What role does と play in this sentence?

Question

What role does と play in the following sentence?

みなさんは、アフリカの平原で、ライオンがのんびりと寝そべっている姿をテレビや映画などで、見たことがあるでしょう。

In my understanding, のんびり is a noun and 寝そべっている is a verb. So interconnecting a noun with a verb with と does not make sense here.

Answer 1

「ライオンがのんびり+ と + [寝]{ね}そべっている」

"the lion is lying sprawled in a carefree manner"

「のんびり」 is an adverb, not a noun. It means "in a carefree manner", "leisurely", etc.

The 「と」 here is an optional particle. You can say 「のんびり寝そべっている」 without using a 「と」.

See definition #5 in デジタル大辞泉.

What does the optional 「と」 do, then? It gives a little more emphasis to the meaning of the adverb 「のんびり」 and the state of the lion at the moment. The difference the 「と」 brings about is quite subtle, but subtlety is a big part of Language and the native speakers' brains will surely pick up the difference.

Other examples:

「しっかり（と）[聞]{き}く」 (to listen carefully)

「ほんのり（と）[甘]{あま}い」 (slightly sweet)

「ピカピカ（と）[輝]{かがや}く」 (to shine brightly)