwarf spiders is shocking, read on.  In arachnid species where sexual cannibalism is demonstrated (these are species where the female kills and ingests the male subsequent to copulation), males go to an even greater extent to ensure paternity.  This makes sense.  If he’s going to die anyway, he might as well do whatever it takes to make sure that his DNA is the prize winning seed for the next generation…even if this means breaking off his copulatory device inside the female genital opening (the human equivalent of a male breaking off his penis inside a females’ vagina in order to prevent future copulations).  Males of the orb-web spider Argiope lobata and the white widow spider Latrodectus pallidus have been shown to do just that^3,4.  Plugging up the female with his copulatory apparatus has the effect of blocking the opening from use by other males subsequent to the sperm transfer, and it has been experimentally shown that such plugging is effective at reducing the paternity share of males that attempt to mate once a plug is in place³.  Further investigation on sexual behavior in this species demonstrated that males that were cannibalized on their first copulation attempt had a much higher probability of damaging  their pedipalps (sexual sperm transfer appendages) than males that escaped (74% vs 15%)³.  Since this self-induced damage has negative consequences for future reproductive bouts (no kidding!), it makes the most sense biologically for a male to undertake it if his chances at securing paternity are high.

Although males of several species in the animal kingdom have developed some foolproof ways in which to ensure the fidelity of their sexual partners (i.e. in order to guarantee paternity of the resultant offspring and maximize fitness), I am much relieved that in our species such methods are almost entirely lacking.  For the most part the chastity belt is a thing of the past, having been replaced by a simple trust in one’s partner or the taking of ceremonial vows in order to maintain a sexually monogamous partnership.  I think that both males and females of our species would agree that such methods are a much more acceptable compromise than either losing your copulatory organ or having your opening plugged up indefinitely.  It’s a tough world out there.

¹Takami, Y., Sasabe, M., Nagata,N., and Sota, T. 2008. Dual function of seminal substances for mate guarding in a ground beetle.  Behavioral Ecology 19: 1173-1178.

²Uhl, G. and Busch, M. 2009. Securing paternity: mating plugs in the dwarf spider Oedothorax retusus (Araneae: Erigoniae). Biological Journal of the Linnean Society 96: 574-583.

³Nessler, S.H., Uhl, G. and Schneider, J.M. 2009. Sexual cannibalism facilitates genital damage in Argiope lobata (Araneae: Araneidae). Behavioral Ecology and Sociobiology 63: 355-362.

⁴Segoli, M., Lubin, Y., and Harari, A.R. 2008. Frequency and consequences of damage to male copulatory organs in a widow spider. The Journal of Arachnology 36: 533-537.

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

Moving is both exciting and stressful at the same time for members of our species…finding (and being able to obtain) a home that meets the needs of a growing family is something that requires careful consideration and a lot of work.  One person’s discard is another person’s dream: as you move out of that one bedroom apartment into a 3 bedroom condo, someone from the 3 bedroom condo may be moving to a 5 bedroom house.  Another individual (likely a first time buyer) is happy to take over your small apartment.  This kind of property exchange represents a complex process that involves several (unrelated) individuals or groups and can ultimately be beneficial for all of them at the same time.  All participants are satisfied with occupying the vacancy left by the previous tenants.  Termed ‘Vacancy chain theory’, the process of a sequential distribution of resources (houses in this example) across multiple individuals¹ occurs in our species as well as in several others.  After all, there can be serious consequences for one’s health and survival for living in quarters that are too cramped: spread of disease and detection by predators are two important ways in which survival can be jeopardized by living in a home that is too small.

Many animals build their own homes out of available materials (e.g. birds use twigs to build nests, snails secrete shells from their mantles, spiders spin their webs from silk glands) making their home construction a process that is independent from conspecifics in the population (i.e. if a larger home is necessary, grab some more supplies and make it happen).  However, there are other animals that rely on the availability of homes in their surrounding environments (akin to the Homo sapien).  Clownfish that inhabit anenmones, tree-cavity nesting birds and hermit crabs that inhabit abandoned snail shells are all examples of animals that make their homes out of discrete, reusable resources that are limited in use to one individual or family at a time.  Individuals of such species may therefore stand to benefit if a new home becomes available.

Researchers examining this phenomenon in the terrestrial hermit crab (Coenobita clypeatus) in Belize found that there were benefits to be gained for several individuals along a vacancy chain when a new shell was introduced to a group².  Crabs ranging in size from large to small were placed in experimental arenas in groups of 8.  A new shell that approximated the size of the largest crab was added to the arena, and the crabs left for 24 hours to proceed with shell inspections and possible shell-switching.  At the end of the trials the crabs that had changed shells were measured for the differences in shell crowding between their old and new homes (overcrowding represents a considerable survival threat for a hermit crab since being unable to fully retract into its shell increases susceptibility to predators).

It was found that an average of 3.2 crabs in each group switched shells, and that a whopping 89% of those that switched shells gained a significant reduction in the amount of shell crowding.  In other words, 3.2 individuals benefitted (aka increased their ability to survive) from the addition of a single resource to the arena, a phenomenon termed the ‘chain multiplier effect’.  More than one individual stands to ‘move on up’ the housing ladder by virtue of the fact that others have as well.  The vacancy chains observed in the hermit crab population were abruptly terminated if one crab had a damaged shell.  This makes sense in the human world too:  damaged homes require a lot more attention and sell for a much lower price, if at all.  Not entirely unlike requesting a home inspection prior to taking that last subject off of your offer to purchase, crabs undertake careful consideration on the qualities of a new home prior to moving in.  The size, weight, internal volume and amount of damage to the shell are all critical to the survival of the crabs living within them, and therefore shell selection is a careful process in this and other hermit crab species³.

The dynamics of home selection in the terrestrial hermit crabs studied here have definite parallels to the struggles faced by the Homo sapienswhen it comes to finding and obtaining a new home.  Vacancy chains can have reverberating benefits down the property ladder in both species.  However, if a home is sub-standard or damaged in some way the benefits of a vacancy chain come to an abrupt halt.  We all look for high quality, non-damaged places to live, especially when there are threats to our well-being created by having outgrown the one we currently inhabit.  A major difference between ‘us’ and ‘them’ in terms of moving up the property ladder is the universal human-based concept of finance.  Hermit crabs have no need for mortgages, loans or banking, no need for individuals to carefully budget the financial cost of the new home.  Once an individual crab has out-competed others for a certain shell, it is that individuals’ home for the taking.  In the human world, out-competing others for a certain home generally translates to making a better offer (a higher price).  In the hermit crab world, out-competing others for a certain home means that an individual physically fought for it and won.  Perhaps it’s a little on the violent side but at least it’s simple…and mortgage free!

¹Chase, I.D. 1991. Vacancy chains. Annual Review of Sociology 17: 133-154.

²Lewis, S.M. and Rotjan, R.D. 2009. Vacancy chains provide aggregate benefits of Coenobita clypeatus hermit crabs.  Ethology 115: 356-365.

³Rotjan, R.D., Blum J. and Lewis, S.M. 2004. Shell choice in Pagurus longicarpus hermit crabs: does predation threat influence shell selection behavior? Behavioral Ecology and Sociobiology 56: 171-176.

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

One of the most common ways that endothermic animals (such as the Homo sapiens) deal with fungal or bacterial invaders is through production of a fever.  Elevated temperatures created within our tissues represent the bodies’ attempt to kill off offending organisms, although a fever is certainly not without its cost to our emotional well-being.  Any parent who has sat at the bedside of infant children overnight as they battle a high fever can empathize that this physiological reaction can be as scary as the bacterial or fungal invaders!  The fever is essentially our bodies’ way of fighting back against the infections that come into it, so in many cases a little fever is actually a good thing.  It shows that our body is responding appropriately to a foreign invader, and that said invader will soon fall victim to the uncomfortable (or lethal) new temperatures to which it is being exposed.  The ability to produce a fever in response to infection is common in organisms that physiologically regulate their internal temperatures (i.e. endotherms), and can be looked at as an advantageous product of evolution.  What about organisms who do not have this built in physiological mechanism?  Ectothermic animals (such as reptiles and insects) cannot physiologically regulate their own body temperatures and consequently rely on their environment in order to do so.  One might therefore assume that such organisms are not able to utilize the health benefits gained from having a fever; however, one would be wrong.

‘Behavioral fever’ is a term used to define the actions of ectotherms who seek to elevate their body temperatures (beyond those of healthy individuals) when they are suffering from an infection.  This can be achieved by basking in areas of elevated temperature, orienting towards solar radiation, shifting from a nocturnal to a diurnal lifestyle or increasing activity levels (such as increased flight in insects)¹.  Behavioral fever has been reported in response to pathogenic infection for a diverse number of taxa including organisms such as scorpions, crustaceans, amphibians and reptiles, suggesting that it is widespread for theraputic use.  Its effectiveness has been confirmed by research examining the process in migratory locusts (Locusta migratoria).  Individuals were inoculated with a fungal infection (or innoculated with a control of distilled water) and allowed to thermoregulate (hang out) in an experimental arena with zones of varying temperatures².  The locusts could freely move between 3 temperature regimes, and they were observed several times per day for up to 8 days subsequent to being infected.  Following the behavioral observations, haemolymph was collected in order to determine the progression of the pathogen and to relate it to the temperatures in which the locusts spent the most time.  Those that had been exposed to the pathogen prior to the experiment exhibited behavioral fevers more frequently than those that had not; and exposure of only 4 hours per day to the higher temperature had a dramatic positive effect on their survival (85%).  The infected individuals did not remain indefinitely in the high temperature areas; they exposed themselves to the high temperatures in frequent periodic bouts, suggesting that this was sufficient to keep the infection to an acceptable level.  Although it did not rid the body of the fungus entirely, the theraputic effects of the behavioral fever allowed infected organisms to live in a normal way (i.e. continued feeding), and had direct positive effects on their rate of survival.

The capability of ectotherms to control their fevers to such a fine-tuned extent represents an awesome advantage in order to cope with pathogenic infection and to maximize survival.  By specifically controlling their exposure to high temperatures infection can be managed without concern for the damaging effects caused by fevers gone awry.  Indeed, there are times when the high fevers produced by endothermic orgnaisms are too high for our own good.  Although they are effective in killing off infection, fevers that are too high present a risk of physiological damage to the host.  For this reason the Homo sapien has developed a variety of ways to cool off a fever when one presents itself (medication is the fastest and most effective).  Unfortnately, members of our species often diminish a fever before it reaches a dangerous level, thereby mitigating its potential positive effects on the infectious organisms in our bodies.  Many of us pill-popping westerners need to remember that a fever is a natural defense mechanism provided by our physiology…the fever is NOT the enemy!  Although it is necessary to prevent temperatures that are too high, leave the medication on the shelf until such time as it is really needed…allow your temple to do what comes naturally.

¹Roy, H.E., Steindraus, D.C., Eilenberg, J., Hajek, A.E., and Pell, J.K. 2006. Bizarre interactions and endgames: entomopathogenic fungi and their arthropod hosts.  Annual Review of Entomology 51: 331-357.

²Ouedraogo, R.M., Goettel, M.S. and Brodeur, J. 2004. Behavioral thermoregulation in the migratory locust: a therapy to overcome fungal infection.  Oecologia 138: 312-319.

Thank you so much for your interest in purchasing my first book, The Nature of Human Nature.

I would be happy to personalize and sign your copy if you leave me instructions to do so in your order.  The cost of the book is $15.95 plus $5 for shipping.

I sure hope you enjoy it, please leave me comments or feedback.  I’d love to hear from you!

Here’s a little more about the book:

The Nature of Human Nature explores the unique position of the Homo sapiens in the animal kingdom.  You might think we’re the only species to do things as unnatural as use contraception or eat junk food…but you’d be wrong!  Join me on a journey of SURVIVAL and REPRODUCTION, from cockroach nutrition to cricket masturbation.  You’ll be happy you did

Here are some reviews:

By Adam Goldstein at the Springerlink Journal Evolution: Education and Outreach:

Link is HERE.

By Science Journalist David Manly:

Another Review by Dr. Greg Gbur, physics professor at UNC and Science Blogger:

Link is HERE.

We humans are really fascinated with our own genitalia, aren’t we? From a very early age, humans are well aware of, and curious about, the special properties that exist in the machinery of our nether regions. Perhaps it’s an unconscious appreciation for the parts of our anatomy that will ultimately catapult our genetic code into future generations, but whether we admit to it or not, everyone engages in solo sex.

Masturbation, we’re told by health professionals, is a natural and necessary process that is enjoyed by males and females, young and old. Although at first glance one might think that masturbation in males is wasteful because genetic blueprints are being released without a potential egg-target, there may be some biological use for this behavior after all. Sperm expulsion by males is a common occurrence in several vertebrate and invertebrate species, and in some cases it’s a strategic maneuver that results in greater reproductive success.

It has been observed in house crickets (Acheta domesticus) that males routinely expel spermatophores at times other than during sex. Spermatophores are packages of sperm that are transferred to the female sperm storage organ, called the spermatheca, during copulation in many invertebrate species. However, not all ejaculates are equal. Female house crickets are more likely to store young sperm in their spermathecas. (A polyandrous sexual system in this species means that multiple males can mate with a female in order to fill up her sperm storage organ.) Young sperm can be superior to older sperm for a number of reasons, including a greater fertilization ability and a higher survival rate in resultant embryos. Due to the fact that younger sperm is dominant in the spermathecas of female crickets, a male’s chances at reproductive success are greatly improved if he provides a relatively new package. How does he preferentially deposit young sperm? By discarding the old sperm, of course. Autonomous sperm ejection (cricket masturbation) is the mechanism by which a male cricket gives himself a younger sperm content and increases his chances of successfully fertilizing his partner’s eggs.

Not all masturbatory emissions are so carelessly discarded. Male marine iguanas in the Galapagos archipelago (Amblyrhynchus cristatus) have developed a strategy for improving their reproductive success via storage of their own ejaculates. There are large size differences between sexually mature marine iguana males. The largest males are the most aggressive, and they hold territories in which the female iguanas nest and feed. Smaller males must somehow sneak into the territories and copulate with females when the larger males are otherwise occupied. Since female iguanas only copulate once per season, smaller males have developed elaborate strategies to get the most bang for their buck (yes, pun intended) during these rare opportunities. It takes approximately 2.8 to 3.1 minutes of intercourse before a male iguana initiates ejaculation. For the large dominant males, this time interval is easily achieved, and such males have a high rate of successful copulations (95 per cent). The story is not as positive for the smaller males, who generally realize a much lower copulatory success rate and are forcefully separated from their female partners by territory-holding males before ejaculation 29 per cent of the time. However, the crafty underdogs are able to increase their fertilization success by masturbating before having sex and storing the prepared viable ejaculate in small pouches near the penis. Storing this extra sperm that can be transferred to the female almost immediately upon mounting her increases the overall fertilization success of the small male iguanas by a respectable 41 per cent. A stored ejaculate strategy may be common in species having a size-dominance hierarchy and sexual interference. Where there’s a will, there’s a way.

So why do humans masturbate? The aged sperm scenario seems like a good reason in some cases; controlling for sperm age is a standard procedure in human in vitro fertilizations. However, this doesn’t explain the absurdly high frequency of masturbation in males, nor its existence at all in females, also observed in other primate species including bonobos and macaques. Although the clitoris is found in all female mammals, scientists have yet to come up with a suitable biological explanation for it, or for the existence of the female orgasm. The long and short of it may be that it’s simply recreational. Let’s call it a day on the science and just enjoy its non-biological value.

*The previous is an excerpt from Dr. Bondar’s first book: ‘The Nature of Human Nature’. Available soon!

Reinhardt K, & Siva-Jothy MT (2005). An advantage for young sperm in the house cricket Acheta domesticus. The American naturalist, 165 (6), 718-23 PMID: 15937751

Wikelski, M., & Baurle, S. (1996). Pre-Copulatory Ejaculation Solves Time Constraints during Copulations in Marine Iguanas Proceedings of the Royal Society B: Biological Sciences, 263 (1369), 439-444 DOI: 10.1098/rspb.1996.0066