Photo by Dan Dan The Binary Man.

Even as plenty of the National Zoo’s other animals are efficiently producing adorable offspring, its giant pandas have generated little more than deflated hopes for another Butterstick. One philanthropist is hoping his investment will change that.

The Washington Post reports that the zoo is set to receive a $4.5 million gift from David M. Rubenstein for its panda reproduction program, sparing the institution from fundraising in difficult economic times and rekindling hopes that maybe — just maybe! — Tian Tian and Mei Xiang will get down to business and actually produce offspring again. All told, the two have put us through six false pregnancies since having given birth to Tai Shan — always known to us as Butterstick — in 2005.

While we’ve got the two of them until 2015, the zoo is only giving them until the end of 2012 to get things done. If they can’t, it has the choice of requesting “one or more replacement pandas from China.” Yikes.

More details:

Female giant pandas usually go into heat for a brief period of a few days. Mei Xiang, who has gone into heat in early winter the past two years, may be doing so again.

Zoo officials said they have recently detected “power walking” in Tian Tian, an indication that he senses his mate is approaching estrus. When that happens, the two will be allowed to try to mate naturally, something they have not been able to do successfully. (Tai Shan was conceived through artificial insemination.)

After having been shipped off to his native China in early 2010, our own Tai Shan has settled in comfortably and become something of a star attraction. All the while, we’ve been left with dashed hopes and expectations. Let’s hope that $4.5 million and a threat of replacement is enough to get Tian Tian and Mei Xiang to be a little more productive in the year to come. (The red pandas have been, after all.)