Out of 23 women who are at the head of S&P 500 companies, six are under pressure of activist investors who are

Six of the 23 top female chief executives in America are under pressure from activist investors who are challenging their leadership and pressing for major changes in their companies.

Nelson Peltz, the renowned billionaire corporate raider behind Trian Fund Management, is targeting three companies with female CEOs - Ellen Kullman at DuPont, Indra Nooyi at PepsiCo and Irene Rosenfeld at Mondelez International (formerly Kraft Foods.)

Also under fire from activists are Marissa Mayer, the CEO of Yahoo!, Meg Whitman at Hewlett-Packard and Mary Barra at General Motors.

The high profile names targeted by Trian in particular led Fortune magazine to ask: 'Does Nelson Peltz have a problem with women?'

Marissa Mayer, the CEO of Yahoo!, (left) and Meg Whitman, the chief of Hewlett-Packard, (right) are both facing challenges to their leadership from activist hedge fund investors

Activist investor Nelson Peltz has targeted DuPont CEO Ellen Kullman, (left) Mondelez CEO Irene Rosenfeld (center) and PepsiCo CEO Indra Nooyi (right)

General Motors CEO Mary Barra is being pressured to buy back company stock and increase dividend payments to shareholders

The New York Times asks, 'Do activist investors - all of them men - see women as softer targets?'

When asked by the Times, all of the investors in question bristled at the suggestion that they had targeted the companies in question because their CEOs are women.

There are just 23 female CEOs among S&P 500 companies - meaning more than one quarter are facing pressure from activist investors.

Research is divided on how female CEOs perform compared to their male peers. Some studies show that their companies have performed better - others show their firms were worse off.

Activist investors buy up a small percentage of a company's stock and seek to improve shareholder value by publicly challenging the CEO and board leaders by demanding changes.

Activist funds have been on the rise since the financial meltdown, with more than $120billion under investment - up from about $35billion in 2008, according to the Economist.

And they have been prolific - mounting challenges to 15percent of all S&P 500-listed companies since 2009, the newspaper reports.

Renowned corporate raider Nelson Pelz's positions in DuPont, Mondelez and PepsiCo led Fortune magazine to ask whether he 'has a problem with women'

Fortune reports that Peltz is pressuring Kullman to break up the 212-year-old DuPont into three companies, believing that they could be more efficient than the conglomerate is now.

He recently won four seats on the company's board of directors.

Nooyi, meanwhile is being pressured to split up PepsiCo's snack food unit and its beverage unit into two separate companies.

Peltz is also pressuring PepsiCo's Frito-Lay brand to combine with Mondelez, which owns brands like Oreo cookies, Toblerone candy and Ritz, Triscuit and Wheat Thins crackers.

He sits on Mondelez's board of directors.

Thus far, the CEOs of all three companies have been able to withstand pressure from Peltz, though he has proved a dogged and persuasive activist throughout his career.

Meanwhile, Mayer - who was herself installed as a result of pressure from an activist investor - is now facing pressure from Starboard Value, led by Jeffrey Smith. The firm has demanded that Yahoo! sell off its Japanese subsidiary and divest from Chinese internet retail giant Alibaba Group. It also said Yahoo! should not make any large purchases to expand its holdings.

Hedge funder J. Kyle Bass is pushing Barra for a seat on the board of GM and is demanding the company buy back some of its stock and increase its dividend to shareholders.

Whitman was forced to give Ralph V Whitworth a seat on the board of HP to stop him from waging a proxy war for control.

Fortune points out that Peltz, worth an estimated $1.65billion, is on his third wife. Claudia Hefner, a former model, is the mother of eight of his ten children - including daughter Nicola, an actress who starred in the latest 'Transformers' movie and 'The Last Airbender.'