Competitive males have higher quality sperm in a monogamous social bee.
BACKGROUND
Reproductive success is determined by the interplay between mating and fertilization success. In social insect species with male-biased sex ratios and queen monogamy, males face particularly strong pre-copulatory sexual selection since they must compete with thousands of other males for a unique mating opportunity. Ejaculate quality is also expected to be under selection, because queens are long-lived and store sperm for life, so males with higher quality ejaculates are expected to provide queens with larger and longer-lived colonies, which in turn may produce more daughter queens (the only direct fitness gains of haplodiploid males). Considering the action of pre and post-copulatory sexual selection on male traits, three scenarios might thus be expected: positive, negative or no association between male mating ability and fertilization success. Here we explored these scenarios in the stingless bee Scaptotrigona aff. depilis, where males gather in large aggregations and queens mate with a single male. Male mating ability was assessed through the capacity of a male to reach an aggregation and persist on it; while sperm viability, sperm number, and sperm morphology were used as proxies for sperm quality.


RESULTS
Sperm viability was associated with persistence time in the aggregation, and males that persisted longer presented shorter spermatozoa and higher variation in sperm length than recently arrived males. However, sperm traits of males that reached aggregations did not differ from those of males collected inside their colonies. In addition, males that persisted longer in aggregations were smaller than other males. Male size and sperm viability were not correlated, suggesting that the observed patterns were not due to trade-offs in male resource allocation.


CONCLUSIONS
Persistence in male aggregations thus seems to select more competitive males with higher quality sperm. Our work is the first one to reveal an association between male competitive ability and fertilization success in a monogamous social insect. This finding sheds important light on the evolution of male traits in social insects and the general mechanisms of sexual selection.
Department of Ecology, University of São Paulo, Institute of Biosciences, Rua do Matão, travessa 14, 321 (05508-090), São Paulo, SP, Brazil. sheina.koffler@usp.br.
Sheina
http://orcid.org/0000-0002-1126-8999
S
Koffler
Department of Animal Sciences, University of Ceará, Center of Agrary Sciences, Fortaleza, CE, Brazil.
Hiara Marques
HM
Meneses
Department of Ecology, University of São Paulo, Institute of Biosciences, Rua do Matão, travessa 14, 321 (05508-090), São Paulo, SP, Brazil.
Astrid de Matos Peixoto
AM
Kleinert
Department of Ecology, University of São Paulo, Institute of Biosciences, Rua do Matão, travessa 14, 321 (05508-090), São Paulo, SP, Brazil.|Vale Institute of Technology, Sustainable Development, Belém, PA, Brazil.
Rodolfo
R
Jaffé
Aggregation; Male competition; Sexual selection; Social insects; Sperm
Nature. 2001 Feb 8;409(6821):681-2
11217847
Evolution. 2001 Feb;55(2):324-9
11308090
Curr Biol. 2002 Jan 22;12(2):121-3
11818062
Proc Biol Sci. 2001 Jan 7;268(1462):77-81
12123301
Curr Biol. 2005 Feb 8;15(3):271-5
15694313
Annu Rev Entomol. 2005;50:395-420
15822204
Braz J Biol. 2005 Aug;65(3):469-76
16341425
Trends Ecol Evol. 2006 Jun;21(6):296-302
16769428
Proc Natl Acad Sci U S A. 2006 Oct 31;103(44):16346-51
17053078
Curr Biol. 2008 Jul 8;18(13):R553-4
18606122
Genet Mol Res. 2009;8(2):690-702
19554768
Proc Biol Sci. 2009 Nov 22;276(1675):3945-53
19710057
Naturwissenschaften. 2010 Mar;97(3):337-43
20012931
Proc Biol Sci. 2010 Oct 22;277(1697):3195-201
20504807
Evolution. 2011 Oct;65(10):3006-12
21967440
Evolution. 2013 Jan;67(1):95-109
23289564
Philos Trans R Soc Lond B Biol Sci. 2013 Jan 21;368(1613):20120042
23339234
Nat Commun. 2014;5:3184
24452310
Naturwissenschaften. 2014 Mar;101(3):261-4
24463620
Evolution. 1992 Apr;46(2):366-375
28564038
Evolution. 1999 Apr;53(2):535-545
28565418
Evolution. 1999 Dec;53(6):1804-1822
28565462
Evolution. 1996 Aug;50(4):1371-1381
28565703
Science. 1977 Jul 15;197(4300):215-23
327542
Monogr Popul Biol. 1978;12:1-352
740003