Dataset for: Mating competition and adult sex ratio in wild Trinidadian guppies

Most experimental tests of mating systems theory have been conducted in
the laboratory, using operational sex ratios (ratio of ready-to-mate male
to ready-to-mate female) that are often not representative of natural
conditions. Here, we first measured the range of adult sex ratio
(proportion of adult males to adult females; ASR) in two populations of
Trinidadian guppies (Poecilia reticulata) differing in ambient predation
risk (high vs. low). We then explored, under semi-wild conditions, the
effect of ASR (i.e. 0.17, 0.50, 0.83) on mating competition patterns in
these populations. ASR in the wild was female-biased and did not
significantly differ between the two populations. The range of ASR in our
experiment was representative of natural ASRs. As expected, we observed an
increase in intrasexual aggression rates in both sexes as the relative
abundance of competitors increased. In support of the risky
competition hypothesis, all measured
behaviors had lower rates in a high
vs. low predation-risk population, likely due to the costs of
predation. In terms of mating tactics, a male-biased ASR
did not lead males to favor forced mating over
courtship, indicating that males did not compensate for the cost
of competition by switching to a less costly alternative mating
tactic. Overall, this study highlights the need for field
experiments using natural ranges of ASRs to test the validity of mating
systems theory in a more complex, ecologically relevant context.