Data for: Morphometric parameters in males of the social wasp Polistes simillimus

Sperm competition and male mating rate are two non-mutually exclusive key evolutionary pressures selecting for larger testes within and across animal taxa. A few studies have tried to disentangle their effects (e.g., testing the role of mating rate in the absence of sperm competition). Under the mating rate hypothesis, particular phenotypes of a given population which are expected to gain more mates (e.g. more ornamented males) are expected to make higher investment in testes size (a proxy for sperm production). We test this prediction in Polistes simillimus, a neotropical paper wasp in which females are single-mated (no sperm competition) and males can mate with multiple partners. According to linear mixed model analyses, testes size was predicted by body size (positive association), sexual ornamentation (negative association), and their interaction (among small males, testes size was positively related to ornamentation but the opposite pattern was observed among large males). We propose that small-bodied well-ornamented males may face the highest risk of sperm depletion. Nevertheless, small-bodied males make relatively higher investment in testes size when sexy (more ornamented). This strategy might be less profitable to large males, as they have overall larger testes. Our results provide strong evidence for the mating rate hypothesis.

Methods
In April 2022, male-producing colonies of P. simillimus (N = 11) were collected in the municipality of Muzambinho, Minas Gerais state, and brought to the University of São Paulo, Ribeirão Preto. In the laboratory, the nests full of pupae (without adult wasps) were individually kept in plastic containers of 5 L. Subsequently, we inspected the nests daily to check for the presence of adult wasps. Newly emerged males were removed and used for morphometric measurements (N = 168 males, mean = 18, SE = ± 14, range = 4 – 41 males per colony). For each newly-emerged male, we measured the testes size, the sexual ornamentation and the body size. Males were anaesthetized by cooling for 2 min then dissected in insect buffer (0.1 M phosphate buffer, pH 7.2) under a stereomicroscope. Testes size was approximated from pictures of the bidimensional testes area. Sexual ornamentation was measured as the proportion of black pigmented vertex, as this seem to be the active signal component mediating female mate choice. We also measured the maximum head width of each male, which is a good correlate of male body size.