Male fawn-footed mosaic-tailed rat reproductive biology

It’s November, and time for Blog 11. I can’t believe that it’s nearly the end of the year already! Last time I wrote, I was asking for your help with crowd funding a project to continue studying the awesome Australian rodent, the fawn-footed mosaic-tailed rats Melomys cervinipes (Figure 1). And I left you waiting patiently to learn all about the reproductive anatomy and biology of male mosaic-tailed rats. So, here you are. The information you’ve been waiting for. There may be some confusing terms, but I’ll provide some information to help make it easier to understand.

Fig. 1. Juvenile fawn-footed mosaic-tailed rat M. cervinipes

Adult breeding males have scrotal or inguinal testes. In layman’s terms, that’s the groin area. The testes are quite large in relation to the body size, being about 2% of the body mass. This is even big for a murid rodent! Regression of the testes can be seen when males are not breeding. This makes sense – if you’re not going to breeding, why waste a heap of time producing sperm that you’re not going to use?

While morphologically similar to other rodents, the glans penis is quite long and wide in comparison to other Australian rodents. In anatomical terms, this is really referring to the rounded head or tip of the penis itself. In M. cervinipes, it is longer and wider than the glans penis of the grassland mosaic-tailed rat M. burtoni (Figure 2), but is actually also a little narrower too. The glans penis has small spines near the tip that disappear towards the base, and there is epidermal folding. The proximal baculum is short and wide. Okay, so what is this “baculum”? Basically, it’s a little isolated bone derived from connective tissue. What does it do? Well, believe it or not, but it aids reproduction by maintaining stiffness during copulation.

Fig. 2. Grassland mosaic-tailed rat M. burtoni. Photo: Russell Best, QPWS, 2009

The seminal vesicles are saccular and well-developed, and have coagulating glands on the inner curves. Their function is to produce the seminal fluid that carries the sperm. The seminal vesicles are bound to the prostate at the base. Active spermatogenic seminiferous tubules are much wider than inactive ones. The sperm averages about 107 μm in length, and is more complex in structure than is seen in Australasian Rattus. The falciform (meaning “curved like a sickle” or hooked) head has two additional elaborate ventral F-actin processes extending from the upper concave surface, joining at the base. The apical hook ultrastructural organization resembles the sperm of Rattus, but there are two ridges of subacrosomal material along the upper convex nuclear margin. Males have lots of types of accessory glands. In juvenile males (Figure 1), pretty much everything is smaller in size, although similar in structure. In addition, while spermatogenesis may occur, it generally doesn’t fully progress past the primary spermatocyte stage.

In the next blog, I’ll move on to looking at the ontogeny and reproduction of mosaic-tailed rats.

You can read more about my research in my publications, listed on my blog. You can also find me on ResearchGate, the James Cook University website, Twitter and Facebook.