Piggybacking tadpoles are epic food beggars

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Tadpoles don’t cry to get their way. But some of them sure can beg.

Each bout of hungry-baby drama among mimic poison frogs (Ranitomeya imitator) occupies both parents for hours. The tadpoles get so crazy-frantic that researchers wanted to know whether the begging is an honest call for help or a histrionic scam.

 Frogs can lay globs of eggs by the thousands and leave them to fend for themselves. But the two-to three-egg clutches of mimic poison frogs (the only known monogamous frogs) get coddled, says Kyle Summers of East Carolina University in Greenville, N.C. Dad repeatedly checks in, sitting on the eggs and shedding some paternal pee if they’re drying out.

When the eggs hatch, dad gives each tadpole a piggyback ride to its own private pool. To find a little rainfall cupped between a leaf and stem, he’ll haul youngsters four meters or so. “A bit of a hike,” Summers says, since dad is only about a centimeter or two long himself.

These baby pools are pretty empty: home to only some algae, maybe some small insects. “The good news is that your offspring are not likely to get eaten; the bad side is that they don’t have anything to eat,” Summers says.

This is where the begging comes in. Frogs can’t make milk like mammals or regurgitate bugs like birds. But this species is one of the rare frogs whose moms, after considerable persuading, will lay an unfertilized egg for the tadpoles’ breakfast.

When parents show up on their weekly visit, a youngster stops regular swimming, noses up to a parent and goes into a frenzy of vibrating its tail. “The parent cannot miss a hungry tadpole,” Summers says.

Bouts of persuasion go on for several hours as the tadpole begs, stops, begs more and then more. Mom often makes several false starts, entering the pool but leaving it without any egg action. During all this, “dad will be the cheerleader,” calling in trills and stroking her, Summers says.

Analyzing tadpole frenzies in the lab, Summers’ then-student Miho Yoshioka found that tadpoles on short rations begged more as hungry weeks dragged on. Parents fed these hungrier tadpoles more reliably than the babies that researchers slipped treats to, Yoshioka, Summers and Casey Meeks report in the March Animal Behaviour. Overall, the researchers conclude, the relentless frenzy shows honest need, not tadpole greed.