Define "Gender"? Or are you trying to say that there are only two 'sexes'? There IS a difference.
Here's a good example of what Gender means to one person.
"So, two criteria substantiate gender: socially determined roles and an internal sense of identity. Neither of these by itself is enough to fully encompass what gender is, but most experts appear to agree that each is a necessary aspect of gender. Therefore, to assess the common claim that gender is unique to humans, we need to look at how other species fare with respect to these two criteria."
https://psmag.com/environment/do-animals-have-gender-roles
Evolution's Rainbow: Diversity, Gender, and Sexuality in Nature and People. Joan Roughgarden. University of California Press, Berkeley, 2004. 474 pp. $27.5
academic.oup.com
By Chloe Rigg Think of lessons you might learn from a standard dad. These lessons may have included: sports, barbequing, or hunting. Personally, one of the most memorable lessons from my father was…
uiwomenscenter.wordpress.com
How do we know that gender is not simply a biological fact? What makes it cultural, rather than analogous to sex-differentiated behavior in animals?
nautil.us
This paper will analyze some of the limitations of using male and female gender classifications when examining an animal’s biological sex and argue that these categories define gender through too n…
thingsofinterestweb.wordpress.com
Society News:Gender benders in the animal kingdom
www.thehindu.com
In an era of increasing gender fluidity, it is worth asking whether gender differences in psychology have been overstated in the past. Non-humans offer helpful hints.
www.psychologytoday.com
Just because it's only been a thing researched more recently, doesn't mean it's also a 'spontaneous' thing that's happening now. Rather now we're looking for it, or paying attention.
So to answer the question - I dunno, which mammal specifically do you want me to focus on, and are we talking the social construct and identity of the mammal, or are we talking what it does with it's reproductive body parts?