▲ability 后接不定式时总是用单数形式，不管它前面修饰它的物主代词或所有格是单数还是复数。你的例子中 ability 前面的物主代词都是单数，下面例子中 ability 前面的物主代词或所有格都是复数，但 ability 也只能用单数。如：

Both players demonstrated their ability to hit the ball hard. 两名球员都显露出用力击球的能力。

Freezing weather in spring hampered the hens’ ability to lay. 春天严寒的天气降低了母鸡的产蛋能力。

He has blind faith (=unreasonable trust) in doctors’ ability to find a cure. 他盲目相信医生有妙手回春的能力。

What impressed me was their ability to deal with any problem. 给我留下深刻印象的是他们处理任何问题的能力。

Humans differ from other mammals in their ability to speak and use tools. 人会说话和会使用工具的能力使其有别于其他哺乳动物。

The main determinant of economic success is our ability to control inflation. 经济方面的成就主要取决于我们控制通货膨胀的能力。

Everyone has the right to good medical care regardless of their ability to pay. 无论支付能力如何，人人都有权得到良好的医疗保健服务。

▲顺便说一句，如果不是后接不定式，与物主代词或所有格连用的 ability 则可以用单数（侧重从整体上考虑其能力）也可以用复数（侧重考虑其多方面的能力）。如：

He has a quiet confidence in his ability. 他对自己胸有成竹，但却并不张扬。

Students do different syllabuses according to their ability. 学生根据自己的能力学习不同课程大纲的课程。

Her low salary is not commensurate with her abilities. 她的薪水很低，与她的能力不相称。

We built up confidence in our abilities. 我们逐步建立起对自己能力的信心。