Image: Insider Monkey.

Every quarter, many money managers have to disclose what they've bought and sold, via 13F filings. Their latest moves can shine a bright light on smart stock picks.

Today let's look at investing giant Bill Ackman, who founded Pershing Square Capital Management in 2003. An investor with roots in real estate, Ackman is an activist, often advocating strongly for big changes at companies in which he has invested heavily. Soon after Ackman invested in the Fortune Brands conglomerate, for example, the company began looking to spin off various divisions -- breaking up into the alcohol-focused Beam and Fortune Brands Home & Security. Beam has since been gobbled up by Japan's Suntory.

Pershing Square's reportable stock portfolio totaled $14 billion in value as of Sept. 30, 2015, spread over a handful of stocks. Its top three holdings -- Valeant Pharmaceuticals International (NYSE:BHC), Air Products & Chemicals, and Canadian Pacific Railway Limited -- make up a whopping 58% of the portfolio's value. Now that's concentration! (Several of his largest positions are based outside the U.S., too, giving the portfolio extra geographical diversification.)

Interesting developments
So what does Pershing Square's latest quarterly 13F filing tell us? Well, for starters, there wasn't much activity. It's a concentrated portfolio with few positions, and the only significant change in the quarter was the addition of a new holding, Mondelez International (NASDAQ:MDLZ), which became the portfolio's fourth-largest holding out of eight.

Mondelez, you may recall, was split off from Kraft Foods in 2012. (Kraft is now Kraft Heinz and part owned by Warren Buffett's Berkshire Hathaway.) Mondelez kept the global snack businesses, including brands such as Cadbury, Nabisco, Triscuit, Dentyne, Oreo, Tang, and Trident. Eight of its brands generate more than $1 billion apiece in revenue annually. Mondelez is free-cash-flow positive, but its revenue has been shrinking in recent years. Ackman has said that the company has room for profit-margin improvement, and he sees solid growth potential in emerging markets.

Despite Mondelez representing the only major change in the portfolio, the biggest news tied to it has been about Valeant Pharmaceuticals, Pershing's top holding. Pershing owns about 5.7% of the company, which has seen its shares fall by close to 50% over the past year, suggesting that 2015 isn't likely to feature Pershing's best performance.

What's going on? Well, the issue of drug pricing (and seeming over­-pricing) has become a hot topic lately and there's a Senate investigation into it -- and into Valeant. That's because Valeant's business model seems to have featured buying drug companies and then hiking the prices of the companies' drugs. For example, it bought the rights to the Isuprel and Nitropress drugs earlier this year and then upped their prices by 525% and 212%, respectively. This practice is not necessarily illegal, but as Warren Buffett's partner Charlie Munger has opined, it's "deeply immoral."

Meanwhile, Citron Research, a short-selling outfit, has accused Valeant of shady dealings with the mail-order pharmacy Philidor. Valeant defended itself in a 90-page document and then ceased doing business with the company. As if all that isn't bad enough, Ackman and Valeant are facing a lawsuit and allegations of insider trading in Botox maker Allergan as they tried (unsuccessfully) to buy the company. (Interestingly, earlier in the year, Ackman referred to Valeant as a "very early stage Berkshire [Hathaway].")

The Valeant story is an ugly one so far, but it's not over. Pershing and Ackman fans, or those interested in the company as an investment, should stay tuned.

We should never blindly copy another investor's moves, no matter how talented he or she may be. But it can be useful to keep an eye on what smart folks are doing. 13F forms can be great places to find intriguing candidates for our portfolios.

Longtime Fool specialist Selena Maranjianwhom you can follow on Twitter, owns shares of Berkshire Hathaway. The Motley Fool owns shares of and recommends Berkshire Hathaway and Valeant Pharmaceuticals. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.