siikukankyou ha sizukana monono, senzyuuneko to no aisyou ha wakaranai.

A natural reading is:

しいくかんきょう は しずかな ものの、せんじゅうねこ との あいしょう は わからない。

Word readings:

• 飼育環境 = しいくかんきょう
• 静かな = しずかな
• 先住猫 = せんじゅうねこ
• 相性 = あいしょう
• 分からない = わからない

ものの means something like although, though, or even though.

So:

• 飼育環境は静かなものの、… = Although the living environment is quiet, ...

It sets up a contrast:

• one point is favorable: the environment is quiet
• but there is still a concern: compatibility with the resident cat is unknown

Compared with けど / けれども, ものの often sounds a bit more formal or written.

Because 静か is a な-adjective.

When a な-adjective directly modifies a noun or a noun-like expression, it uses な:

• 静かな部屋
• 静かな環境
• 静かなものの

Here, ものの is attaching after the adjectival form, so 静かな is the correct shape.

So:

• 静かものの = incorrect
• 静かなものの = correct

This is a very common question.

With な-adjectives, both patterns can exist in some contexts, but before conjunction-like expressions such as ものの, the form without だ is very common:

• 静かなものの
• 便利なものの
• 簡単なものの

So 静かなものの is the natural standard form here.

You can think of it as the adjective connecting smoothly into the concessive expression ものの.

It means compatibility with the resident cat or how well it gets along with the cat already living there.

Breakdown:

• 先住猫 = the cat that was already in the home first
• と = with
• の = turns that whole phrase into something that can modify a noun
• 相性 = compatibility

So:

• 先住猫との相性 = compatibility with the resident cat

This X との Y pattern is very common:

• 犬との関係 = relationship with a dog
• 家族との時間 = time with family
• 人との相性 = compatibility with people

Because と shows the relationship partner, and の links that phrase to the noun 相性.

So the structure is:

• 先住猫と = with the resident cat
• 先住猫との = with-the-resident-cat’s / involving the resident cat
• 先住猫との相性 = compatibility with the resident cat

You can think of の here as turning the phrase before it into a modifier for the noun that follows.

The sentence has two topics/contrast points:

• 飼育環境は = as for the living environment...
• 先住猫との相性は = as for compatibility with the resident cat...

This creates a contrast like:

• The environment is quiet
• but compatibility with the resident cat is unknown

The first は introduces one aspect. The second は introduces another aspect.

Japanese often uses は this way to compare or contrast different points about the same situation.

飼育環境 means the keeping environment or living environment for the animal.

In a sentence like this, it usually refers to the home conditions where the animal is being kept or would be kept. In pet adoption contexts, it often means something like:

• the home atmosphere
• whether it is quiet or busy
• whether the environment is suitable for the animal

So 飼育環境は静かなものの means:

• The living/keeping environment is quiet, but...

This is an important nuance.

• 知らない = do not know a fact / have no knowledge of it
• 分からない = do not know / cannot tell / it is unclear

Here, 相性 is not really a simple stored fact. It is something that may only become clear after the animals interact. So 分からない is much more natural.

• 先住猫との相性は分からない = Compatibility with the resident cat is unknown / hard to tell

Using 知らない here would sound less natural.

Japanese often leaves that unstated.

Here, 分からない does not strongly focus on who does not know. It simply means that the compatibility is unknown or cannot be determined yet.

Depending on context, you could understand it as:

• we don’t know
• the staff don’t know
• it’s not known yet
• it’s hard to tell

In English, the best translation is often impersonal:

• Compatibility with the resident cat is unknown.

By itself, 先住猫 does not explicitly mark number.

So depending on context, it could mean:

• the resident cat
• resident cats

However, in many pet-adoption contexts, it is often understood as the existing cat in the home, so English may use the singular naturally.

Japanese nouns often do not show singular/plural unless the context makes it necessary.

Yes, it is a complete and very natural Japanese sentence.

Japanese often omits information that is obvious from context. Here, several things are left unstated, such as:

• whose living environment it is
• who does not know
• whose compatibility is being judged

But the core grammar is complete:

• 飼育環境は静かなものの
• 先住猫との相性は分からない

That is fully natural Japanese, especially in short descriptive writing such as pet profiles, notices, or adoption information.

It has a fairly neutral, slightly formal, informational tone.

Reasons:

• ものの sounds more written/formal than casual けど
• the vocabulary is somewhat technical/descriptive: 飼育環境, 相性
• the sentence presents facts in a careful, objective way

This makes it sound like something from:

• an adoption profile
• a shelter description
• a veterinary or care note
• a written assessment of an animal

Yes. A more casual version might be:

飼育環境は静かだけど、先住猫との相性は分からない。

This means the same thing, but:

• だけど is more conversational
• ものの sounds more formal or written

So the original sentence is not strange at all; it just sounds a bit more polished or official.