Moon phases influence encounters of anurans in the Brazilian semi-arid region of Piauí

As fases lunares influenciam o encontro de anuros na região brasileira do semiárido do Piauí

Abstract

The lunar cycle is associated with the behaviour of amphibians, influencing vocalization, reproductive behaviours, and environmental choices. In the present work, we aimed to answer some questions regarding the influence of lunar phases and their interference with the behaviours of semiarid anuran species in Piauí, Northeastern Brazil. The work was carried out in the municipality of Floriano (S 6°30’; W 43°42’), and an active search was carried out for the inventory, with weekly campaigns from June 2018 to June 2019. The days were transformed into Julian days, and the sampling corresponding to 365.25 Julian days (JDN) were transformed into degrees (JDN*360/365). The absolute occurrence by species was classified as constant, accessory, and accidental. In the circular statistics the Rayleigh test (Z) was applied to verify that there is no randomness, with the sample being considered unidirectional if the calculated interval corresponds to z ≥ z (α) and P=0.01. Sample sufficiency was estimated using the RAO’S (U) spacing test, with the value considered random when U<U (α) and P= 0.01. During the 13 months of sampling, 1,921 individuals were recorded in 12 species. Rhinella diptycha was the species with the highest record (constant species), and Physalaemus albifrons was the one with the lowest record of occurrence (accidental species). It was possible to deduce that most of the species (60%=6 of 10 species evaluated) present a greater occurrence between 270°–0°–90°, that is, the lunar albedo with less clarity. A similar pattern was observed in the Atlantic Forest in a previous study. Four species were considered constant; two were considered accessory species and six, species with accidental occurrence. Studies with the lunar synodic cycle are important, as they help to understand the mechanisms of interaction between anuran species and the environment.

Keywords:
Cerrado; lunar cycle; reproductive behaviour; vocalization

Resumo

O ciclo lunar está associado ao comportamento dos anfíbios, influenciando a vocalização, os comportamentos reprodutivos e as escolhas ambientais. No presente trabalho, objetivamos responder questões referentes à influência das fases lunares e sua interferência nos comportamentos de espécies de anuros do semiárido no Piauí, Nordeste do Brasil. O trabalho foi realizado no município de Floriano (S 6°30’; W 43°42’). Foram realizadas buscas ativas para o inventário, com campanhas semanais de junho de 2018 a junho de 2019. Os dias foram transformados em dias julianos e a amostragem correspondente a 365,25 dias julianos (DJN) foi transformada em graus (DJN*360/365). A ocorrência absoluta por espécie foi classificada em constante, acessória e acidental. Na estatística circular foi aplicado o teste de Rayleigh (Z) para verificar que não há aleatoriedade, sendo a amostra considerada unidirecional se o intervalo calculado corresponder a z≥z (α) e P=0,01. A suficiência amostral foi estimada pelo teste de espaçamento RAO’S (U), sendo o valor considerado aleatório quando U<U(α) e P= 0,01. Durante os 13 meses de amostragem foram registrados 1.921 indivíduos em 12 espécies. Rhinella diptycha foi a espécie com maior registro (constante) e Physalaemus albifrons foi a que apresentou menor registro de ocorrência (acidental). Foi possível deduzir que a maioria das espécies (60%=6 de 10 espécies avaliadas) apresenta maior ocorrência entre 270°–0°–90°, ou seja, no albedo lunar com menor luminosidade. Padrão similar foi encontrado para anfíbios da Mata Atlântica em estudo anterior. Quatro espécies foram consideradas constantes; duas foram consideradas espécies acessórias e seis espécies acidentais. Estudos com o ciclo sinódico lunar são importantes, pois auxiliam na compreensão dos mecanismos de interação entre espécies de anuros e o ambiente.

Palavras-chave:
Cerrado; ciclo lunar; comportamento reprodutivo; vocalização

1. Introduction

The ontogenetic development and evolutionary history of organisms and their variations associated with ecological interactions shape individuals and their populations (Martinez et al., 2018; Vidal‐García et al., 2014), molding endogenous and exogenous cycles that interfere with periods of reproduction, foraging, dispersion, occurrence, and migration (Margalef, 1983; Zimecki, 2006). Lunar phases influence the circadian cycle of living beings and studies on the influence of the lunar cycle on anurans have been occurring since 1960, with new research being developed more recently. However, understanding the lunar synodic cycle and the behaviour of anurans has not yet been sufficient to establish the behavioural pattern of anurans and the lunar cycle (Church, 1960; Lima et al., 2021). The behaviour of anurans regarding the vocalization and lunar clarity (Tuttle et al., 1982), reproductive behaviour, and breeding environment choices related to the lunar cycle (Baugh and Ryan, 2010), migration, and behaviour coupled to the lunar synodic cycle (Grant et al., 2009; Arnfield et al., 2012; Vignoli and Luiselli, 2013; Henrique and Grant, 2019) were studied with emphasis on a particular species in the space-time relationship and the influence of the lunar synodic cycle.

According to Lima et al. (2021), after analysing 37 species, 1,691 individuals in 882 days of research, the dark albedo of the moon influences the favourable foraging, vocalization, and reproduction behaviour for 78% of the species that made up the study in a region of the Atlantic Forest of Brazil. This country is home to six Biomes (Amazonia, Caatinga, Cerrado, Atlantic Forest, Pampa, and Pantanal) forming a territorial extension of 8,510,345,538 km², with 3.6 million km² of maritime territory and four time zones (IBGE, 2023). It is in this highly diverse territorial context, we find the state of Piauí with 251,529 km², consisting primarily of Caatinga and Cerrado with several transition areas (ecotones) and enclaves of cocais forests landscapes (Ivanov, 2020).

The state of Piauí is predominantly covered by xerophytic, deciduous vegetation, a feature that, in the Caatinga biome, gives rise to the so-called “white forest,” named for the loss of leaves during the dry season. The other major biome in the state is the Cerrado, whose vegetation formations include Campos Limpos and Cerradões (dense, medium-stature woodlands) characterized by twisted trunks and shrubs, thick bark, and deep root systems. In Piauí, this vegetation occurs under a tropical climate in the northern portion of the state and a semiarid climate in the south (Brasil, 2006; Floriano, 2023). The municipality of Floriano lies precisely within the transition zone between these vegetation types, containing enclaves of both Caatinga and Cerrado and exhibiting a predominantly semiarid climate.

In the present research, we seek to answer whether reproductive phenology is influenced by lunar phases, whether the influence is species-specific, or if it reaches the community with an equal degree of intensity, and if the same conditions of influence from the Atlantic Forest affect species from the ecotonal area of Piauí.

2. Materials and Methods

2.1. Geographic region

Data collection was carried out in the municipality of Floriano (S6°30’ W43°42’), a southwestern mesoregion of the state of Piauí, Northeastern Brazil, with an area of 3,676 km², limited by the municipalities of Amarante to the north, Itaueira and Flores do Piauí to the south, Francisco Ayres, Nazaré do Piauí and São José do Peixe to the east and Jerumenha to the west. The phytophysiognomy is Cerradão and Campo Cerrado vegetation. The climate is hot semi-arid tropical, and the annual rainfall mean is 950 mm with average temperatures between 29 and 30 °C (Floriano, 2023).

2.2. Sampling design

The anuran community inventory was carried out by active search, that is, visual and auditory search in which one or more researchers actively search for anurans in the most diverse habitats (litterfall, reproductive sites, lotic and lentic environments, rocky outcrops, hollow trunks, closed and open vegetation area) and shelters commonly used by the anurans (Lima and Pederassi, 2015).

2.3. Focal area and sampling period

The anuran activity was monitored in weekly efforts, at night, between 7:00 and 9:00 pm, from June 2018 to June 2019 in the Federal University of Piauí on the Amílcar Ferreira Sobral Campus – CAFS (S 6°30’9.4” W 43°42’23.6”).

2.4. Statistical and ecological analyses

2.4.1. Uniformity and accuracy of the sample period

The days were transformed into Julian days number using the formula JDN = (1461 * (Y + 4800 + (M - 14)/12)) / 4 + (367 * (M - 2 - 12 * ((M - 14)/12))) / 12 - (3 * ((Y + 4900 + (M - 14)/12) / 100)) / 4 + D - 32075 where day (D) is 1 to 31, month (M) is 1 to 12, and year (Y) is 1801 to 2099 (Fliegel and Van Flandern, 1968). In this way, the days were considered as intervals from noon until the subsequent noon, disregarding time intervals of weeks, months, and years. Thus, the sampling corresponded to 365.25 Julian days. The Julian day number (JDN) were transformed into degrees (JDN*360/365) (Costa-Jr, 2000).

2.4.2. Absolute and relative frequencies

The absolute frequency of individuals per sampling period was recorded, and the relative frequency of individuals was calculated using Prevalence (Equation 1), which consisted of:

$$P=\left(n/N\right)*100$$ (1)

where the n = number of specimens found and N = total number of species (Silveira Neto, 1976; Lima et al., 2022).

2.4.3. Constancy

To calculate Constancy, the absolute values of specimens and species were submitted to the Formula 2:

$$C=p*100/N$$ (2)

where p = number of samples containing the species and N= total number of samples, these classified as C=>50% – Constant, C=25–50% – Accessory, and C=<25% –Accidental (Silveira Neto, 1976).

2.4.4. Frequency polygon

The Frequency Polygon was established through the absolute frequency of specimens per species to the lunar synodic cycle to demonstrate the effects between linear interpretation and circular analysis of sample occurrence (Zar, 2010).

2.4.5. Circular statistics

The zero azimuth was the new moon, the angular direction being clockwise in the module, the mean vector (μ) was established through the mean location of the peak occurrence of the anuran species to the lunar influence, the length of the mean vector (r) was calculated to determine the direction of unidirectionality. The Rayleigh test (Z) was applied to verify that there is no randomness causing bias, with the sample being considered unidirectional if the calculated interval corresponds to z≥z (α) and P=0.01. Sample sufficiency was estimated using the RAO’S (U) spacing test, with the value considered random when U<U (α) and P= 0.01. If there is not enough data to meet the corollary of the test, the sample is considered insufficient (IS). To develop the tests, we used the ORIANA 4.02 program (Batschelet, 1981; Jammalamadaka and Sengrupta, 2001).

3. Results

During 46 weeks in 13 months (June 2018 to June 2019), 1,921 observations and 12 species were recorded, with Rhinella diptycha being the species with the highest number of records and Physalaemus albifrons the lowest. The absolute frequency of records per species for the entire sample period is shown in Figure 1.

Figure 1
Absolute frequency of occurrence of anuran species during the 46-week sampling period between June 2018 and June 2019.

When submitting the absolute occurrences per species to the constancy calculation, we classified the distribution as constant for Rhinella diptycha (100%), Leptodactylus vastus (95%), Scinax ruber (71%), and Scinax x-signatus(54%); between 25% and 50% accessory distribution for two species Boana raniceps (30%), and Pleurodema diplolister (26%); the other species showed accidental distribution behavior with a frequency of occurrence below 25% being Leptodactylus macrosternum (21%), Physalaemus cuvieri (13%), Rhinella mirandaribeiroi (13%), Leptodactylus fuscus (8%), Dendropsophus minutus (4%), and Physalaemus albifrons (2%) (Figure 2).

Figure 2
Relative frequency of occurrence by species and classification according to distribution behavior.

We considered species with a constant distribution that ones with occurrence above 50%. This research showed four species with constant distribution behaviour (54% to 100%): R. diptycha (100%), L. vastus (95%), S. ruber (71%), and S. x-signatus (54%). To evaluate their occurrences concerning the lunar synodic cycle, the data were submitted to the frequency polygon, in which we inferred that R. diptycha and L. vastus did not show predominance in a specific moon's phase while S. ruber and S. x-signatus predominate in the new and waning phases. Nonetheless, there is no absence of species in any phase. The other species with a frequency of occurrence of less than 50% did not allow us to evaluate using the frequency polygon (Figure 3).

Figure 3
Occurrence by lunar phase of species with constant distribution behavior.

The absolute frequency of occurrence of the 12 species during the 46 weeks and the respective synodic cycles of the moon were transformed into Julian days to meet the corollaries of distribution on a circular axis, making it possible to evaluate all species due to the greater accuracy of the mathematical model (Table 1).

Table 1
Constancy of the species and its occurrence according to the moon vector.

Distribution behavior (RF%)	Species	Mean Vector (Degrees)	Observations	Z Test	U Test	P Value
Constant (100)	Rhinella diptycha (Cope, 1862)	98.745	1243	13.94	350.73	P<0.01
Constant (95)	Leptodactylus vastus Lutz, 1930	50.96	270	0.26	319.26	P<0.01
Constant (71)	Scinax ruber (Laurenti, 1768)	58.45	160	3.59	309.85	P<0.01
Constant (54)	Scinax x-signatus (Spix, 1824)	169.96	125	10.85	303.21	P<0.01
Accessory (30)	Boana raniceps (Cope, 1862)	210.29	19	0.41	176.55	P<0.05 *
Accessory (26)	Pleurodema diplolister (Peters, 1870)	32.52	16	4.24	198	P<0.01
Accidental (21)	Leptodactylus macrosternum Miranda-Ribeiro, 1926	251.08	19	1.08	195	P<0.01
Accidental (13)	Physalaemus cuvieri Fitzinger, 1826	220.99	11	1.31	202.60	P<0.01
Accidental (13)	Rhinella mirandaribeiroi (Gallardo, 1965)	21.14	7	1.68	143.64	P<0.10*
Accidental (13)	Leptodactylus fuscus (Schneider, 1799)	134.53	19	8.31	215.36	P<0.01
Accidental (4)	Dendropsophus minutus (Peters, 1872)	74.06	2	0.82	0.0	IS
Accidental (2)	Physalaemus albifrons (Spix, 1824)	0.0	1	0.0	0.0	IS

• Values Calculated to establish unidirectionality with the lunar synodic cycle through the Rayleigh Test (Z) z≥z (α). RAO’S spacing (U) U<U (α). IS = insufficient sample; RF% = Relative frequency.
• ^*
  P values higher than expected.

When evaluating the constant distribution behaviour, the species S. x-signatus is the only one that presents an average occurrence for a full moon (light albedo) with a mean vector of 169.96° (Figure 4).

Figure 4
Scinax x-signatus represented on a circular axis with a mean vector of 169.96° and unidirectionality facing the full moon (light albedo).

Another species with constant distribution behaviour, R. diptycha, presented an average vector of occurrence for the crescent moon, that is, its albedo corresponds to the part of the illuminated lunar disk, and in the present research, it corresponded to 98.74° (Figure 5).

Figure 5
Rhinella diptycha represented on a circular axis with a mean vector of 98.74° and unidirectionality facing the crescent moon (partially light albedo).

The other two species with constant distribution behaviour were related to the dark albedo of the moon between the new and first quarter phases with respective average vectors of 50.96° for L. vastus and 58.45° for S. ruber (Figures 66B).

Figure 6
(A) Leptodactylus vastus represented on a circular axis with a mean vector of 50.96°; (B) Scinax ruber mean vector of 58.45°, both with unidirectionality facing the crescent moon (partially light albedo).

The species with accessory distribution behaviour were presented oppositely to the lunar angular axis, with B.raniceps having an average vector of 210.29°, which corresponds to the albedo of the transition from the full moon (light albedo) to the waning moon (light albedo darkening). For P.diplolister, the average vector corresponded to 32.52°, the albedo brightening to the first quarter. In both cases, the intensity of the reflected light is reduced, and the environment is partially dark (Figures 77B).

Figure 7
(A) Boana raniceps represented on a circular axis with a mean vector of 210.29°, the transition between the full and waning moon; (B) Pleurodemadiplolister on a mean vector of 32.52°, transition from new to crescent. Both are partially light albedo.

As for the six species with accidental distribution behaviour, D. minutus and P. albifrons had insufficient samples to establish a possible relationship of dependence on the lunar cycle (Table 1).

The last two species with accidental behaviour have a divergent relationship regarding their dependence on the lunar angle, with R. mirandaribeiroi being related to the dark albedo of the moon with an angle of 21.14° (new moon) (Figure 8A), while L. fuscus was associated with the crescent moon (light albedo) at an average angle of 134.53° (Figure 8B).

Figure 8
(A) Rhinella mirandaribeiroi, mean vector 21.14°, strong influence of the new moon; (B) Leptodactylus fuscus mean vector 134.53°, influence of the waxing moon.

4. Discussion

The Cerrado Biome is one of the 25 global biodiversity Hot Spots (Myers et al., 2000), which requires continued study to understand the interaction of biodiversity. The state of Piauí is included in this context, especially the municipality of Floriano, the site where the research was carried out, consisting of cerrado, campo cerrado, and cerradão (Brasil, 2006). This geographical condition and fragility of this biome would already be sufficient for the developing this research. However, the situation is worsened as studies related to anurans in Piauí, for the most part, stopped listing species to different landscapes of its territory (Silva et al., 2007; Loebmann and Mai, 2008; Roberto et al., 2013; Dal Vechio et al., 2013; Andrade et al., 2014, 2022).

Considering the need to understand better the interactions of the anuran community in this area with a large sampling gap, we sought to develop research relating to the dependence of species occurrence on the lunar synodic cycles and the occurrence of 12 species for the sampling period of 48 weeks, which is within the expected range, according to previous citations, which establish lists from 12 to 31 species occurring through research in different regions of Piauí. Therefore, we can deduce that studies with a sample universe of species that corresponds to approximately 50% of the number of species predicted for the region do not differ from other work that related anurans and the lunar cycle, such as Tuttle et al. (1982) that worked on hylids linking the species with lunar clarity, as well as Lima et al. (2021) that record the lunar occurrence for 37 species.

Although additional abiotic factors such as rainfall and atmospheric pressure were not included in the analyses, the sampling design—covering 12 complete and consecutive lunar synodic cycles—was intended to provide robustness to the dataset. Because these environmental variables naturally covary, and cannot be experimentally isolated in field conditions, our approach focused on capturing the overall pattern of vocal activity across a broad temporal scale. Thus, even without explicitly modelling each abiotic factor, the extended and continuous sampling period helps to integrate their potential influence and reduce the impact of short-term stochastic variation.

In the present research, it was possible to deduce that most species (60% = 6 of 10 species evaluated) presented a prevalence of occurrence between 270° – 0° – 90°, which represents the period of low lunar albedo. This finding suggests that the influence of the synodic cycle is a widespread phenomenon. To contextualize the scope of our research, and demonstrate that lunar influence studies are valuable regardless of the number of species investigated, we note that Grant et al. (2009) worked on the reproductive phenology of 12 anuran species in Italy and the United Kingdom, while Henrique and Grant (2019) studied the displacement of just one species, L. latrans, influenced by the lunar synodic cycle in the municipality of Eldorado do Sul, RS, Brazil.

Understanding the interaction mechanisms of anuran species with the lunar synodic cycle will allow us to promote actions that will mitigate anthropogenic interference, providing increased conservation of the anuran community. Even species that are usually present in anthropic environments, such as R. diptycha and L. Vastus (Soares et al., 2011), which had a constant distribution behaviour, showed a tendency to increase in occurrence during the dark albedo of the moon (L. vastus 50.96° and R. diptycha 98.74°) therefore, even though there is an adaptation to the synanthropic condition, the behavioural information for occurrence during the dark albedo of the moon prevailed.

Among the 40% of species with behaviour that is not related to dark albedo, three species (L. macrosternum, P. cuvieri, and B. raniceps) are associated with partially light albedo (251.08°, 220.99° and 210.29°– Waning Moon). We highlight that the three species are found in anthropized areas, dry open environments, temporary puddles, streams, and lakes (Mai and Loebmann, 2010; Freitas, 2011), which denotes the peculiar adaptation of these species in the most varied environments.

The environments occupied by anurans, increasingly anthropic, with intense public lighting, may mean that the partially light albedo, like the waning moon, already offers sufficient conditions for camouflage. Dias et al. (2019) described several situations involving losses and adaptations of anurans in illuminated environments, which requires a deeper understanding of the behaviour of anurans depending on the lunar synodic cycle and this relationship with environments altered by artificial lighting.

Among the anurans studied, L. fuscus was associated with the crescent moon (light albedo) at an average angle of 134.53°. When we correlated this lighting condition with the species' behaviour during the reproductive period, we speculated that the lighting should have little effect since this species reproduces inside burrows (Heyer, 1969). Martins (1988), when studying the reproductive behaviour of L. fuscus for 34 nights, observed that the male digs the burrow and sings near its entrance until the female approaches. Considering this reproductive behaviour associated with shelter, the lunar albedo represents less exposure of the species to the environment, as they have the burrow to protect themselves and continue the reproductive process.

The only species in this study directly associated with the lunar light albedo was S. x-signatus, with an average vector of 169.9° (full moon), commonly found in homes and popularly known as bathroom frog (Freitas, 2011), a behaviour that configures the species’ synanthropism and demonstrates a probable change from native behaviour to a home associated species, away from predators, close to the preys attracted by the light and humidity, a common condition in Brazilian bathrooms.

In the present study, we observed that reproductive phenology is influenced by lunar phases, regardless of light or dark albedo, as the records of the species studied here present unidirectionality of occurrence associated with dependence on lunar albedo, except for two species that had insufficient sampling. The degree of influence of the lunar synodic cycle is conspicuous to the species. Therefore, occurrence behaviour is not associated with the community but rather with the population level. More than 50% of the anuran species in the semi-arid region of Piauí – similarly to the pattern reported for the Atlantic Forest by Lima et al., (2021) –, occur during the dark albedo of the lunar phases. Thus there appears to be a similar general pattern in response to the lunar synodic cycle across regions, despite strong differences in species richness and in species-specific responses.

To conclude, efforts are need to be made to understand if species that present synanthropism and species with a strong dependence on the lunar synodic cycle are at risk of becoming extinct due to the urban, rural, and agricultural expansion.

Acknowledgements

The authors thank Kléssia Denise S.S. Souza for the line drawings in the figures.

Data Availability Statement

All data analysed in the research are available at (Mendeley Data), and can be accessed via https://data.mendeley.com/datasets/mz7shw4jt9/1.

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Edited by

• Editor:
  Takako Matsumura Tundisi

Publication Dates

• Publication in this collection
  19 June 2026
• Date of issue
  2026

History

• Received
  31 Aug 2025
• Accepted
  19 Jan 2026