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Response to Comment on “Females engaging in adaptive hybridization prefer high-quality heterospecifics as mates”

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Abstract

Braun et al. contend that we did not account for survival, but we did. Differential survival does not alter our conclusions, which were also robust to removing anomalous families. They ignore the study system’s natural history justifying our fitness measures, while failing to account for our behavioral data. We stand by our conclusion that females adaptively choose among heterospecific males.

Hybridization is adaptive if its fitness benefits outweigh its costs (1). This occurs for spadefoot toads, Spea bombifrons. Hybridization with S. multiplicata produces offspring with low fertility (males are sterile; females are less fecund). However, under some circumstances, hybrid tadpoles are more likely to survive than pure S. bombifrons tadpoles (1, 2), and S. bombifrons females prefer to mate with heterospecifics when hybridization has this net fitness benefit (1) (Fig. 1). We examined whether S. bombifrons females further optimize fitness of their hybrid offspring in this crucial tadpole stage by preferring S. multiplicata males that sire higher-fitness tadpoles versus mating randomly with S. multiplicata. We found that the pulse rate of S. multiplicata male calls predicted hybrid tadpole fitness (3). Critically, S. bombifrons females preferred pulse rates of S. multiplicata males corresponding with fitter offspring only when hybridization was favored (13).

Fig. 1 Spadefoot breeding ponds and tadpoles.

Ponds vary in depth and duration. (A) In wet years, when ponds last longer, slowly developing S. bombifrons tadpoles have time to metamorphose. (B) In dry years, the same pond can dry before S. bombifrons tadpoles metamorphose. (C) Tadpoles die if they fail to metamorphose (here, all “pebbles” are dead or dying tadpoles), but larger tadpoles desiccate more slowly and could survive if ponds refill from rain. Because hybrids develop faster than S. bombifrons, female S. bombifrons have higher net fitness by hybridizing in shallow ponds (1, 2). [Photo credits: David Pfennig, (A) and (B); Karin Pfennig (C)]

Braun et al. (4) contend that our conclusions are unsupported because we did not identify a quadratic relationship between survival and male pulse rate [we investigated a linear effect and found none (5)]. However, they do not account for parent size and condition (3), which should be included whenever evaluating relationships between fathers’ sexual signals and offspring fitness (6, 7). Using a logistic regression with tadpole survival/death as the response and family as a random effect reveals that the top three models include a quadratic effect of pulse rate (ΔAICc ≤ 0.98), and the fourth-best includes male condition only (ΔAICc = 1.70) [the fifth-best was the null model (ΔAICc = 4.58)] (8). Given these small AICc differences [<2 (9)], a quadratic relationship between pulse rate and offspring survival is suggested but not definitive.

Braun et al. also do not articulate how a quadratic relationship between survival and male pulse rate undercuts our findings. Differential survival weakens our conclusions if the pattern of survival opposes that of growth (i.e., if better-growing tadpoles are more likely to die). However, tadpoles were more likely to survive when from families with better average growth [using our original fitness metric, PC1 from a principal components analysis of snout-vent length (SVL), Gosner stage (GS), and mass]: logistic regression with family as a random effect; likelihood ratio test, χ2(1) = 5.52, P = 0.019; the same holds when separately examining SVL [χ2(1) = 4.99, P = 0.025] and GS [χ2(1) = 6.03, P = 0.014] [mass, which is confounded with SVL: χ2(1) = 3.35, P = 0.067]. Thus, our original analyses appear conservative.

Braun et al. do not account for our mate choice data. In two-choice tests, S. bombifrons females preferred heterospecific calls associated with higher tadpole fitness, and females only expressed those preferences when hybridization was favored (13). A curvilinear relationship between sire signal and offspring fitness would still demonstrate that male sexual signals predict hybrid offspring fitness, and females would still be expected to prefer heterospecific mates that sire high-fitness offspring. Specifically, if the relationship between male calls and offspring fitness were curvilinear, females should disfavor extreme pulse rates relative to those we used (6). Such “preference function” data (6) would provide a nuanced perspective on sexual selection exerted by S. bombifrons females on S. multiplicata males, but would not undercut our study.

Braun et al. also take issue with our measures of tadpole growth “as the sole fitness metric in a hybrid system” and argue that growth reflects heterosis (i.e., enhanced fitness of hybrids relative to both pure species). However, Braun et al. do not acknowledge crucial details motivating our study. The spadefoot natural history and known fitness consequences across life stages, which we described (3), justifies our focus on tadpole growth (see also above). Spadefoot tadpoles develop in desert ponds (Fig. 1). Growth (in terms of size) and development time (measured using GS) have life-or-death consequences (Fig. 1C). Owing to intermediate––not heterotic––growth rates of hybrid tadpoles, hybrids have higher fitness than S. bombifrons (but not S. multiplicata) tadpoles in rapidly drying ponds where S. bombifrons may fail to metamorphose (1, 2). Moreover, tadpole size predicts patterns of adult size, and greater size can increase pre- and post-metamorphic survival (1012). We based our study on these details, and predicted that S. bombifrons females would only express preferences for high-quality heterospecifics in environments and populations where hybridization confers a net advantage. Our predictions were met (3). Braun et al.’s criticisms cannot account for these behavior data nor their relationship to what is known about hybrid fitness in this system [(3) and references therein].

Braun et al. assert that our statistical analysis of fitness was inappropriate. They state that regression coefficients for fathers’ pulse rate as a predictor of PC1 do not differ from zero in three out of five models when family 9 is removed. However, they fail to note that the two best-fit models are not only robust to removing family 9 [P < 0.05, ΔAICc = 2.70, compared to ≥3.54 for remaining models; table S3 of (3)], but also to removing families 4 and 15. Our results therefore did not depend on outlier families (see also Table 1).

Table 1 Summary of relationships between our individual fitness proxies and sire pulse rate.

Non-nested candidate models were obtained using dredge from “MuMIn,” and all contained family as a random effect (13). We refer to models with regression coefficients for sire pulse rate that were significantly different from zero (P < 0.05) as “top models,” because all of those models had smaller ΔAICc scores than models that found no significant relationship with pulse rate or that did not include pulse rate. All of the top models identified inverse relationships with sire pulse rate. Other models did not contain sire pulse rate or found no significant relationship with sire pulse rate; the single exception identified a significant inverse relationship between GS and sire pulse rate (ΔAICc = 4.60), but we do not refer to it as a “top model” because two models that did not contain pulse rate had smaller ΔAICc values. Marginal R2 of pulse rate for linear mixed-effect models was obtained using r.squaredGLMM in package “MuMIn” (13); we do not provide R2 for GS because pseudo-R2 values for ordered logit models are not comparable to those for linear regressions. We examined our top models without three families that had both low survival and sires with more extreme values of pulse rate: family 9 (sired by the male with the fastest pulse rate) and families 4 and 15 (sired by the males with the slowest pulse rates). We report the number of models that were robust to their removal [i.e., those that still found that regression coefficients for pulse rate were significantly different from zero (P < 0.05)]. SVL and mass are conflated, but we present both for completeness.

View this table:

Braun et al. additionally contend that PC1 is difficult to interpret. Setting aside that mass, SVL, and GS all loaded positively onto PC1 and explained 91.6% of the variation [table S1 of (3)], we examined the variables separately. These analyses support our original findings: Sire pulse rate is significantly inversely associated with size and development rate in the best-fitting, non-averaged models, which are robust to the removal of outlier families (Table 1). Given Braun et al.’s focus on condition, we examined condition measurements that are appropriate for amphibians, and found that condition is not associated with sire pulse rate (Table 1). However, as noted above, size and development rate are better assays of fitness––particularly survival––in the context of spadefoot hybridization (Fig. 1).

The natural history, known fitness consequences of hybridization, evidence that sexual signals predict hybrid fitness, and behavior data across the different environments and populations cannot be ignored. Taken together, they demonstrate adaptive mate choice among heterospecific males.

References and Notes

  1. Braun et al. state that we excluded three families with zero survivorship from our analyses [despite their note 4], but they confound survivorship with absence of offspring: One female did not produce eggs, and the other two produced few eggs, none of which developed. Including these families would conflate female fecundity with offspring survival. Notably, Braun et al. also exclude these families, so their rationale for including them in their figure 1A is unclear.
  2. G. G. Rosenthal, Mate Choice (Princeton Univ. Press, 2017).

  3. When we analyze survival as family-level proportions and include parent measures using a linear model, we find that the top model contains only a quadratic term of pulse rate, the second-best contains only sires’ condition (ΔAICc = 3.00), and the third-best was the null model (ΔAICc = 3.86).
  4. K. P. Burnham, D. R. Anderson, Model Selection and Multimodel Inference: A Practical Information-Theoretic Approach (Springer, ed. 2, 2002).

  5. K. Bartoń, R package version 1.42.41 (2018).

Acknowledgments: We thank J. Umbanhowar for statistical consultation, and D. Pfennig, J. Kingsolver, G. Calabrese, P. Kelly, E. Laub, M. Reynolds, and B. Loflin for discussion. Supported by NSF grant IOS-1555520; C.C. was also supported by the NSF GRFP (DGE-1650116).

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SpaceX launches Starlink app and provides pricing and service info to early beta testers

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SpaceX has debuted an official app for its Starlink satellite broadband internet service, for both iOS and Android devices. The Starlink app allows users to manage their connection – but to take part you’ll have to be part of the official beta program, and the initial public rollout of that is only just about to begin, according to emails SpaceX sent to potential beta testers this week.

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The initial Starlink public beta test is called the “Better than Nothing Beta Program,” SpaceX confirms in their app description, and will be rolled out across the U.S. and Canada before the end of the year – which matches up with earlier stated timelines. As per the name, SpaceX is hoping to set expectations for early customers, with speeds users can expect ranging from between 50Mb/s to 150Mb/s, and latency of 20ms to 40ms according to the customer emails, with some periods including no connectivity at all. Even with expectations set low, if those values prove accurate, it should be a big improvement for users in some hard-to-reach areas where service is currently costly, unreliable and operating at roughly dial-up equivalent speeds.

Image Credits: SpaceX

In terms of pricing, SpaceX says in the emails that the cost for participants in this beta program will be $99 per moth, plus a one-time cost of $499 initially to pay for the hardware, which includes the mounting kit and receiver dish, as well as a router with wifi networking capabilities.

The goal eventually is offer reliably, low-latency broadband that provides consistent connection by handing off connectivity between a large constellation of small satellites circling the globe in low Earth orbit. Already, SpaceX has nearly 1,000 of those launched, but it hopes to launch many thousands more before it reaches global coverage and offers general availability of its services.

SpaceX has already announced some initial commercial partnerships and pilot programs for Starlink, too, including a team-up with Microsoft to connect that company’s mobile Azure data centers, and a project with an East Texas school board to connect the local community.

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Erratum for the Report “Meta-analysis reveals declines in terrestrial but increases in freshwater insect abundances” by R. Van Klink, D. E. Bowler, K. B. Gongalsky, A. B. Swengel, A. Gentile, J. M. Chase

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S. Rennie, J. Adamson, R. Anderson, C. Andrews, J. Bater, N. Bayfield, K. Beaton, D. Beaumont, S. Benham, V. Bowmaker, C. Britt, R. Brooker, D. Brooks, J. Brunt, G. Common, R. Cooper, S. Corbett, N. Critchley, P. Dennis, J. Dick, B. Dodd, N. Dodd, N. Donovan, J. Easter, M. Flexen, A. Gardiner, D. Hamilton, P. Hargreaves, M. Hatton-Ellis, M. Howe, J. Kahl, M. Lane, S. Langan, D. Lloyd, B. McCarney, Y. McElarney, C. McKenna, S. McMillan, F. Milne, L. Milne, M. Morecroft, M. Murphy, A. Nelson, H. Nicholson, D. Pallett, D. Parry, I. Pearce, G. Pozsgai, A. Riley, R. Rose, S. Schafer, T. Scott, L. Sherrin, C. Shortall, R. Smith, P. Smith, R. Tait, C. Taylor, M. Taylor, M. Thurlow, A. Turner, K. Tyson, H. Watson, M. Whittaker, I. Woiwod, C. Wood, UK Environmental Change Network (ECN) Moth Data: 1992-2015, NERC Environmental Information Data Centre (2018); .

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Max Q: NASA makes key discovery for future of deep space exploration

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Max Q is a weekly newsletter from TechCrunch all about space. Sign up here to receive it weekly on Sundays in your inbox.

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We’ve got an amazing lineup, including newsmakers we regularly cover here. NASA Administrator Jim Bridenstine will be there, as well as U.S. Space Force commanding office Jay Raymond, and Rocket Lab CEO Peter Beck, to name just a few. Tickets are available now, so sign up ASAP to get the best price possible.

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OSIRIS-REx probe 'tagging' the surface of asteroid Bennu

Image Credits: NASA

NASA has managed to collect a sample from the surface of an asteroid in a first for the agency. The sample collection came courtesy of NASA’s OSIRIS-REx robotic exploration probe, which was built by partner Lockheed Martin. OSIRIS-REx still has some work to do at Bennu, the asteroid from which it collected the sample, but next year it’ll begin heading back with its precious cargo intended for study by scientists here on Earth.

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Relativity Space has tons of promise in terms of its 3D-printed rockets, but it still hasn’t actually reached the launch stage. It did however secure a key government contract, with Lockheed Martin selecting its rocket for a forthcoming mission to test fluid management systems for NASA.

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