C'è una coda vestigiale, cioè alcune ossa della coda all'interno del corpo, proprio come gli umani. I porcellini d'India non hanno bisogno di una coda perché non ne hanno bisogno per l'equilibrio o la presa, il loro habitat naturale è abbastanza piatto, e potrebbero usare tane costruite da altri animali. Potrebbero occasionalmente saltare, ma non fanno molto nel senso di arrampicarsi perché l'ambiente non lo rende necessario. I criceti hanno una coda molto piccola, mentre i ratti e i topi hanno una coda di buone dimensioni rispetto al loro corpo. Le cavie si sono evolute per perdere la coda, o i ratti e i topi si sono evoluti per avere code più lunghe? Certamente la preferenza femminile perpetuerà certe caratteristiche rispetto ad altre.

Because it doesn’t need one.

In more detail: one of three reasons.

1. Maybe the ancestors of guinea pigs were once rat-like creatures with tails. Because of the environment in which they lived, shorter tails conferred a survival advantage over long ones,so each generation tended to have a shorter tail until eventually external tails vanished. (But I’ll bet they still have tailbones inside them, just as humans do).
2. Because guinea pigs once preferred mates with shorter tails, and a shorter tail did not actually put them at any disadvantage. Same end result as 1. The male peacock’s tail is a more obvious example of evolution driven by sexual preference — it puts the male peacock at a considerable disadvantage in survival terms, but since he doesn’t do anything to help with eggs and chicks, all that matters is that enough male peacocks live long enough to fertilize the females’ eggs, and that the females choose the males with the biggest and most beautiful plumage.
3. Because an ancestor of the guinea pig suffered a (rare) major and dominant mutation which suppressed tail development. This did not put it or its descendants at any disadvantage, so the dominant gene spread until it was universal. (c.f. the Manx cat).