By now the trend is certainly clear: Consumers increasingly read their e-mails on their mobile devices rather than on their desktop PCs.  In the fourth quarter of 2013, 64.75% of e-mails in the U.S. were opened on smartphones or tablets versus 35.25% on desktop computers, according to the “U.S. Consumer Device Preference Report: Q4 2013” by e-mail marketing technology provider Movable Ink, which tracked the e-mails of more than 100 of its clients, including retailers. Those rates show a shift toward more mobile e-mail opens from the quarter prior, in which consumers opened 60.81% of e-mails on a smartphone or tablet versus 39.19% on a desktop.

“Now, more than ever, mobile optimization is critical for e-mail marketers,” says Vivek Sharma, co-founder and CEO of Movable Ink. “Ensuring every message renders properly and is designed with mobile in mind is just the first step.” Retailers can, for example, create e-mails that display different messaging depending on the device a customer is using, Sharma says. Taking that further, retailers can also make clicks on a smartphone result in different actions than clicks on a desktop, he adds. “For instance, a click on a mobile phone could launch the phone app to call the customer service hotline or the nearest store location,” he says.

In addition to continuing a mobile e-mail trend, the end of 2013 also marked a turning point in U.S. consumers’ habits overall. For the first time, the majority of consumers in each of the 50 states opened e-mails on mobile devices, not desktops, Movable Ink says. Texas claims the highest percentage of smartphone e-mail opens, 57.60%. The report does not break out tablet e-mail open rates by state.

Among mobile devices, smartphones are the most popular for opening e-mails, accounting for 48.23% of all opens in the fourth quarter, compared with 16.52% for tablets and 35.25% for desktops, Movable Ink says.

Apple Inc. iOS devices dominate devices running Google Inc.’s Android, which is not a surprise as study after study and retailer after retailer say that consumers on Apple mobile devices make far greater use of their devices for accessing the web and apps and for buying merchandise. In the fourth quarter of 2013, 35.39% of all e-mails were opened on an iPhone, 14.54% iPad, 12.47% Android smartphones, 1.80% Android tablets (excluding Kindle Fire), 0.20% BlackBerry smartphones, 0.17% Windows Phone smartphones, 0.10% Amazon.com Inc.’s Kindle Fire tablet (which runs Android), and 35.33% desktop/laptop PC, Movable Ink says.