Directional selection on body size but no apparent survival cost to being large in wild New Zealand giraffe weevils

Abstract

When an individual's reproductive success relies on winning fights to secure mating opportunities, bearing larger weapons is advantageous. However, sexual selection can be extremely complex, and over an animal's life the opportunity to mate is influenced by numerous factors. We studied a wild population of giraffe weevils (Lasiorhynchus barbicornis) that exhibit enormous intra and intersexual size variation. Males bear an elongated rostrum used as a weapon in fights for mating opportunities. However, small males also employ sneaking behavior as an alternative reproductive tactic. We investigated sexual selection on size by tracking individual males and females daily over two 30‐day periods to measure long‐term mating success. We also assessed how survival and recapture probabilities vary with sex and size to determine whether there might be a survival cost associated with size. We found evidence for directional selection on size through higher mating success, but no apparent survival trade‐off. Instead, larger individuals mate more often and have a higher survival probability, suggesting an accumulation of benefits to bigger individuals. Furthermore, we found evidence of size assortative mating where males appear to selectively mate with bigger females. Larger and more competitive males secure matings with larger females more frequently than smaller males, which may further increase their fitness.

Social selection is density dependent but makes little contribution to total selection in New Zealand giraffe weevils

“…There was no linear or quadratic social selection in either sex when not taking into account variation in body size, density, or sex ratio (linear social selection = 0.479, se = 0.563, χ 2 = 0.029, p = 0.864; sex interaction = −0.494, se = 0.591, χ 2 = 0.700, p = 0.403; quadratic social selection = 0.485, se = 0.498, χ 2 = 0.268, p = 0.604; interaction = −0.674, se = 0.527, χ 2 = 1.633, p = 0.201). As previously found [25] both sexes were under approximately equal positive linear direct selection for body size (linear direct selection = 0.232, se = 0.382, χ 2 = 11.477, p < 0.001; sex interaction = 0.038, se = 0.391, χ 2 = 0.009, p = 0.923; quadratic direct selection = −0.470 se = 0.669, χ 2 = 1.551, p = 0.213, sex interaction = 0.395, se = 0.671, χ 2 = 0.347, p = 0.556). The strength of social selection did not depend on the size of the focal individual for either sex (focal and rival body size interaction = −0.200, se = 0.368, χ 2 = 1.781, p = 0.182; sex interaction = 0.093, se = 0.380, χ 2 = 0.060, p = 0.806), nor was it different among different classes of male (contrast between female and large male = −0.197, se = 0.218, contrast between female and small male = −0.014, se = 0.204, χ 2 2 = 1.262, p = 0.532).…”

“…We located aggregations of adult giraffe weevils on karaka trees ( Corynocarpus laevigatus ), which were subsequently used for behavioural observations. The observations and data collection used in the current study are described in full in a previous study [25] with the data available online [28], but we briefly outline them again here.…”

“…Observations took place over two periods between November 22 and December 22, 2013 (31 days, N = 120 females, 132 males), and January 22 to February 23, 2014 (33 days, N = 301 females, 366 males). For the analysis we excluded individuals only seen once, and those who were first seen in the last week of each observation period (following [25]). This left a dataset of 1234 records of 155 different females and 236 different males.…”

“…We also predicted that smaller males would be less affected by the body size of rivals, as they can readily switch between fighting with similar-sized rivals and “sneaker” tactics that allow them to gain copulations without direct competition [27]. Finally, following previous work which found positive size-assortment among mating pairs [25], we predicted that there would be positive assortment for body size in the individuals present on trees in both sexes, causing social selection to reduce the overall strength of selection on body size.…”

“…We aimed to fill gaps in our knowledge surrounding the contribution of social selection to total selection, and the conditions it is strongest, in a wild population of New Zealand giraffe weevils ( Lasiorhynchus barbicornis ; Coleoptera: Brentidae). Both sexes are extremely variable in size [21–23], males bear an elongated rostrum used as a weapon during contests for mates [24], and body length is under positive linear, but not quadratic, selection in males and females [25]. As giraffe weevils form aggregations on trees and compete for access to mates, we predicted that social selection for body size would be negative, where the presence of larger rivals reduces a focal individual’s fitness (following [26]).…”