The complex behaviours of humans

Photo credit: Pinterest.com

For the final blog in this series, it seems fitting to address the most complex and bizarre of all creatures’ reproductive behaviours, the Human.

Whilst most of the things we do with our spouses or partners seem very normal to us if we take a truly objective view in relation to other animal’s behaviour, it seems we over-complicate reproduction to such an extent that, as a species, we have even developed methods and ideologies to allow individuals to opt out on reproduction all together.

It is both our pre mating behaviour and copulation that seem to produce perplexing results. Once humans reach sexual maturity most of their time and energy is spent on being appealing for a potential mate. Jobs, houses, cars, clothes, perfumes, make-up and most other items we possess are all designed to be appealing towards a mate. The strangest part of this behaviour is Humans spend most of their lives behaving this way but then pretend that it is not for the purpose of reproduction.

Photo credit: pinterest.com

Sexual selection is one of the main driving forces in mate choice with humans (Miller, 2000). Power and money seem to be an important part of mate selection for females suggesting the more money a male makes the better he can provide and a female’s youth and appearance seem important for male mate choice, suggesting youth indicates a higher chance of fertility (Buss, 1989).

Of course is not always the case and as particular socio-demographics would show males don't always need large resources to produce offspring and females don't always need youth for mate selection.

photo credit: 420dialog.com

The other unusual characteristic Humans possess is copulation. Despite males not having detachable phallus or females the ability to select or reject suitable semen, Humans have a whole bag of strange behaviours when it comes to copulation. Each individual has their own preference in the way copulation occurs including monogamy, bisexuality, homosexuality, violence and even multiple mating at one time (Schmitt, 2005). Some individuals may use one of these methods throughout its life span while others may choose multiple behaviours with in its life.

Copulation is such a driving force for humans it has overpowered instinctive reproduction all together. With the use of temporary and permanent contraception, Humans have evolved such behaviours that reproduction is now a by-product of copulation rather than the intended outcome.

References:

Buss, D.M. 1989, "Sex differences in human mate preferences: Evolutionary hypotheses tested in 37 cultures", Behavioral and Brain Sciences, vol. 12, no. 1, pp. 1-14.

Schmitt, D.P. 2005, "Sociosexuality from Argentina to Zimbabwe: A 48-nation study of sex, culture, and strategies of human mating", Behavioral and Brain Sciences, vol. 28, no. 2, pp. 247-275.

Miller, G.F. 2000, The mating mind: how sexual choice shaped the evolution of human nature, Doubleday, New York.

Life in the fast lane

Father and young. Photo credit: wikipedia.org

 The Barbary macaque (Macaca sylvanus) are the only macaque's to live outside of Asia. They are also one of the only macaques where the males show exceptional parental care, including care towards young that are not biologically their own. The females however are rather promiscuous and will intake multiple matings with different males while in her peak fertility stage (Pfefferle et al., 2008). 

Two baby Barbary macaques. Photo credit: flickr.com

Like many animal species the females will make a particular call whilst in the midst of copulation. It was first thought that the calls were an indication to the male that the female were at a higher fertile phase but that now may not be the case (Pfefferle et al., 2008). A study has shown that the loud vocal calls made by the female during copulation are actually to help the male macaque ejaculate. Each time the female would make a call the male would increase his thrust rate during copulation, resulting in a higher rate for ejaculation. If the female didn't make any noise than the male may not ejaculate. The more intense the calls the faster the thrust from the male resulting in ejaculation (Pfefferle et al.,2008). It is believed that the vocal calls during copulation evolved by females for the purpose of sperm competition and to cause paternity confusion amongst the males. The loud vocal calls indicate to other males that ejaculation has occurred and with high levels of promiscuity by females, each male only knows that they may be the possible biological  donor. This then leads to higher paternity care from the males towards young that may not be their own (Pfefferle et al., 2008). 




References:
Pfefferle, D., Brauch, K., Heistermann, M., Hodges, J.K. & Fischer, J. 2008, "Female Barbary macaque (Macaca sylvanus) copulation calls do not reveal the fertile phase but influence mating outcome", Proceedings of the Royal Society B: Biological Sciences, vol. 275, no. 1634, pp. 571-578.

The double life of the Komodo Dragon

Photo credit: livescience.com

The Komodo dragon ( Varanus komodoensus) comes from the islands of Indonesia and is the largest extant lizard. Although a regular in zoo's and the likely source for the folk tails of dragons, it was only recently that new light was placed on the Komodo dragon and their reproductive behaviours.




It has been discovered that Komodo dragons can reproduce via a process called parthenogenesis, this is where a female is able to produce offspring without fertilisation (Watts et al., 2006). What is even more incredible is that the female can switch from both sexual reproduction with a male and asexual reproduction via parthenogenesis.
When a female reproduces through parthenogenesis she can only produce homozygous ZZ or WW. The ZZ chromosomes turn into viable fertile males while the WW chromosomes fail to develop, leaving only male offspring (Watts et al., 2006).

Photo credit: travelalltogether.com

Although the Komodo dragons population numbers in the wild are low, their reproductive plasticity could be an advantage, where females are able to find a new niche environment and start new populations without needing males already present within the population.  It may also be advantageous if population sizes are low and mate choice is hard to come by (Watts et al., 2006).

Photo credit: zooborns.com

References:
Watts, P.C., Buley, K.R., Sanderson, S., Boardman, W., Ciofi, C. & Gibson, R. 2006, "Parthenogenesis in Komodo dragons", Nature, vol. 444, no. 7122, pp. 1021-1022.

Argonauts and their detachable phallus

The species of Octopus known as Argonaut is quite a unique species in its self. Being the only extant genus in the family Argonautidae. They can be found throughout the world in tropical waters. They also display extreme sexual dimorphism, where females will reach sizes of up to 50cm including their shell.(depending on which species) but males will only reach a size of 2cm and lack a shell (Finn and Norman., 2010). The males also lack dorsal tenticals which on the female is used to create the egg case AND males also obtain a detachable penis. 

Photo credit: Finn and Norman 2010, (a) female argonaut (Argonauta nodosus),
 (b) chambered nautilus (Nautilus pompilius)

 When copulation occurs, males will use their detachable penis which is called a hectocotylus, and insert it into the females palliative cavity. Once the penis is inserted he then detaches himself from the hectocotylus leaving it inserted within the female. 
Sadly after the male performs this unusual technique he will die afterwards, whereas the female is able to house and store the detachable penis full of sperm for weeks until her eggs are fully developed for reproduction. She can lay thousands of eggs which can consist of three different developmental stages. The eggs are incubated in a type of shell produced by the female until hatching occurs (V. Laptikhovsky and A. Salman., 2002).

References :

• Finn, J.K. & Norman, M.D. 2010, "The argonaut shell: gas-mediated buoyancy control in a pelagic octopus", Proceedings: Biological Sciences, vol. 277, no. 1696, pp. 2967-2971.

• Laptikhovsky, V. & Salman, A. 2003, "On reproductive strategies of the epipelagic octopods of the superfamily Argonautoidea (Cephalopoda: Octopoda)", Marine Biology, vol. 142, no. 2, pp. 321-326.

The uncomfortable truth of duck copulation

How many of us have gone to a pond and thrown chunks of bread into the water to feed the ducks? Now how many of us have ever wondered how ducks copulate? Well sadly ducks display a rather aggressive approach towards their potential partner and often rape, not only by the chosen male but by other “want to be donors” is the likely outcome.

Image: 3 male Argentine Lake ducks accosting a
female. photo:mentalfloss.com 

Ducks are one of only 3% of avian species to pose an external phallus.  It is believed this is due to the fact that ducks are aquatic birds and the potential for their semen to be damaged by water is high (Brennan et al., 2000). So unlike most other birds who just rub their cloaca’s together for fertilization, male ducks have evolved to obtain an external phallus and use penetration to ensure greater potential for fertilization (Coker et al., 2002).  Now where I think things become really interesting is that the female duck have also evolved a rather unique reproductive tube to allow her to choose which semen she would like to use for her next offspring.

The male Argentine lake duck has one of the largest penises in proportion to body size of any animal. Not only is it rather large it is also coiled and also contains barbs at the base of his penis and at the tip are tiny spines that resemble a bottle brush flower. These small spines are believed to help push out other males semen. The female Argentine lake duck ovarian tubes accommodate for the unique penis and are also coiled though in the opposite direction. As it coils around there are also small chambers and pouches known as “dead ends” that function to exclude the phallus and help eject unwanted semen (Brennan et al., 2000).

Image: Arrow shows the female vagina,
 star shows male phallus. Image: Brennan et al., 2000.

It is believed that the genital morphology has evolved by an arms race in copulation behaviour and inter-specific selection by the female (Brennan et al., 2000). The larger the male duck’s phallus the more aggressive copulation is on the female.  As the male becomes more aggressive towards the female the female has evolved to counteract this behaviour by a complex vagina, allowing the female to retain control of fertilization.

References:

Brennan, P.L.R., Prum, R.O., McCracken, K.G., Sorenson, M.D., Wilson, R.E. & Birkhead, T.R. 2007, "Coevolution of male and female genital morphology in waterfowl", PloS one, vol. 2, no. 5, pp. 418.

Coker, C.R., McKinney, F., Hays, H., Briggs, S.V. & Cheng, K.M. 2002, "Intromittent organ morphology and testis size in relation to mating system in waterfowl", The Auk, vol. 119, no. 2, pp. 403-413.

The Banana Slug

The Banana slug (Ariolimax spp.) is a terrestrial slug located in the temperate forests of North America from San Diego up to Alaska. They get their common name from the striking yellow colouring that resembles a banana.  As with the majority of molluscs, the banana slugs are hermaphroditic, though what has captured a reputation of bizarre sexual behaviour is what is known as apophallation (Leonard et al., 2002). This is where a slug will amputate the penis of either the partner or its self by chewing it off (Leonard et al., 2007).

The penis of the banana slug is located at its head and when erect can reach a whopping 6 inches, which is almost the same size as its body. The female reproductive tract is located at the upper side of the body. Copulation occurs by intromission of the penis into the female reproductive tract. Both slugs will be in a position that resembles a yin yang, with head swinging and a chewing motion from the mouth.  This is when intromission occurs and the entire process including courtship can last over several hours (Leonard et al., 2007). After intromission it is believed that if the penis is not the right size, in particular too large for the opening, then it can become stuck in the reproductive tract. And of course, the only alternative is to chew it off.  Generally the recipient will chew the penis off its partner but there are reports of apophallation to be reciprocal if both gain intromission. Apophallation is a slow process and erosion of the penis is visible before amputation is complete. Once the penis is severed it is usually eaten by the partner. The amputated penis cannot regrow thus the individual will remain a female for the duration of its life (Leonard et al., 2002). 

Image of Banana Slug during copulation.

References:

 Leonard, J., Pearse, J.  & Harper, A. 2002, "Comparative reproductive biology of Ariolimax californicus and A. dolichophallus (Gastropoda; Stylommiatophora)", Invertebrate Reproduction & Development, vol. 41, no. 1, pp. 83-93.

Leonard, J.L., Westfall, J.A. & Pearse, J.S. 2007, "Phally polymorphism and reproductive biology in Ariolimax (Ariolimax) buttoni (Pilsbry and Vanatta, 1896) (Stylommatophora: Arionidae)", American Malacological Bulletin, vol. 23, no. 1, pp. 121-135.