Gemara

96b - Question: If an animal became weaker Machmas Melachah (due to working), what is the law?

Objection (Rav Chilkiya brei d'Rav Avya): This implies that if it was Mes (died) Machmas Melachah, he would be liable. This is unreasonable. He borrowed it to work!

Answer (Rava): Not only if an animal became weaker Machmas Melachah, rather, even if it died, he is exempt, for he borrowed it to work!

A case occurred in which Reuven borrowed a lamp from Shimon, and it broke.

Rava: Bring witnesses that you did not use it abnormally, and you are exempt.

Rav Papa said the same to one who borrowed a bucket, and it broke.

Version #1: Reuven borrowed a cat from Shimon, and mice ganged up on it and killed it.

Question (Rav Ashi): Is this like Mesah Machmas Melachah?

Answer (Rav Mordechai): Yes. (He was not negligent for putting it in a dangerous place. A proper cat would not be killed by mice.)

Version #2: Reuven borrowed a cat from Shimon, and it ate too many mice, got overheated and died.

Question (Rav Ashi): Is this like Mesah Machmas Melachah?

Answer (Rav Mordechai): Yes.

Rishonim

Rif and Rosh (8:5): When it Mesah Machmas Melachah, if the Shomer cannot bring witnesses that he did not use it abnormally (to exempt himself), he swears to exempt himself.

Rambam (Hilchos She'elah 1:1): If one borrowed an animal to plow and it died while plowing, he is exempt. If it died before or after plowing, or he rode on it or threshed with it and it died at the time, he is liable. The same applies to all similar cases. If one borrowed an animal to go to the place Ploni, and it died under him on the way, or he borrowed a bucket to draw water and it cracked while filling it up, or he borrowed an ax to chop wood and it broke due to the chopping, he is exempt, for he borrowed it to do this job, and he did not deviate.

Rivash (423): The Torah explicitly obligates a borrower if the animal died, i.e. normally, even though the angel of death would have killed it anywhere. If it died due to the job, the lender was negligent for lending it for a job it could not bear. The Rambam exempts only if it died while working. This is why he said 'under him', i.e. while he was riding it; otherwise, we would not know that it died due to the task. Or, perhaps he means that the sickness started while he was riding, and it died within 24 hours. It seems that this is his opinion. The Ramban and Rashba disagree, for the Gemara said due to the task, not at the time of the task. Why should we exempt at the time of work? Perhaps it was sick due to something else! Rather, we must know that it died due to the task, e.g. from weariness. This is even if it died afterwards, for the lender was negligent.

Magid Mishneh: The Ramban and Rashba are correct.

Rambam (4): If the animal became leaner due to the job, he is exempt, and swears that it was due to the job.

Rosh (4): Mesah Machmas Melachah is when it tripped and fell while walking, and the borrowed did not deviate and was not negligent. Or, it became tired and overheated and died. However, if it died on the way and he did not notice weariness due to the exertion of walking, he cannot swear that it died due to the work. Perhaps it would have died had it been standing at its feeding trough! If it became tired and weak due to the journey, even if it did not die right away, rather, it kept getting weaker over a long time, this is Mesah Machmas Melachah.

Rosh (5): Rava required the one who borrowed a lamp to bring witnesses, because many people were around, like Isi ben Yehudah (who says that one may not swear in such a case - 83a). Alternatively, he meant that he should bring witnesses, or swear.

Terumas ha'Deshen (328, cited in Beis Yosef CM 340 DH Reuven): If one borrowed armor and weapons to fight with his city against the enemy, and the enemy conquered and took all the weapons, he is exempt. This is Mesah Machmas Melachah. Even though it did not deteriorate or break due to the task, since the loss was due to the task, it is like Mesah Machmas Melachah. We learn from the mice that ganged up on the cat and killed it. Tosfos asked why Rav Ashi was unsure about this, and answered that he asked whether taking it to a place where many mice are found is negligence. If not, surely he is exempt, even though it did not die due to its task of catching mice and eating them.

Poskim

Shulchan Aruch (CM 340:1): If one borrowed an animal or Metaltelim, he is exempt for any mishap due to the job for which he borrowed it. This is not only if it became leaner, rather, even if it died. This is if he did not deviate from the job for which he borrowed it. If he deviated, even if he did a lighter task, we attribute the death to the change. Deviation applies even in the job for which he borrowed it, e.g. he borrowed it to plow, and he plowed day and night, unlike normal practice.

SMA (4): The Rosh and Tur (308:4, 311:4) permit changing to a job no harder, and all the more so to an easier job! It is unreasonable to be more stringent about a borrower, who gets all the benefit, than a renter. The Torah exempted Mesah Machmas Melachah! We must say that even though one may deviate, if we can attribute the death to the change, we do.

Shulchan Aruch (3): If one borrowed an animal to go on a certain path, and bandits or wild animals killed it on the path, this is Mesah Machmas Melachah.

Beis Yosef (DH Kosav ha'Ramah): The Ramah says so. This is like the cat that ate too many mice, even though the eating itself did not kill it, just it became sick and died due to this. The Tur says that the Rosh disagreed. The Rosh did not write this; he just told his Talmidim.

Rema (3): Some disagree, for even without going, such an Ones could occur.

Source: Bedek ha'Bayis citing the Rosh.

Rema (ibid): If one borrowed a cat to chase mice, and mice ate it, this is Mesah Machmas Melachah, for the Ones was due to the task.

SMA (8): The Tur exempts when it died from eating too many mice, even though it is not totally Machmas Melachah. The cat was borrowed to chase mice, not to eat them. The Rema should have taught this, and all the more so he is exempt if mice killed the cat. Perhaps he holds like Version #1 (and obligates when the cat died from overeating). Or, he holds like the Terumas ha'Deshen, that the bigger Chidush is when mice ate the cat.

Shach (6): The Darchei Moshe and SMA (8) bring the Terumas ha'Deshen, who learns from the mice that ganged up on the cat, that if one borrowed weapons and the enemy conquered, he is exempt. This is wrong. The cat was lent to chase mice by itself. If it could not, the owner was negligent (for lending something inadequate for the job). Kelim do not make war by themselves. The soldiers' defeat is due to themselves, so they are liable! The Terumas ha'Deshen holds like the Mechaber. The opposing opinion that the Rema brings is primary, and it would obligate the soldiers!

Rema (ibid.): If an animal became weary while working and became overheated and died later or tripped and died, this is Mesah Machmas Melachah. This is only if he noticed weariness while it was working. If not, he cannot swear that it died Machmas Melachah. Perhaps in any case it would have died!

Question (Bedek ha'Bayis): This is astounding. Do we obligate the Shomer due to a doubt? We should say that since it died on the road, presumably it was Machmas Melachah. He needs to swear only that it died on the road to be exempt.

Answer (Shach 7): The Shomer must swear to exempt himself. He cannot swear if he is unsure, therefore he must pay. R. Yerucham and the Terumas ha'Deshen bring the Rosh; this is primary.

Nimukei Yosef (Bava Kama 39a DH Chaayvin): The Ro'oh says that that if one borrowed a donkey for his Shali'ach to ride on, and he does not know whether or not it died Machmas Melachah, he is exempt, for perhaps he was never liable. If he borrowed it to ride on it himself and gave it to a Shali'ach, he was liable immediately for deviating, and he does not know whether he is exempt due to Mesah Machmas Melachah, therefore, he must pay.

Ketzos ha'Choshen (4): This shows that normally, we do not obligate a Shomer if he cannot swear because he does not know.