Bava Metzia 79: Partial Use, Partial Problems, Partial Restitution

Our daf begins with a debate about when and how one should be reimbursed for renting someone a donkey that becomes sick or mad.  We learn details about the importance of renting a donkey capable of walking straight.  We also learn that donkeys are not available for rent in every place.  Further, if a person has rented a specific donkey, the owner is not necessarily responsible for replacing that donkey if it does not 'work out'.  The rabbis discuss how much of the fee should be returned, and how much money might be earner from the sale of the carcass of this animal.  Finally, we learn about the impact of receiving payment of the principal of the donkey over the course of a yovel, a Jubilee Year.

The rabbis move on to question a similar circumstance: what should be done if a rented boat sinks halfway through the journey?  Who is reimbursed, and when, and with how much money?  The rabbis speak about choosing specific boats - and specific bottles of wine - as exchanges.  What if a person unloads more of his own cargo into a rented boat part way through the journey?  Is this breaking the initial contract?  Was extra rope used to tie up the extra cargo?

Our daf ends with a conversation that begins with the consideration of extra weight placed on an animal due to carrying food.  The rabbis then claim that an animal that is contracted to be ridden by a man can only be ridden by a man; one contracted for a woman can only be ridden by a woman.  Or a girl.  Or a pregnant woman.  Or a nursing woman.  And even though significant extra weight is placed on the animal in these cases - sometimes even two people are riding the animal - there can be no grievance.  The rabbis even mention a woman who is pregnant and nursing.  

Why would the rabbis be concerned about the sex of the person sitting on an animal but not about extra weight?  A woman who is nursing could be niddah, and yet she would still be permitted to sit on the animal.  So this is not about an animal becoming tamei.  And it is not about protecting an animal from additional weight.  So what is this practice about?