Ontogeny of mosaic-tailed rats ... let's "develop" some ideas

Welcome to January 2020! It’s a new year and that means a new start to blogging again. It’s amazing how quickly time flies by as it’s already mid-January! But, we’ll get back into the swing of things. 

Fig. 1. Juvenile fawn-footed mosaic-tailed rat Melomys cerviniipes

So, I’ve brought you up to speed with the reproductive biology of male and female mosaic-taild rats Melomys cervinipes (Figure 1). There was a lot of minute detail in that, but I hope it gives you a sense that are quit complicated little beasts. So, I’m going to carry on the story now by focusing a little more closely on the ontogeny of mosaic-tailed rats.

But first off, what do we mean by ontogeny? Ontogeny refers to the development of an individual organism, or specific anatomical or behavioural features of an organism, from the earliest stage to maturity (and, in some cases, beyond even that). Basically, it refers to how an organism develops over its lifetime, and what factors affect its development. Ontogeny is one of the four questions posed by Niko Tinbergen to understand an animal’s behaviour. Not only does an animal’s behaviour change over time, but the anatomy and physiology underlying that behaviour (i.e. the behavioural machinery) can also change.

Fig. 2. Adult male fawn-footed mosaic-tailed rat with russet red pelage

In general, we don’t really know much about the ontogeny of mosaic-tailed rats, outside of their reproductive biology. One of the major ontogenetic stages is sexual maturation, which normally occurs around 45-60g body mass or 95-120mm body length. By the time sexual maturity is reached the adult pelage is nearly completely attained, changing from a smokey-grey (Figure 1) to a brown-russet red (Figure 2). The reproductive anatomy also changes at this time, with the testes descending in males. In females, the nipples become prominent and the vagina becomes perforate. For females, sexual maturation is also the time the oestrous cycle commences. Females have a long cycle (10-24 days), with a relatively short period of receptivity (1-3 days of oestrus).

Fig. 3. Baby fawn-footed mosaic-tailed rat. Photo: David Wilson

Obviously, the most momentous ontogenetic developmental stages occur to an individual while it is in the womb. Mosaic-tailed rat gestation lasts 27-40 days, which is really long for rodent in general. Interestingly, mosaic-tailed rats tend to give birth to twins, a boy and girl, although they can have up

to four pups (I’ve only ever seen three). In mosaic-tailed rats, the young (Figure 3) are born with fully erupted incisors, unlike in some other murids. This enables them to cling to their mother’s nipples, which they do quite tenaciously for the first 7-10 days. Development is rapid following birth, with the pups having eyes open by about 5-7 days, and being fully furred by 10 days. They start eating solid food around 10 days of age, and weaning generally takes place around 20 days of age.

In the next blog, we’ll take a more in-depth look at what we know about the ecology 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.