*652 ORDER

Abraham Ramirez and brothers Jorge and Moisés Lopez-Ontiveros pleaded guilty to conspiring to manufacture, to possess with intent to distribute, and to distribute more than 1,000 marijuana plants. See 21 U.S.C. §§ 846, 841(a)(1). All three filed notices of appeal, and we consolidated their appeals for briefing and disposition. Their appointed attorneys now assert that the appeals are frivolous and move to withdraw under Anders v. California, 386 U.S. 738, 87 S.Ct. 1396, 18 L.Ed.2d 493 (1967). We invited the defendants to respond, see Cir. R. 51(b), but received only a letter signed by a man purporting to speak for Ramirez. We will not consider this non-lawyer’s filing because litigants in federal court must represent themselves or retain licensed counsel. 28 U.S.C. § 1654; Elustra v. Mineo, 595 F.3d 699, 704 (7th Cir. 2010); see also United States v. Taylor, 569 F.2d 448, 451 (7th Cir. 1978). We therefore limit our review to the potential issues identified in the attorneys’ facially adequate briefs. United States v. Aslan, 644 F.3d 526, 531 (7th Cir. 2011).

Law enforcement officers discovered Ramirez and the Lopez-Ontiveros brothers growing marijuana in the Chequame-gon National Forest in Wisconsin’s North-woods. Fertilizer and camping supplies, along with loaded semi-automatic rifles and pistols, were also found at the site. Investigations revealed that the three defendants and several others had been paid to live together in the forest to cultivate and harvest the marijuana and to protect the plants from thieves.

In exchange for their guilty pleas, the government dropped charges that the defendants possessed firearms during the marijuana conspiracy. See 18 U.S.C. § 924(c)(l)(A)(i). The district court sentenced all three defendants to the ten-year statutory minimum for the marijuana count. See 21 U.S.C. § 841(b)(1)(A).

The attorneys do not indicate whether they have spoken with their clients about challenging their pleas. See United States v. Konczak, 683 F.3d 348, 349 (7th Cir. 2012); United States v. Knox, 287 F.3d 667, 670-71 (7th Cir. 2002). Still, these omissions do not require denial of the An-ders motions. The briefs and transcripts of the plea colloquies show that the district court accepted the pleas after substantially complying with the requirements of Federal Rule of Criminal Procedure 11. See Konczak, 683 F.3d at 349; United States v. Blalock, 321 F.3d 686, 688 (7th Cir. 2003). The district court explained to all three defendants the rights they would relinquish by pleading guilty, warned them of the consequences of their pleas, ensured the pleas were voluntary, and determined that factual bases for the pleas existed. See Fed.R.CRIm.P. 11(b). On the present record, it would be frivolous for any of the defendants to challenge the voluntariness of his plea. See Konczak, 683 F.3d at 349.

All three Anders briefs address whether the defendants’ sentences could be challenged as unreasonable. Each attorney concludes that such a challenge would be frivolous. We agree, because the defendants’ sentences were the lowest permitted by statute; any argument that the sentences were not reasonable would be frivolous. See United States v. Cooper, 461 F.3d 850, 856 (7th Cir. 2006); see also United States v. Johnson, 580 F.3d 666, 673 (7th Cir. 2009).

We GRANT the motions to withdraw and DISMISS the appeals.