Having sex two times in a row is exhausting (for plants too!)

For humans, having sex once – let alone twice – a day is tiring.  Although we may need a nap afterwards to recoup, taking the time and energy to have sex is worth it!  Besides the obvious feel-good perks, sex can actually improve our health by relieving stress, boosting our immune system, burning calories, and helping us sleep better.

Everyone gets tired after sex - especially if
 you 'do it' twice in a row!
Image from: http://www.lifedynamix.com/


However, sex may be far more exhausting and less rewarding in plants.  Unlike humans, studies have suggested that plants are not capable of consecutive sex sessions and sex can even decrease their wellbeing; investment in reproduction may result in losses to vegetative growth and maintenance. 

 

Fittingly, Bernett, Laemmerzhal, and Rockwood (2009) set out to test how draining sex actually is for herbaceous plants.  They used a large population of bloodroot (Sanguinaria canadensis), a rhizomatous, perennial herb.  The population was located in a 100 year old temperate deciduous forest in Virginia, within which they set up a 50 x 100m sampling quadrat.



In spring 2003, Bernett et al. tagged all flowering bloodroot individuals within the quadrat with yellow flags.  After flowering was complete, they returned to count the fruits and seeds produced by the tagged individuals.  They also measured the area of the solitary leaf produced by each plant.



Flowering individuals of a bloodroot
(Sanguinaria canadensis) population

In spring 2004, Bernett et al. returned to the population for more tagging: individuals that only flowered in 2003 kept their original yellow tags, individuals that flowered in 2003 and 2004 were retagged with orange flags, and individuals that only flowered in 2004 were tagged with pink flags.  For each flowering individual, the same counts and measurements were recorded as in 2003.

 

Surprisingly, there was a dramatic reproductive and vegetative difference between plants that flowered in 2004 only versus those that flowered in both 2003 and 2004.  Significantly fewer plants flowered in both years than in 2004 only.  Plus, plants that only flowered in 2004 produced significantly more fruits, more seeds, and 20% larger leaves than those plants that flowered consecutively.



Figure 1 from Bernett et al. demonstrating the signficatntly reduced
leaf area in plants that reproduced consecutively

The results clearly demonstrate that each reproduction event is tiring and expensive – most plants can't even flower two years in a row.  Plus, plants that did flower consecutively had to steal vegetative resources to do so, and were far less capable of producing offspring.

Although a ‘roll in the hay’ (or two) for humans leaves us out of breath for a few hours post-action, we typically feel good afterwards and embrace many unexpected health benefits.  For many plants, however, sex can leave them out of breath for a year or more, decreasing offspring output and vegetative size. 

Consecutive sex in plants, then, hardly seems worth it.  Successive reproduction reduces future plant livelihood by reducing leaf size; smaller leaves result in decreased surface area required for light capture and decreased photosynthetic cells responsible for energy production.  Plus, producing flowers in the second reproductive year is often energetically wasteful; these flowers often fail to produce fruits or seed.

Or is it worth it after all?  Consecutive reproduction events – draining or not – help to evolve the species more rapidly; more reproduction events result in more recombination, which leads to more potential for new genes.  This will ensure that bloodroot individuals are more environmentally adaptive on our ever-changing planet.

Sources:

http://www.webmd.com/sex-relationships/features/10-surprising-health-benefits-of-sex?page=3

http://www.lifedynamix.com/articles/Anti-Aging/Sleep_Program.html
Bernett CC, Laemmerzhal A, Rockwood LL (2009) Reduction in reproductive output and leaf size in Sanguinaria canadensis as a cost of reproduction. Journal of the Torrey Botanical Society, 136:4, 457-464.