Polygyny carries costs in both sexes in Trinidadian guppies

According to sexual selection theory, males should benefit more from mating with multiple partners than females do, as male investment into offspring production is typically lower. For females, empirical evidence indeed often shows diminishing returns or even costs of mating multiply. For males, the assumption often seems to be "the more, the better" - i.e., a steady increase of male reproductive success with mate number - but experimental tests of it are rare. Here we used a laboratory experiment with Trinidadian guppies (Poecilia reticulata), known for being promiscuous, to assess how pairing males weekly with 4 vs. 7 females affects both sexes reproductive performance (n = 32 polygynous males and 170 monogamous females). Increased polygyny delayed females reproductive onset by 9% and tripled their risk of reproductive failure. High-polygyny males fathered offspring with 49% more females and had 73% higher daily reproductive output. Yet, they needed 19% longer to initiate pregnancy, and only accumulated more offspring than low-polygyny males after two months. This study suggests that male mating performance is not unlimited. Especially when high extrinsic mortality selects for fast reproduction, less polygyny might be advantageous, and the strength of sexual selection perhaps more similar between the sexes than often assumed.