Sometimes, the Ladies Get to Flaunt Too

Generally, when imagining sexual dimorphism in animals, the automatic thought is, "Oh yeah! The guy has to be super cool and work extra hard in order to impress the ladies so they can get laid and go off and get laid some more!" And to that I say:

Black Widow Spiders. 

Check mate. 

But seriously, there always has to be exceptions to the "majority rule." Sexes evolve courtship and dimorphism depending on a number of different factors: 

Who's the main parent taking care of the offspring? 

What's the male to female ratio?

How often can the female make eggs?

How easy is it for the male to fertilize those eggs?

What's the success of those eggs finding success and becoming offspring?

And on and on and on and on....

(I kept thinking back to that paper I read when researching inbreeding that goes a bit into why some species would choose inbreeding because of such ratios, a link to which can be found on that post.)

In the example of the Pipefish, this is only one instance where the generally accepted "rule" is turned on its head. For a good reason, though. Per normal anatomy, the female is the one that produces the eggs. However, the male acts as the incubator, in the end expending more energy toward raising the offspring and ensuring their survival. Contrary to the male needed to find a suitable female to inseminate, the female needs to find a suitable male to carry her eggs to term. This leads to the males being the "choosy" partner of the species, carrying the eggs in his brood pouch and therefore willingly making himself more vulnerable and using more of himself in the two weeks it takes for the eggs to hatch. 

*Widebody Pipefish - Australia

To bring it all together, it's generally accepted that the female is the "choosy" partner because, in most cases, the female is the one putting more care and energy into the act of reproduction (caring for/rearing offspring, protecting/incubating eggs, less chances of reproduction within her lifetime, etc.). Sexual dimorphism tends to reflect that. However, when reproduction/child-rearing is done any other way, it would makes sense that males and females would take on different physical traits.

I mean, which one is the female Emperor Penguin? Both parents take a relatively equal role in raising offspring and there's not a large emphasis on competition for mates.  

Male deer and goats are known to have large horns and antler because of the emphasis on competition for territory, mating, herd dominance, and resources. 

Yet even then, when looking at Reindeer, both males and females grow antlers, again because of the role each parent plays in raising offspring (mothers needing that critical protection during the harsh weather around time of their pregnancy). 

Altogether, whether it's the male or the female or neither that presents more intense or flamboyant traits can be a key indicator to the social and reproductive nature of a species. 

Links

https://www.petoskeynews.com/story/news/local/gaylord/2016/11/03/reminder-to-hunters-to-differentiate-between-deer-elk/44868603/

https://fishesofaustralia.net.au/home/species/4039

https://encyclopediaofarkansas.net/media/female-and-male-black-widow-spiders-8404/

https://www.livescience.com/64764-photos-emperor-penguins.html

https://www.chesapeakebay.net/S=0/fieldguide/critter/pipefish