The role of conspecifics in egg laying decisions in Jefferson’s salamander, Ambystoma jeffersonianum

Background/Question/Methods

Although many studies have shown that animals avoid ovipositing where there are conspecific eggs or larvae, other research has shown that females of some species are more likely to lay their eggs at sites where conspecifics’ eggs are present. Theory predicts that conspecific attraction should be highest at intermediate densities, where there are enough conspecifics to provide adequate cues that habitat quality is high, but not so many that there are negative effects from aggregation. In Jefferson’s salamanders, Ambystoma jeffersonianum, egg masses appear to be aggregated, and females attach their eggs to sticks and vegetation in ponds. We tested the hypotheses that conspecific attraction in this species affects females’ decisions about where to lay their eggs, and that females would prefer sites with intermediate densities of conspecifics’ eggs. To determine how existing egg masses affected the probability of new egg masses being deposited, we set out grids of bamboo stakes, which served as egg-laying substrates, in two ponds. For ten days we monitored the stakes, counting newly laid egg masses. We used a maximum log likelihood model to assign preference values for stakes with differing numbers of existing egg masses and AIC tests to identify the best model.

Results/Conclusions

Our final analysis yielded a model that included five categories of stakes, grouped according to the number of eggs masses they held: zero; one; two or three; four, seven or eight; and five or six egg masses. Stakes with one egg mass had the highest preference value (0.39), indicating that females were most likely to lay eggs on these stakes. This value was more than twice the values for stakes with zero (0.17) or more than three egg masses (0.18) and differed significantly from them. Females’ preference for stakes with two or three egg masses (0.26) was weaker than their preference for stakes with one mass, and stronger than their preferences for empty stakes or those with more than three egg masses, but did not differ significantly from the preference for any other stakes. These data indicate that females use existing egg masses as a cue to identify preferred egg-laying substrates and support the intermediate density hypothesis. Future work should investigate variation in larval survival at different egg mass densities in order to discover the selective advantage of aggregation in this species.