Revista de Biología Tropical, ISSN: 2215-2075, Vol. 71(S4): e57188, diciembre 2023 (Publicado Nov. 01, 2023)

First mercury and stable isotope assessment from an unusual

mass stranding of rough-toothed dolphins (Steno bredanensis)

(Artiodactyla: Delphinidae) in Azuero peninsula, Pacific coast of Panama

Dalia C. Barragán-Barrera1, 2* https://orcid.org/0000-0003-4023-9908

Lissette Trejos Lasso3, 4 https://orcid.org/0000-0002-2495-0452

Betzi Pérez-Ortega3, 5 https://orcid.org/0000-0001-5414-6329

José Julio Casas4, 6, 7 https://orcid.org/0000-0001-9951-0542

Roberto Santamaria Valverde6 https://orcid.org/0000-0001-7371-8273

1. Instituto Javeriano del Agua, Pontificia Universidad Javeriana, Bogotá, Colombia; daliac.barraganbarrera@gmail.com

(Correspondence*)

2. R&E Ocean Community Conservation Foundation, Oakville, ON, Canada; daliac.barraganbarrera@gmail.com

3. Fundación Panacetacea Panamá, Ciudad de Panamá, Panamá; ltrejos@miambiente.gob.pa; betziperez@yahoo.com

4. Ministerio de Ambiente, Avenida Ascanio Villalaz, edificio 500, Ancón, Panamá.; ltrejos@miambiente.gob.pa;

jcasas@miambiente.gob.pa

5. Biology Department and Redpath Museum, McGill University, Montreal, Canada; betziperez@yahoo.com

6. Facultad de Ciencias del Mar, Universidad Marítima Internacional de Panamá – UMIP, Ciudad de Panamá, Panamá;

jcasas@miambiente.gob.pa; santamariaroberto43@gmail.com

7. Estación Científica Coiba AIP, Ciudad de Panamá, Panamá; jcasas@miambiente.gob.pa

Received 11-VIII-2022. Corrected 09-XII-2022. Accepted 26-V-2023.

ABSTRACT

Introduction: Small cetaceans are good bioindicators of environmental contamination; however, knowledge

about their ecotoxicological status in Central America is scarce. In Panama, access to samples from wild popula-

tions to determine the ecotoxicological status of oceanic dolphins is limited; therefore, stranding events provide

an alternative for obtaining samples. In April 2016, a rare mass stranding event occurred in the Azuero Peninsula

(Pacific coast of Panama), where 60 rough-toothed dolphins (Steno bredanensis) stranded, including ten which

died on the beach.

Objective: To assess total mercury (THg) concentrations, and δ13C and δ15N stable isotope values in rough-

toothed dolphins for the first time in this region.

Methods: Nine skin samples were collected from adults, stored in 70 % ethanol, and posteriorly analyzed to

determine THg concentrations and stable isotope values.

Results: THg concentrations ranged from 4 764 to 18 689 ng g-1 dry weight (dw) (mean = 12 841; SD = 5 083 ng

g-1 dw), δ13C values ranged between −16.8 and −15.2 ‰ (mean = −16.2; SD = 0.6 ‰), and δ15N values ranged

between 14.3 and 15.9 ‰ (mean = 15.0; SD = 0.5 ‰).

Conclusions: High THg concentrations reported for this species in the Azuero Peninsula are consistent with

values reported for rough-toothed dolphins in other areas worldwide, such as the central-northern Rio de Janeiro

State in Brazil and La Guajira in the Colombian Caribbean. Elevated mercury (Hg) concentrations may be related

to the rough-toothed dolphin diet, which according to δ15N values found here, appears to be based mainly on

high trophic level prey that bioaccumulate more Hg in their tissues compared to lower trophic level organisms.

However, additional dietary studies would be required to support these findings. Continuing monitoring of

https://doi.org/10.15517/rev.biol.trop..v71iS4.57188

SUPPLEMENT • SMALL CETACEANS

2Revista de Biología Tropical, ISSN: 2215-2075 Vol. 71(S4): e57188, diciembre 2023 (Publicado Nov. 01, 2023)

INTRODUCTION

Mercury (Hg) is a global contaminant

that is biomethylated in aquatic sediments by

microorganisms (Alcala-Orozco et al., 2019;

Wiener & Suchanek, 2008) from the inorganic

form (Hg2+) to the organic methylmercury

(CH3Hg+; MeHg) form. The organic MeHg

form is more toxic and bioaccumulates in

aquatic organisms through an individual’s life,

causing deleterious effects (Bossart, 2011; Cor-

rea et al., 2014; Reif et al., 2015; Schwacke et al.,

2002; Wiener et al., 2003). Top predators are

particularly vulnerable to the harmful effects

because Hg biomagnifies through aquatic tro-

phic chains (Bosch et al., 2016).

Because of their role as top predators,

dolphins tend to bioaccumulate high Hg levels

in their tissues, so they can be bioindicators of

contamination worldwide (e.g., Aubail et al.,

2013; Barragán-Barrera, Luna-Acosta et al.,

2019; Cáceres-Saez et al., 2015). However, in

Central America little information is available

on the toxicological status of dolphins. The few

Hg assessments that have been conducted in

this region have focused mainly on fish from

the Pacific coast of Costa Rica and Nicaragua,

examining Hg concentrations in four elasmo-

branchs, as well as 23 other freshwater and

marine fishes (Elliot et al., 2015; Sandoval et

al., 2015). Likewise, for feeding ecology stud-

ies based on stable isotopic data, one study has

traditional dietary analysis, as well as contamination levels in fish and dolphins, is necessary to understand the

dolphins’ ecotoxicology in Panama.

Key words: Steno bredanensis; dolphins; cetaceans; contamination; heavy metals; ecotoxicology; Panama.

RESUMEN

Primera evaluación de mercurio e isótopos estables de delfines de dientes rugosos (Steno bredanensis) pro-

venientes de un varamiento masivo inusual en la península de Azuero, Costa Pacífica de Panamá

Introducción: Los pequeños cetáceos son buenos bioindicadores de la contaminación ambiental; sin embargo, el

conocimiento acerca de su estado ecotoxicológico en Centroamérica es escaso. En Panamá, el acceso a muestras

para determinar el estado ecotoxicológico de delfines oceánicos es limitado; por lo tanto, los varamientos pro-

veen una alternativa para obtener muestras. En abril de 2016, un raro evento de varamiento masivo ocurrió en la

Península de Azuero (Pacífico panameño), en el cual 60 delfines de dientes rugosos (Steno bredanensis) vararon

incluyendo diez que murieron en la playa.

Objetivo: Determinar los niveles de mercurio total (THg), e isótopos estables de δ13C y δ15N en los delfines de

dientes rugosos por primera vez en la región.

Métodos: Nueve muestras de piel de adultos fueron colectadas, almacenadas en etanol al 70 %, y analizadas pos-

teriormente para determinar THg e isótopos estables.

Resultados: Las concentraciones de THg variaron entre 4 764 y 18 689 ng g-1 de peso seco (dw) (promedio=

12 841; DE= 5 083 ng g-1 dw), los valores de δ13C entre −16.8 y −15.2 ‰ (promedio= −16.2; DE= 0.6 ‰), y los de

δ15N entre 14.3 y 15.9 ‰ (promedio= 15.0; DE= 0.5 ‰).

Conclusiones: Los altos niveles de THg reportados para esta especie en la Península de Azuero son consistentes

con los reportados en la piel de los delfines de dientes rugosos en otras áreas del mundo, como en el estado de

Río de Janeiro en Brasil y La Guajira en el Caribe colombiano. Las altas concentraciones de mercurio (Hg) pueden

estar relacionadas con la dieta de los delfines de dientes rugosos, la cual, de acuerdo a los valores de δ15N encon-

trados aquí, parece estar basada en presas de alto nivel trófico que acumulan más Hg en sus tejidos. Sin embargo,

estudios dietarios adicionales son requeridos para confirmar estos resultados. Un monitoreo continuo de la dieta

usando análisis tradicionales, así como de los niveles de contaminación en peces y delfines, es necesario para

entender la ecotoxicología de los delfines en Panamá.

Palabras clave: Steno bredanensis; delfines; cetáceos; contaminación; metales pesados; ecotoxicología; Panamá.

Nomenclature: SMT1: Supplementary material Table 1; SMF1: Supplementary material Figure 1.

Revista de Biología Tropical, ISSN: 2215-2075, Vol. 71(S4): e57188, diciembre 2023 (Publicado Nov. 01, 2023)

been conducted with demersal elasmobranchs

in the Costa Rican Pacific basin (Espinoza et

al., 2015). To date, the only published cetacean

study in Central America which determined

both total Hg (THg) and stable isotope mea-

surements was conducted on common bottle-

nose dolphins Tursiops truncatus (Montagu,

1821) from the Bocas del Toro Archipelago in

the Panamanian Caribbean (Barragán-Barrera,

Luna-Acosta et al., 2019).

The lack of ecotoxicological studies focused

on cetaceans in Panama is mainly due to the

difficulty in obtaining tissue samples from

wild populations, particularly for species with

oceanic habits like the rough-toothed dolphin,

Steno bredanensis (G. Cuvier in Lesson, 1828).

This species is found mainly in oceanic waters

in tropical latitudes (Jefferson, 2018), with

some occurrences in nearshore waters of oce-

anic islands (Baird, 2016; Oremus et al., 2012).

Access to tissue samples of oceanic species is

challenging, particularly where no long-term

marine mammal monitoring programs have

been established. Therefore, stranding events

provide a good alternative to obtaining samples

from oceanic dolphins.

On the night of April 19th, 2016, a group

of 60 rough-toothed dolphins was reported in

a rare mass stranding event on the Pacific coast

of Panama. This event occurred on Ostional

beach (“Playa Ostional”), in the Tonosí dis-

trict of the Azuero Peninsula, 340 kilometers

southwest of Panama City (Fig. 1). The event

was unusual because this species strands less

frequently than other marine mammal spe-

cies (Mackey et al., 2003), and most strand-

ings in Panama have been reported as isolated

individuals (May-Collado et al., 2017). In the

early morning of April 20th, 2016, experienced

marine biologists and veterinarians from the

Environmental Ministry of Panama (MiAmbi-

ente), the International Maritime University of

Panama (UMIP by its acronym in Spanish), the

Universidad de Panamá, the Aquatic Resources

Authority (ARAP by its acronym in Spanish),

Fundación Panacetacea (non-governmental

organization), and local fishermen attended to

the stranded animals. Ten of the 60 individuals

died on the beach (Fig. 1), and the remaining

animals were rescued and moved to deeper

waters. According to their total lengths, nine

of the ten deceased individuals were classi-

fied as adults (> 255 cm) and one as a calf (<

1m) (Mackey et al., 2003; Reeves et al., 2008).

Necropsies were conducted on the adults in

situ, and basic morphometric data and tissue

Fig. 1. Location of Ostional Beach in the Azuero Peninsula, Pacific coast of Panama, where the mass stranding of rough-

toothed dolphins (Steno bredanensis) occurred in April 2016. On right above, a photo of the mass stranding. On right below

a species description illustrated by Emmanuel Laverde © www.arteyconservacion.com

4Revista de Biología Tropical, ISSN: 2215-2075 Vol. 71(S4): e57188, diciembre 2023 (Publicado Nov. 01, 2023)

samples were collected. Sex was determined by

external examination, identifying five females

and four males. The calf was transported to the

laboratory to conduct the necropsy and posteri-

or analyses. To prevent sampling biases related

with tissue decomposition, fresh or moderate

carcasses with body condition code 2 or 3 (Kui-

ken & Hartmann, 1991; Geraci & Lounsbury,

1993) should be used (Méndez-Fernández et

al., 2020). Therefore, because all animals were

freshly dead (body condition code 2; Geraci &

Lounsbury, 1993), samples were used for this

ecotoxicological assessment.

Although the liver is considered the main

storage organ for Hg (e.g., Mackey et al., 2003),

recent analyses in small cetaceans have shown

that skin also reflects the concentration of

Hg in the internal organs (e.g., Aubail et al.,

2013; Cáceres-Saez et al., 2015; Fontaine et al.,

2015). Therefore, nine skin samples collected

from the adults (stored in 70 % ethanol at -20

°C) were used to conduct the Hg and stable

isotope assessments. Because the remoteness

of the stranding area, ethanol was the prefer-

able storage method; however, in cetaceans’

skin, ethanol could affect the composition of

the δ13C stable isotope concentration by show-

ing depletion (e.g., Hidalgo-Reza et al., 2019;

Kiszka et al., 2014). Nevertheless, the magni-

tude of ethanol effects on this isotope in dol-

phins’ skin has not been fully confirmed like

a linear relationship as has been assessed in

other taxa (Kiszka et al., 2014). To address the

potential issues related to ethanol preservation

on the δ13C composition results, lipids should

be removed, since they are depleted in δ13C

(De Niro & Epstein, 1978; Tieszen et al., 1983).

Previously to this, samples were covered with

aluminum foil and left on a bench to let the

ethanol evaporate. Posteriorly, samples were

washed ten times with distilled water, which

was evaporated at 45 °C over 48 h, and samples

were then ground and freeze-dried.

To extract lipids for isotopic analyses, the

whole sample (up 50 mg each) was delipidated

as follows: 4 ml of cyclohexane was added,

next, the sample was agitated constantly for

10 min, centrifuged at 4 500 rpm for 5 min,

and the lipid supernatant was discarded. This

process was repeated three times, and then the

sample was dried at 45 °C in an oven for 48 h.

Finally, around 0.02 – 0.04 mg lipid-free sample

was weighed in a tin cup to perform posterior

stable isotope analyses in a continuous flow

mass spectrometer (Delta V Plus with a Con-

flo IV Interface, Thermo Scientific, Bremen,

Germany) coupled to an elemental analyzer

(Flash 2000 or EA Isolink, Thermo Scientific,

Milan, Italy). The usual δ notation relative to

Vienna PeeDee Belemnite Standard for δ13C

and atmospheric N2 for δ15N, in parts per thou-

sand (‰), was used to express the results (Mén-

dez-Fernández et al., 2020). Measurements in

duplicates of internal laboratory standards

(acetanilide) during each autorun indicated an

experimental precision (SD) of 0.03 for δ13C

and 0.09 for δ15N. To determine if lipid extrac-

tion was efficient, the C:N ratio was assessed

using the percent C and N elemental composi-

tion, in which values lower than four indicate

good lipid removal (Lesage et al., 2010).

As described in Vélez et al. (2021), THg

concentrations were measured using an atomic

absorption spectrometer AMA-254 (Altec ©

Advanced Mercury Analyzer-254). To control

the analytical quality of THg measurements,

these were repeated at least two times until

there were analytical differences below 10%.

Additionally, blanks were run at the beginning

of the analytical session, and certified reference

material (CRM) TORT-2 (Reference Material

of lobster hepatopancreas marine certified by

the National Research Council of Canada) were

used after blanks and every four analyses. The

CRM measured concentration was 251 ng g-1 (n

= 2) and showed good precision with a percent-

age of recovery of 93 %. THg measurements are

presented in ng g-1 on a dry weight basis (dw)

and the detection limit was 0.05 ng.

The results found all nine samples (females:

n = 5; males: n = 4) collected from the rough-

toothed dolphins showed detectable concentra-

tions of THg, with a mean of 12 841; SD = 5

083 ng g-1 dry weight (dw) ranging between

4 764 to 18 689 ng g-1 dw (SMT1). Regarding

the stable isotopes, the C:N ratios reflected an

Revista de Biología Tropical, ISSN: 2215-2075, Vol. 71(S4): e57188, diciembre 2023 (Publicado Nov. 01, 2023)

efficient lipids removal (SMT1), so the results

showed that δ13C values ranged between –16.8

and –15.2 ‰ (mean = –16.2; SD = 0.6 ‰), and

δ15N values ranged between 14.3 and 15.9 ‰

(mean = 15.0; SD = 0.5 ‰) (SMT1).

This study presents the first assessment of

THg and stable isotope measurements in rough-

toothed dolphins from Central America, which

is considered to be a species of “Least Concern”

but with an “unknown population trend” by

the IUCN Red List (Kiska et al., 2019). Bycatch

and direct harvest have been considered threats

to this species (Avila et al., 2018; Kiska et al.,

2019), and metal exposure may be considered

as a potential hazard. Several negative effects

have been associated to Hg in marine mam-

mals, such as immunotoxicity (Desforges et al.,

2016), neurotoxicity (Krey et al., 2015), repro-

ductive, endocrine, heart, and kidney damage

(Bossart, 2011; Correa et al., 2014; Kershaw

& Hall, 2019; Schwacke et al., 2002), as well as

cancer (Béland et al., 1993; Martineau et al.,

1994). The high concentrations that rough-

toothed dolphins bioaccumulate may warrant

special attention and qualifies them as a bioin-

dicator species in oceanic waters. However, the

rough-toothed dolphin is a highly migratory

species, so THg levels reported in their tissues

do not necessarily reflect the local Hg levels.

Studies in several areas worldwide have

found rough-toothed dolphins to have high

THg levels (SMT2). For instance, in La Guajira

(Colombian Caribbean), an assessment of Hg

concentrations in the skin of five dolphin spe-

cies (Atlantic spotted dolphin Stenella fronta-

lis Cuvier, 1829; common bottlenose dolphin;

common dolphin Delphinus sp.; rough-toothed

dolphin; and spinner dolphin Stenella longiros-

tris (Gray, 1828)) showed the highest values

for rough-toothed dolphins (THg-skin-mean

= 16 817; SD = 3 815 ng g-1 dw; n = 3; Bar-

ragán-Barrera, Farías-Curtidor, Luna-Acosta

et al. 2019, Barragán-Barrera, Farías-Curtidor,

Chávez-Carreño et al., 2019). Similarly in Bra-

zil, specifically in the central-northern Rio de

Janeiro State, rough-toothed dolphins showed

the highest values in their muscle (THg-mean

= 10 150; SD = 6 230 ng g-1 dw; n = 9), in

comparison to muscle of coastal species like the

Franciscana (Pontoporia blainvillei (Gervais &

d’Orbigny, 1844); THg-mean = 1 920; SD = 960

ng g-1 dw; n = 16) and Guiana dolphin (Sotalia

guianensis (Van Bénéden, 1864); THg-mean =

3 910; SD = 2 160 ng g-1 dw; n = 28) (Baptista et

al., 2016). The same pattern was observed in the

southern Rio de Janeiro State in Brazil, where

rough-toothed dolphins showed the highest

THg values in their liver (THg-mean = 594 800;

SD = 200 300 ng g-1 dw; n = 3) in comparison

to the liver of the coastal form of common

bottlenose dolphin (THg-mean = 4 380; SD =

2 470 ng g-1 dw; n = 10) and the offshore Atlan-

tic spotted dolphin (THg-mean = 8 130; SD =

10 470 ng g-1 dw; n = 3) (Lemos et al., 2013).

The high THg levels likely reflect the

rough-toothed dolphins high-trophic prey pref-

erences, which appear to be indicated by the

isotopic values enriched in δ15N that have been

reported in dolphins’ skin here and in other

areas worldwide (SMT3). Examples include

the southern Rio de Janeiro State in Brazil

(δ15N-mean = 14.5; SD = 0.1 ‰; n = 3; δ15N-

mean-autumn = 18.1; SD = 0.5 ‰; N = 4; δ15N-

mean = 18.6; SD = 0.2 ‰; n = 5; Paschoalini

et al., 2021; Troina et al., 2020, 2021), La Gua-

jira in the Colombian Caribbean (δ15N-mean

= 12.8; SD = 0.1 ‰; n = 3; Barragán-Barrera,

Farías-Curtidor, Chávez-Carreño et al., 2019),

and Moorea Island in the Society Archipelago

(δ15N-mean = ~ 14.7 ‰; N = 35; Kiszka et al.,

2010). The rough-toothed dolphin diet consists

of cephalopods and fish of various sizes, includ-

ing large and carnivorous fish with high trophic

levels like black skipjack (Euthynnus lineatus

Kishinouye, 1920), mahimahi (Coryphaena hip-

purus Linnaeus, 1758), and ribbonfish (Trichi-

urus lepturus Linnaeus, 1758) (Ortega-Ortiz

et al., 2014; Pitman & Stinchcomb, 2002; West

et al., 2011). Unfortunately, we didn’t find any

content in the carcasses’ stomachs, so insights

about dolphins’ diet in Panamanian Pacific

waters is still unknown.

The δ13C values reported here are depleted

in δ13C, which suggests oceanic habits. These

isotopic measurements are similar to those

found in skin samples stored in ethanol of

6Revista de Biología Tropical, ISSN: 2215-2075 Vol. 71(S4): e57188, diciembre 2023 (Publicado Nov. 01, 2023)

rough-toothed dolphins from La Guajira in the

Colombian Caribbean (δ13C-mean = −14.7;

SD = 0.2 ‰; n = 3; Barragán-Barrera, Farías-

Curtidor, Chávez-Carreño et al., 2019), and the

Society Archipelago (δ13C-mean = ~ −14.9 ‰;

n = 35; Kiskza et al., 2010) where the species

has neritic habits (Farías-Curtidor & Barragán-

Barrera, 2017, Farías-Curtidor & Barragán-

Barrera, 2019; Oremus et al., 2012). Indeed,

the species has been reported in coastal waters

along the Caribbean of Honduras and Panama

(Barragán-Barrera et al., 2015; Kuczaj & Yeater,

2017). However, some δ13C values reported

for rough-toothed dolphins in the Panama-

nian Pacific basin are similar to those reported

for frozen skin samples of common dolphin

oceanic form (Delphinus delphis Linnaeus,

1758) in the Gulf of California, Mexican Pacific

(δ13C-mean = −18.3; SD = 0.2 ‰) (Elorriaga-

Verplancken et al., 2020). Nevertheless, this

interspecific comparison should be interpreted

with caution due potential bias derived from

ethanol preservation on our samples (Kiskza et

al., 2014). Thus, until more information about

the effect of ethanol on rough-toothed dolphin

skin samples, as well as their potential prey and

the isoscapes is provided, it is not possible to

assess the ecological habitats of rough-toothed

dolphins in the Pacific basin of Panama.

Further monitoring is needed to assess

the rough-toothed dolphins’ feeding ecology

in the Panamanian Pacific basin, including

the assessment of their diet through stomach

content analysis or direct feeding behavior. For

isotopic analysis, it is highly recommended to

collect samples and storage frozen. Addition-

ally, complementary analyses that include the

characterization of isotopic content of organic

material content at the base of local food webs,

in order to determine local carbon sources as

well nitrogen reference levels, are necessary.

This study provides the first contribution of

ecotoxicological knowledge on a little-known

cetacean predator found in Central America,

providing important baseline data to under-

stand the feeding ecology as well as the con-

tamination of dolphins in the region.

Ethical statement: the authors declare that

they all agree with this publication and made

significant contributions; that there is no con-

flict of interest of any kind; and that we fol-

lowed all pertinent ethical and legal procedures

and requirements. All financial sources are fully

and clearly stated in the acknowledgments sec-

tion. A signed document has been filed in the

journal archives.

Author Contribution: DCBB, LTL, BPO

conceptualized the study. DCBB, LTL acquired

funding. LTL, BPO, RSV collected samples. JJ

provided logistic support. DCBB performed lab

and statistical analysis, and wrote the draft and

revisions of this manuscript. BPO participated

in editing some versions of this manuscript. All

authors approved the final version.

See supplementary material

a11v71s4-MS1

ACKNOWLEDGMENTS

We especially thank the Ministry of Envi-

ronment (MiAmbiente) of the Republic of

Panama, the National Aeronaval Service of

Panama (SENAN), and the International Mari-

time University of Panama (UMIP) for their

logistic support. Special thanks to the commu-

nity of the Ostional Beach in Tonosí for their

rapid response and help in the rescue effort.

We thank P. Bustamante, C. Churlaud, and

M. Brault-Favrou for facilitating Hg analyses,

and G. Guillou for conducting the mass spec-

trometer analyses. This study was carried out

under the scientific permit SE/AO-1-16 from

(MiAmbiente) granted to Fundación Panac-

etacea Panamá. The Vicerrectoría de Investiga-

ciones from Pontificia Universidad Javeriana

is acknowledged by providing a Postdoctoral

Grant (Call 2021-2) to D. Barragán (2022), who

also thanks to the Instituto Javeriano del Agua

for its support during the Postdoctoral stay.

This study was supported partially by the Small

Grant in Aid of Research from the Society for

Revista de Biología Tropical, ISSN: 2215-2075, Vol. 71(S4): e57188, diciembre 2023 (Publicado Nov. 01, 2023)

Marine Mammalogy (D. Barragán, 2019). We

thank Kristin Rasmussen for her English revi-

sion to the manuscript, Alejandra Duarte for

her support preparing the map, and Emmanuel

Laverde from Arte y Conservación (www.artey-

conservacion.com) for making the dolphin

illustration. Finally, we thank the anonymous

reviewers whose comments improved the final

version of this manuscript.

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