Size-dependent alternative male mating tactics in the yellow dung fly, Scathophaga stercoraria

Abstract

Whenever males can monopolize females and/or resources used by females, the opportunity for sexual selection will be great. The greater the variation among males in reproductive success, the greater the intensity of selection on less competitive males to gain matings through alternative tactics. In the yellow dung fly, Scathophaga stercoraria, males aggressively compete for access to receptive, gravid females on fresh dung. Larger males are better able to acquire mates and to complete copulation successfully and guard the female throughout oviposition. Here we demonstrate that when an alternative resource is present where females aggregate (i.e. apple pomace, where both sexes come to feed), smaller males will redirect their searching for females from dung to the new substrate. In addition, we identify a class of particularly small males on the alternative substrate that appears never to be present searching for females on or around dung. Smaller males were found to have a mating 'advantage' on pomace, in striking contrast to the pattern observed on dung, providing further support for the existence of an alternative male reproductive tactic in this species.

Relative fitness of alternative male reproductive tactics in a mammal varies between years

“…Body mass influences reproductive success in many species, including those with ARTs (e.g. brook trout Salvenius fontinalis , Blanchfield, Ridgway & Wilson 2003; horseshoe crabs Limulus polyphemuus , Brockmann & Taborsky 2008; dung flies Scathophaga stercoraria , Pitnick et al. 2009).…”

Alternative mating tactics in the yellow dung fly: resolving mechanisms of small-male advantage off pasture

“…Male S. stercoraria were separated into small and large groups by eye based on experience (but later measured; see below) and intermediate males were discarded. The small-male class was meant to encompass those males that avoid dung and instead primarily adopt the patroller tactic, whereas large males reflected body sizes of that typically observed on dung [45].…”

“…The socio-sexual environment of S. stercoraria on pomace differs from that on dung in several important respects: (i) pomace resource is much larger and less defensible; (ii) distribution of S. stercoraria is more dispersed (i.e. less densely populated); (iii) sex ratio is approximately equal; (iv) agonistic malemale interactions are absent; and (v) the vast majority of matings on pomace are obtained by a smaller size class of males typically not present on dung or in cattle pastures, despite males of all body sizes being abundant [45]. These findings indicate that the S. stercoraria mating system is characterized by alternative patroller-competitor mating tactics, although the underlying mechanism(s) giving rise to the small-male mating advantage at feeding sites are unknown.…”

“…These findings indicate that the S. stercoraria mating system is characterized by alternative patroller-competitor mating tactics, although the underlying mechanism(s) giving rise to the small-male mating advantage at feeding sites are unknown. To explain this pattern, Pitnick et al [45] proposed three non-mutually exclusive hypotheses. We have adapted and broadened these hypotheses into the following: (i) small males are specialized for the patroller tactic and more efficiently search for or capture females on pomace; (ii) females preferentially mate with small males (or are more resistant to mating with large males) in this non-competitive environment; and/ or (iii) large males have lower mating propensity and/or are less persevering when females are resistant to mating.…”

“…Males provide no courtship and females are thought to have little or no control in determining mates, although Borgia [44] suggests that females sometimes preferentially orient towards larger males when approaching the pat. By pairing with the largest males, Borgia [44] found that females were better protected from costly takeover attempts of rival males which frequently occur throughout copula and oviposition [39,42,45]. Once oviposition is complete, females leave dung to feed and produce their next egg clutch.…”