When a female giant panda's thoughts briefly¹ turn to love each year, her ears perk² up. The solitary³ animals can discern the love calls of different suitors, new research suggests.

This skill could help females, who are fertile only two or three days out of the year, decide which males to mate with, says Ben Charlton, a biologist at Zoo Atlanta, who led the new study. Gaining a better understanding of how females pick their mates might, in turn, help zoos breed the notoriously picky and endangered animals.

Previous research suggested that scent⁴ plays a critical role in female choice. But the male's bleat⁵ may also be involved in panda mating rituals, since males produce them far more frequently during breeding season and females often respond back with chirps⁶ of their own.

Working with captive animals at China's Research and Conservation Center for the Giant Panda, Charlton's team played audio recordings⁸ of bleats⁹ of different males to females.

To determine whether females could tell one bleat from another, Charlton played six consecutive¹⁰ calls from one male, followed by the call of a second male, then another call from the first male.

After each successive call, females tended to spend less and less time looking for the male suitor. But when a different male's call blared out, females' ears perked¹¹ up and they spent significantly more time looking toward an audio speaker 20 metres away, compared to the previous call. Critically, a repeat of the first panda's call again elicited¹² less interest.

Because females showed renewed interest in the novel bleat, this indicates that females could tell the difference between the calls of two males, Charlton says.

The pitch the calls, however, didn't seem to make a difference for female pandas. When Charlton digitally modified the bleats to the same pitch, females still perked up to the new sound. Not so, when Charlton eliminated another vocal¹³ feature called amplitude¹⁴ modulation¹⁵, where pandas rapidly raise and lower the volume of their bleats, producing a whinnying sound.

Charlton hasn't yet proven that females use these bleats to choose their mate, but showing that females can tell one bleat from another is a first step.

In anticipation¹⁶ of a female's fleeting¹⁷ fertility, males will often trail a single female for the preceding month, bleating¹⁸ all the while. The ability to discriminate¹⁹ calls could help females avoid Johnny-come-latelys who haven't paid their dues, Charlton says. "I think there's also female preference for more familiar males because they are likely to be of higher quality given that they have out-competed other males."

Mimicking²⁰ this familiarity in zoos could also encourage unwilling²¹ pairs of pandas to mate, Charlton says. After researchers discovered that scent plays an important role in panda mating, zoos began exposing pandas to the smells of their partners-to-be.

Devra Kleiman, a behavioural ecologist who has worked with the pandas at the Smithsonian National Zoological Park in Washington, DC, says "I firmly believe that pandas can distinguish individuals they know."

After a series of unsuccessful matings between the zoo's first pandas, a female named Ling-Ling and a male named Hsing-Hsing, the zoo brought in another male from London to mate with Ling-Ling, Kleiman recalls. "They got into a horrible fight and he really injured her seriously. The next day she was almost in a catatonic state."

Kleiman then played Ling-Ling a recording⁷ of Hsing-Hsing's bleat. "She just started bleating in response," Kleiman says. "I was astounded²²."

熊猫妹妹每年都会坠入一次爱河，可时间只有区区几天。每到这个时候，她的耳朵会变得非常灵敏。最新研究表明，那些喜欢独来独往的熊猫女郎有一种本领：能够识别不同求爱者的声音。

该研究项目的负责人、亚特兰大动物园的生物学家本·查尔顿说，熊猫女郎的这种本领对她们很有帮助，因为她们每年的有效受精时间只有那么两三天，如果没有这种本领，怎么知道自己最倾心的是哪位熊猫帅哥呢？弄清楚熊猫女郎的择偶标准还有助于减轻育种人员的工作压力。要知道这些濒危动物在择偶方面的高标准和喜欢挑三拣四的“猫病”，在动物世界里早就传遍四乡六里了。

以前的相关研究曾认为，气味在熊猫小姐的择偶标准中占据关键地位。现在看来，熊猫先生如希望洞房花烛夜的美梦成真，还得有一副好歌喉才行。科学家发现，一到繁殖季节，熊猫先生就更喜欢歌唱了，而熊猫小姐往往也会用她们的婉转歌喉和他们对歌。

查尔顿率领的研究小组在中国大熊猫保护与研究中心对圈养起来的大熊猫开展相关研究工作。他们先把几个熊猫哥儿们的不同唱腔录制下来，然后分别放给几位熊猫姑娘欣赏。为了弄清楚熊猫姑娘是否真能区分熊猫哥儿们各自的唱腔，查尔顿想了一个绝招：先播放其中一位老兄的情歌，连续六声，紧接着播放另一位老兄的一声唱腔，然后又播放一次第一位老兄的情歌。

熊猫姑娘们在聆听第一个哥们的六遍情歌时，其注意力表现出逐渐下降的倾向，也就是说，听第一遍时，双眼会放电，听到第六遍时，放电量明显下降；而就在她们突然听到一首不同的情歌时，她们都会不约而同地再次竖起耳朵，给予新的高度关注，同时她们的眼睛还会四处寻找，给远在20米外的喇叭抛媚眼。更为关键的是，这时候继续播放第一个熊猫哥们的那首“老掉牙”的情歌时，她们的性趣还是没有第一次听到时那么高了，看样子熊猫姑娘虽很挑剔，但并非性冷淡。

查尔顿说，这应能表明，熊猫女士们确实能分辨不同求爱者的情歌，因为她们对新情歌表现出新的兴趣。

查尔顿还发现，熊猫女士们可不管求爱者的情歌的调子如何，只知道对不同哥们唱都表现不同的性趣。为了证明这一点，查尔顿用数字变调技术，把不同熊猫歌王的情歌都调成相同的调子，结果熊猫姑娘们的表现和没有变调子的时候一样。查尔顿还故意把音调变成AM格式，同时故意把熊猫哥们的情歌弄得忽高忽低，简直像野马嘶鸣一样，难听死了。结果，她们不干了，懒得理会他们。

查尔顿虽然还没有证明，熊猫小姐最后会不会根据求婚者的歌声来决定是否以身相许，但既已表明她们能够区分不同求爱者的歌声，起码算是证明了关键的第一步。

熊猫女士的受孕时间可谓转瞬即逝。结果呢，可怜的熊猫先生为了得到那宝贵的圆房时刻，只能提前一个月向他的梦中情人频频发起攻势，紧追不放，一路追逐还得一路唱情歌。查尔顿说，如果熊猫女士不懂得识别求爱者的歌声，那是很难保证不会让半路杀出来的某个程咬金捡个大便宜的。查尔顿认为，熊猫姑娘是不会乱抛绣球的，她们只会把绣球抛给她们熟悉的心上熊，那么谁才是她的心上熊呢？只能是在她最喜欢的某方面胜过其他追求者的帅哥靓仔了。

查尔顿认为，既然明白了熊猫姑娘的心，今后在动物园里就可以通过模拟熊猫姑娘喜欢的歌声的办法，去蒙骗她们，让她们嫁给本来不将其放在眼里的可怜哥们，给咱地球多留些熊猫子孙。这倒不是查尔顿心眼有多坏，原来，早在科研人员发现气味在熊猫婚嫁中所起的关键作用的时候，已经有人想到用这种损招去蒙骗挑剔得不行的熊猫公主了，使她们以为嫁到了好郎君。

华盛顿特区史密斯宋尼亚（Smithsonia）国家动物园的行为生态学家德芙拉·克莱曼说：“熊猫一定能区分她们所认识的不同熊猫小伙子的，对此我深信不疑。”

美国国家动物园为了让该动物园的第一对大熊猫相好，该想的办法的都想了，可他们俩就是宁死不从。这对大熊猫，公主叫玲玲，王子叫兴兴，是1972年尼克松总统访华之后中国人民出于好意赠送给美国人民的。可是，他们俩简直前世有仇似的。没办法，动物园方面只好从英国给玲玲公主找来另一位熊猫王子，希望这一次能花好月圆。克莱曼回忆说：“真是万万没想到，他俩一见面就打架，打得很可怕，英国熊猫王子把玲玲狠狠给揍了一顿。第二天玲玲就吓得不行，精神高度紧张。”

后来，克莱曼给玲玲播放了兴兴的情歌，没想到玲玲这次唱出了她心中的歌来对歌，真让克莱曼感到非常震撼。