Horn growth in male pronghorns Antilocapra americana:: selection for precocial maturation in stochastic environments

Pronghorns Antilocapra americana (Ord, 1818), the sole member of a family unique to North America, grow rapidly and reproduce at an early age. Recent studies have found male pronghorns can grow large horns by 2 to 3 yrs of age. This pattern contrasts with many other ungulates, and it has profound implications for life history strategies. We examine 5 hypotheses that might explain precocial horn growth: (1) sampling bias, (2) nutrition, (3) phylogenetic inertia, (4) reproductive benefits conveyed by rapid horn growth alone, and (5) rapid horn growth as part of a suite of characteristics acquired due to precocial maturation. Hypotheses 1, 2 and 3 do not imply any natural selection, whereas hypotheses 4 and 5 do. We reject hypotheses 1, 2 and 4, and we cannot evaluate hypothesis 3. We conclude that hypothesis 5 most likely explains precocial maturation in male pronghorns, and mortality related to frequent, severe weather events may drive this pattern. We suggest several experiments to further examine relationships between age, size, and horn growth.