<< Chapter < Page Chapter >> Page >
a graph showing a positive correlation between the average phallus length vs. the number of pouches in the corresponding female’s vagina
Comparison of average phallus length vs. the number of pouches in the corresponding female’s vagina shows a positive correlation. This provides evidence of co-evolution (Adapted from Brennan et al. 2007).

This provides comparative evidence for the sexual conflict between sexes in waterfowl, as species that do have elaborate genitalia have been shown to be more likely to practice forced copulation while those that do not are less likely to have convoluted genitals .

It has been suggested that perhaps these forced copulations are due to overcrowding and not a natural part of the mallard’s reproductive tactics; however, research has concluded that the drakes practice forced copulation in normal population conditions in the wild (Evarts 1987). It would be uncertain whether or not the extra-pair drakes were copulating with the females when forcibly mounting the hens (see [link] ), but because of the multiple parentage observed in mallard clutches, 17-25%, and the lack of brood parasitism , it was concluded that the actions were extra-pair copulations (Evarts 1987). Normally in birds, extra-pair copulations are consensual and initiated by the female leading to greater fitness in her offspring; however, in mallards this may be a direct loss of fitness for the female if these extra-pair chicks are actually from an inferior male. In this case, both the female and her mate have lost some amount of fitness while the extra-pair male has increased his own.

a group of male ducks waiting to mount a female, one male has already mounted the female.
A group of drakes waiting to mount a female. Photo taken from<http://www.flickr.com/photos/26421824@N03/3413720963/sizes/m/>.

Forced copulations are distinct from regular copulations in that females show significant resistance to them by making displays, flying, hiding, diving, running, or swimming away (Evarts 1987). If the hen is mated, then her mate will peck at the aggressor(s) unless there are too many to deal with. Various studies have shown that in mallards, drakes will defend their own mates against other aggressors until she has laid her eggs. However, afterwards, the drake will join groups of other paired or unpaired males to attempt forced extra-pair copulations with fertile females that have yet to lay their clutches, which maximizes their own reproductive successes (McKinney 1983).

Therefore, female hens that have developed labyrinthine vaginas exert greater control over their reproductive fitness. Assuming that all drakes have elaborate phalluses, even the unfit ones would be able to forcibly inseminate hens that did not have this trait, so elaborate vaginas went to fixation. Bluhm and Gowaty describe this phenomenon well with the constraints theory : “[which] says that when individuals reproduce with nonpreferred partners, they will have offspring of lower viability than when individuals reproduce with preferred partners” (2004). They found by experimentation that this was true in mallard hens by controlling which partners the hens mated with and comparing offspring surviving to 45 days after hatching. This would support a mate-choice hypothesis for the co-evolution of mallard sexual organs wherein indirect benefits, the good genes of preferred males, would be what the females are selecting for (Adler 2010).

Get Jobilize Job Search Mobile App in your pocket Now!

Get it on Google Play Download on the App Store Now




Source:  OpenStax, Mockingbird tales: readings in animal behavior. OpenStax CNX. Jan 12, 2011 Download for free at http://cnx.org/content/col11211/1.5
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'Mockingbird tales: readings in animal behavior' conversation and receive update notifications?

Ask